Telecom & Networks
Antenna Systems Measurement and Troubleshooting
Table of Contents
Course Synopsis …………………………………………………………………… i
Part 1: Electromagnetic Propagation, Antennas and Transmission Lines
Electromagnetic Propagation ………………………………………………………
Antennas ……………………………………………………………………………..
Path Effects ………………………………………………………………………….
Transmission Lines ………………………………………………………………….
Waveguides ………………………………………………………………………….
7
8
15
31
38
51
Part 2: Antenna System Characteristics and Performance …………………
Antenna Characteristics …………………………………………………………….
Transmission Line Characteristics …………………………………………………
Distributed and In-Building Antenna Systems……………………………………..
Filtering Systems …………………………………………………………………….
58
59
81
86
91
Part 3: Antenna System Measurement and Troubleshooting………….......
Potential Antenna System Problems ……………………………………………..
Antenna System Measurement and Interpretation ………………………………
110
111
123
Part 4: Laboratory Practice ………………………………………………………
147
APPENDIX A: Anritsu Site Master Distance-to-Fault Application Note …...….
161
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We understand that waves on pond are a vertical movement of medium, water, and that
these waves propagate away from a disturbance. Propagation implies advancement
through regenerating itself, and the fall of one wave on a pond creates the wave in front
of it.
But what is the medium for radio waves? Radio signals travel by Electromagnetic
Propagation. Only a Magnetic Field or only an Electric field would die off quickly with
distance but when an electric field changes rapidly (and radio frequency), in creates a
magnetic filed. Likewise, a rapidly changing magnetic field creates an electric field, so
the energy is constantly transferred between and electric and magnetic fields, each
creating the wave in front of it.
The voltage in a transmitting antenna creates the electric field and the current in a
transmitting antenna creates the magnetic field. Likewise, the electric field induces a
voltage in the receiving antenna and the magnetic field induces a current in the
receiving antenna.
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The electric field (E) and magnetic field (H) in a propagating wave are always at 90
degrees to one another. The orientation of the electric field defines the polarity of the
wave. Vertical and horizontal polarity are the most common, although circular polarity
(rotates in a spiral) is also possible.
The wavelength is the distance between two adjacent peaks in the electric or magnetic
field wave because this is the distance that the wave propagates in once cycle.
Therefore the wavelength can be calculated with the simple formula
l=c/f
where c is the speed of propagation in the medium (speed of light in free space,
approximately 3 x 108 m/s).
If the wave propagates in a transmission line or waveguide, the propagation speed will
be slower by a percentage called the velocity factor. For example, if a transmission line
has a velocity factor of 60%, the wave will propagate at about 2 x 108 m/s, so it is
important to consider this when calculating the wavelength in a transmission line.
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The phenomenon of resonance is used in transmission lines, waveguides, filters and
some types of antennas. Whenever certain points of a wave are constrained, the wave
will tend to oscillate at specific frequencies which are multiples of half wavelengths
between the constrained points.
For example, in a waveguide, the copper walls of the waveguide short-circuit the
electric field and therefore constrain the electric field to be zero at those points. In a
dipole, the open-circuit ends constrain the current to be zero at those points.
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dB is a convenient way to express gain, loss and power levels in a communication
system because it can easily handle very large and very small numbers, and because
multiplications and divisions become additions and subtractions.
An exponent is the number of times that a number (e.g. 10) is multiplied by itself. By
definition, any number to the exponent 0 equals 1. Therefore 100 = 1, 101 = 10 and 103
= 1000 etc. A negative sign in the exponent means “one divided by”, so 10-1 = 0.1, 10-3
= 0.001 etc.
We can also have fractional exponents. For example 100.3 = 2 and 10-0.3 = 1/2 etc.
Logarithms are simply the inverse of exponents of 10. If y = 10X, then log (y) = x. In
other words, the logarithm of a number means “10 to the exponent what gives that
number?” For example, log(1000) = 3 because 3 is the exponent of 10 which gives
1000 (103 = 1000). Likewise log (½) = -0.3 because -0.3 is the exponent of 10 which
gives ½.
Decibels are defined as 10 times the logarithm of a power ratio. Therefore a ratio of
1000 is 30 dB and ½ is -3 dB. We can also define decibels with respect to a voltage
ratio but since the power is proportional to the voltage squared (for constant
impedance), dB is 20 times the log of a voltage ratio.
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dB is used as a comparison between two power levels (or voltage levels using a
modified formula). For example, the gain of an amplifier or antenna is expressed in dB
because we are comparing the output power to the input power. Likewise, the
attenuation of a device is given in dB because we compare the input power to the
output power.
It is often convenient to use dB to express an absolute power level instead of just a
comparison between two powers. In this case, we define dBm as being the power
compared to 1 mW. Therefore 0 dBm = 1 mW, 3 dBm = 2 mW, 30 dBm = 1 W etc.
An important advantage of dB is that gains and losses can be added and subtracted.
That is because a multiplication in the linear scale, becomes an addition in the
logarithmic scale. Division in the linear scale is equivalent to subtraction on a
logarithmic scale.
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One of the simplest and effective practical antennas is the dipole. If a twin-wire
transmission is fanned-out at the end, if forms a dipole. If the total length of the dipole
is one half wavelength, then the dipole is resonant.
The current in an antenna creates the magnetic field portion of the electromagnetic
wave. Since the dipole is open at the ends, the current must be zero at these points.
This is the constraint which causes the dipole to resonate.
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Impedance is an important concept in antennas, transmission lines and equipment. It is
simply defined as the ration of Voltage to Current amplitude. If the voltage and current
are in-phase, the impedance is equivalent to a resistance. If they are phase shifted, the
impedance is either capacitive or inductive. A resonant dipole has a 50-75 ohm
resistive input impedance (sometimes called Radiation Resistance).
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The antenna pattern diagram shows how well an antenna radiates power in each
direction. For example, a dipole radiates very little power along its axis and maximum
power broadside to the axis. This forms a doughnut shaped radiation pattern. In
practice, radiation patterns are rarely shown three dimensionally. For simplicity, a
vertical or horizontal cross-section is drawn.
In both cases, the farther the blue line is from the origin (centre), the more power is
radiated in that direction.
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Antennas can have gain, but how is this possible when they are passive devices? The
laws of conservation of energy state that the total output power cannot be greater than
the total input power. Antenna gain does not increase the total power, but describes the
antennas ability to concentrate that power in a useful direction.
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By concentrating energy in a useful direction, the antennas radiates in that direction as
if a higher power was applied to a non-directional antenna.
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Antenna gain is a measure of antenna directivity and corresponds to the amount that
the input power would have to be increased to obtain the same signal strength in the
direction of maximum radiation if the directional antenna were replaced by an isotropic
(perfectly non-directional) antenna.
ERP (or EiRP) is the effective (isotropic) radiated power when you apply the antenna
gain to the actual RF power input to the antenna. The ERP is the equivalent power into
a dipole (or isotropic) antenna, which would provide the same signal strength as the
actual power produces in the maximum direction of the directive antenna.
Another measure of directivity is the beamwidth, measured in degrees. This is the
angle within which the gain remains less than 3 dB below the maximum gain.
The front-back ratio is the dB difference between the maximum gain of the antenna and
the gain in the opposite direction (180 degrees away).
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Array antennas use phase addition and cancellation to shape the antenna pattern. If
two dipoles are spaced ½ wavelength apart and fed in-phase, a receiver directly in front
of the antennas will be equidistant and therefore receive signals from both antennas inphase.
This arrangement provides 3 dB more forward gain than a single dipole.
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If, however, the receiver is at an angle to the side, the paths length from each antenna
will be different and the signals from each antenna will be somewhat out-of-phase and
the resulting signal will be weaker. At 90 degrees to the side, the phases will cancel
completely, providing a strong null (if the signals amplitudes are the same).
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What would happen if the antennas elements were fed out of phase? For example, if we
lengthened the feed-cable for the lower element, signals leaving this element would be
delayed with respect to the upper element.
Directly in front of the out of phase signals will result in a partial cancellation.
Conversely, at a certain angle below the horizon line, the signals having travelled
different distances will be in-phase and their voltages will add
This configuration is called Electrical Down-tilt.
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Yagi Antennas use at least 3 elements. The Driven Element is connected to the
transmission line and does the primary radiation. About one tenth of a wavelength
behind the Driven Element, and slightly longer, is the reflector element. Director
Elements are slightly shorter than the Driven Element and are periodically spaced in
front (direction of maximum radiation) of the Driven Element.
Yagi Antennas are relatively inexpensive and can produce typical gains of 7-15 dB.
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A parabola is a shape which has two important mathematical properties:
1. Any waves emitted from a specific point, called the focal point (or focus), are
reflected as parallel waves away from the reflector.
2. The total distance from the parabola and to a normal line is constant regardless
of the angle of reflection (FP-A-B = FP-C-D = FP-E-F).
This means that all reflected waves will be in-phase. The result is a highly direction
radiation pattern, sometimes called a pencil beam. Secondary side-lobes are caused by
imperfections such as finite feed-point size.
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Each antenna has certain characteristics, advantages and disadvantages. The gain of
an antenna refers to its ability to concentrate the transmitted or received energy in a
specific direction and not an amplification as would be the case with and amplifier.
When the characteristics of the antenna are not available, the above formula can be
used to approximate the gain as a function of the frequency and dish diameter for a
parabolic microwave antenna.
The gain of a multi-element antenna is roughly proportional to the number of elements
(if the number of elements is doubled, the gain increases by approximately 3 dB)
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Adaptive antenna systems use complex pattern steering algorithms to customize the
radiation pattern for individual mobile users. As the mobile moves within the service
area, the antenna system automatically adjusts it’s radiation pattern to maximize the
signal from that mobile. If there is interference, the adaptive antenna system will adjust
it’s radiation pattern to obtain a null in the direction of the interference.
There are many different approaches to multi-user adaptive antenna systems but in
TDMA systems, the antenna can actually re-steer itself between timeslots.
Note that this pattern steering is done electronically and there are no actual moving
parts.
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We saw earlier how an array of antennas elements can produce a directed pattern, and
how changing the phase relationship between the elements can change the direction of
maximum gain.
This same principle is used by adaptive antennas but the phase and amplitude of each
element is controlled in real-time to maximize the signal strength and/or minimize the
interference.
Complex adaptive algorithms are used to constantly adjust the phase and amplitude of
each element to produce the best quality signal. Depending on the number of elements
and the technique used, the beam can have multiple lobes and multiple nulls to
simultaneously track more than one mobile or cancel more than one interferer.
Performance improvement can be significant in a dense urban environment but at the
cost of greatly increased complexity.
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Reflected Wave :
• Phenomenon of wave bouncing off a surface. The reflected wave remains in the
same medium as the incident signal.
• The angle of reflection is equal to the angle of incidence.
• After reflection, there is a polarity change in the wave.
• The coefficient of reflection is the propensity of a surface to reflect a wave. A
coefficient of reflection of 1 means that 100% of the wave is reflected, and less
that 1 means that some of the wave is refracted through or absorbed by the
surface.
Refracted Wave :
• The bending of a wave as it passes through a surface between two materials with
different refractive indexes.
• The Refractive Index is the ratio of the propagation speed in a vacuum to the
propagation speed in the medium.
• The more dense the medium, the higher its refractive index and the smaller the
angle of refraction.
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Diffracted Wave :
• Bending of a wave as it passes over an obstacle
• Based on Huygens principal which treats each point on the wave front as a point
source of a new spherical wave.
Scattered Wave :
• Random redirection of a wave by reflection and refraction by very fine particles in
the atmosphere.
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5
The inherent free-space attenuation of wave refers to the progressive fading of the
signal as a function of the distance transmitter, caused by dilution of the power density.
The power from the transmitting antenna is spread over the surface of the wave front
which expands in area as it propagates away from the transmitter. As the area of the
wave front increases, the power density (power per square meter of wave front)
progressively decreases. The receiving antenna only receives the tiny portion of the
transmitted power contained in its aperture. Less received power means weaker signal.
The signal power decreases with R2 (where R is the distance from the transmitter) in
rural areas due to inherent attenuation, but this rate can increase to R3 or R6 in urban
areas or elsewhere where no line-of-site path exists.
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•
•
•
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Radio Line-of-sight propagation (slightly more than geometric line of sight)
Line-of-sight depends on the height of the antennas and the curvature of the earth
Primarily used for VHF, UHF frequencies and above (including Microwave)
When the signal travels on more than one path (direct and reflected), the
received signal is the sum of both paths. The difference in path lengths between
the direct and reflected paths, and its resulting phase shift, causes signal fading.
This phenomenon is called sporadic or multi-path fading.
Sporadic Fading :
• In some situations, can cause a loss or slight gain in signal.
• Fades are random in duration and depth due to random changes in path
characteristics.
• Varies in time and with environmental conditions.
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-
Space diversity involves at least two receive antennas for the single transmit signal.
When a path suffers from multi-path fading, the fade is location-specific and the
probability is very small that two points (separated by a few meters) would be
simultaneously faded. Since the path distance and reflection point will be slightly
different over each path, the delay will be different and the point at which the direct and
reflected paths cancel will be different.
On the receive side, a comparator selects the best signal, or alternatively, the signals
can be phase-aligned and combined to further improve reliability.
Point-to-point systems typically use vertically spaced diversity antennas while most
mobile systems use horizontally spaced diversity antennas at the site.
The technique which uses a comparator generally requires two complete receivers.
The selection of the strongest signal is done by comparing the level of each output and
activating a selector. Other combining methods allow some common receiver parts.
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When Space Diversity is not possible, usually due to restricted space at a site, some
diversity improvement is possible with Polarization Diversity. The antenna array
contains diagonally crossed elements. The elements oriented at -45 degrees are all fed
from one feed line while the elements oriented at +45 degrees are fed from another
feed line.
Since the index of reflection can vary according to the polarization of a signal, multi-path
fading between the two differently polarized antennas tends to be somewhat decorrelated (not occurring at the same time on both antennas)
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An ideal (lossless) transmission line can be modelled as a long string of tiny series
inductors and parallel capacitors.
Each capacitor charges and discharges through the inductors as a voltage and current
wave propagates down the transmission line
The inductance is due to the conductors. The capacitance is caused by the dielectric
(insulator) between the conductors.
Actual transmission lines are continuous (there are no discrete capacitors and
inductors) so L and C refer to the inductance and capacitance per unit length
respectively
A more realistic model would also contain series and parallel resistors to account for
transmission line losses.
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The Characteristic Impedance of a transmission line is can be thought of as it’s natural
impedance.
Imagine that we had an infinitely long transmission line and connected a DC voltage to
one end.
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When the DC voltage is applied, the first capacitor will quickly charge through the first
inductor, then the second capacitor will begin to charge, and so on. A wave-front of
voltage and current will propagate down the transmission line away from source.
At the source, the transmission line looks like a resistor, meaning that we will measure a
constant DC current IS flowing into the transmission line after we apply the voltage VS..
The Characteristic Impedance would of this transmission line would be Z0.= VS / IS. The
Characteristic Impedance depends on the ratio of Inductance to Capacitance in the
transmission line.
The velocity at which the wave-front propagates down the transmission line is slightly
slower than the speed of light, depending to the dielectric constant of the insulator.
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If the Transmission line is not infinite, then the actual impedance (V/I at any point) may
vary along the line. But (only) if the source and load resistance is equal to the
Characteristic Impedance, the actual impedance will be constant everywhere along the
line.
The impedance must be constant for maximum power transfer and to prevent the power
from being reflected back.
Of course transmission lines are normally use with AC (RF) voltages and currents, but
the same phenomenon occurs.
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Real transmission lines have some conductor resistance, and dielectric conductance
which transform a portion of the currents and voltages into heat. Additionally, some
energy may be radiated (the transmission line, especially open-lines, may act
somewhat like an antenna).
The loss in dB is proportional to the length of the transmission line and is usually
specified in dB/100ft or in dB/100m.
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Magnetic Field
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Two parallel wires make an efficient transmission line (twin-line). The RF voltage and
currents produce electric and magnetic fields around the line. While efficient at lower
frequencies, the fields around the transmission line can be influenced by nearby
conductive or ferrous materials.
The Characteristic Impedance depends on the ratio of conductor separation to
conductor diameter.
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Electric Field
Magnetic Field
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A Coaxial Cable is more practical than an open-line because the Magnetic and Electric
Fields are confined within the cable, which means that the cable can be directly
attached to metallic objects without affecting the transmission line characteristics.
The Characteristic Impedance depends on the ratio of inner conductor diameter to the
inside-diameter of the outer conductor (shield).
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Whenever there is a change in impedance in a transmission line, some of the
propagating wave is reflected. The voltage of the reflected wave will add or subtract
from the forward (incident) wave depending on how their phases align.
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The reflected wave adds and subtracts from the incident wave as their phases align inphase and out-of phase at fixed points on the line. At specific locations (every halfwavelength) the reflected wave will be in-phase with the incident wave and the voltages
will add.
In other locations (one quarter wavelength away from the peaks), the reflected wave will
always be 180 degrees out o phase with the forward wave, partially cancelling it. This
fixed pattern of amplitude peaks and nulls is called a standing wave.
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There are 2 common ways to measure this impedance mismatch: VSWR and Return
Loss.
VSWR is a ratio of the voltage peaks to voltage nulls of the standing wave in the
transmission line. When 0% of the power is reflected, there is no standing wave and
the VSWR is 1:1. If 100% of the power is reflected then the nulls drop to zero volts and
the VSWR is infinity. Therefore the VSWR is always 1:1 or greater.
Return Loss is simply the ratio of reflected to incident power, expressed in dB. Here the
higher the return loss the better.
2005-05-17
Antenna Systems and Troubleshooting
49
Telecom & Networks
: '* *
VSWR
1.00
1.01
1.02
1.03
1.04
1.05
1.06
1.07
1.08
1.09
1.10
1.11
1.12
1.13
1.14
1.15
1.16
1.17
1.18
1.19
1.20
1.21
1.22
1.23
1.24
1.25
1.26
1.27
1.28
1.29
1.30
Return
Loss
(dB)
46.1
40.1
36.6
34.2
32.3
30.7
29.4
28.3
27.3
26.4
25.7
24.9
24.3
23.7
23.1
22.6
22.1
21.7
21.2
20.8
20.4
20.1
19.7
19.4
19.1
18.8
18.5
18.2
17.9
17.7
2005-05-17
Trans
Loss
(dB)
.000
.000
.000
.001
.002
.003
.004
.005
.006
.008
.010
.012
.014
.016
.019
.021
.024
.027
.030
.033
.036
.039
.043
.046
.050
.054
.058
.062
.066
.070
.075
Power
Trans.
(%)
100.0
100.0
100.0
100.0
100.0
99.9
99.9
99.9
99.9
99.8
99.8
99.7
99.7
99.6
99.6
99.5
99.5
99.4
99.3
99.2
99.2
99.1
99.0
98.9
98.9
98.8
98.7
98.6
98.5
98.4
98.3
&
Power
Reflect
(%)
.0
.0
.0
.0
.0
.1
.1
.1
.1
.2
.2
.3
.3
.4
.4
.5
.5
.6
.7
.8
.8
.9
1.0
1.1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
VSWR
1.32
1.34
1.36
1.38
1.40
1.42
1.44
1.46
1.48
1.50
1.52
1.54
1.56
1.58
1.60
1.62
1.64
1.66
1.68
1.70
1.72
1.74
1.76
1.78
1.80
1.82
1.84
1.86
1.88
1.90
1.92
+
Return
Loss
(dB)
17.2
16.8
16.3
15.9
15.8
15.2
14.9
14.6
14.3
14.0
13.7
13.4
13.2
13.0
12.7
12.5
12.3
12.1
11.9
11.7
11.5
11.4
11.2
11.0
10.9
10.7
10.6
10.4
10.3
10.2
10.0
Trans
Loss
(dB)
.083
.093
.102
.112
.122
.133
.144
.155
.166
.177
.189
.201
.213
.225
.238
.250
.263
.276
.289
.302
.315
.329
.342
.356
.370
.384
.398
.412
.426
.440
.454
Power
Trans.
(%)
98.1
97.9
97.7
97.5
97.2
97.0
96.7
96.5
96.3
96.0
95.7
95.5
95.2
94.9
94.7
94.4
94.1
93.8
93.6
93.3
93.0
92.7
92.4
92.1
91.8
91.5
91.3
91.0
90.7
90.4
90.1
Power
Reflect
(%)
1.9
2.1
2.3
2.5
2.8
3.0
3.3
3.5
3.7
4.0
4.3
4.5
4.8
5.1
5.3
5.6
5.9
6.2
6.4
6.7
7.0
7.3
7.6
7.9
8.2
8.5
8.7
9.0
9.3
9.6
9.9
VSWR
1.94
1.96
1.98
2.00
2.50
3.00
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
7.50
8.00
8.50
9.00
9.50
10.00
11.00
12.00
13.00
14.00
15.00
16.00
17.00
18.00
19.00
20.00
30.00
Antenna Systems and Troubleshooting
Return
Loss
(dB)
9.9
9.8
9.7
9.5
7.4
6.0
5.1
4.4
3.9
3.5
3.2
2.9
2.7
2.5
2.3
2.2
2.1
1.9
1.8
1.7
1.6
1.5
1.3
1.2
1.2
1.1
1.0
1.0
.9
.9
.6
Trans
Loss
(dB)
.468
.483
.497
.512
.881
1.249
1.603
1.938
2.255
2.553
2.834
3.100
3.351
3.590
3.817
4.033
4.240
4.437
4.626
4.807
5.149
5.466
5.762
6.040
6.301
6.547
6.780
7.002
7.212
7.413
9.035
Power
Trans.
(%)
89.8
89.5
89.2
88.9
81.6
75.0
69.1
64.0
59.5
55.6
52.1
49.0
46.2
43.7
41.5
39.5
37.7
36.0
34.5
33.1
30.6
28.4
26.5
24.9
23.4
22.1
21.0
19.9
19.0
18.1
12.5
Power
Reflect
(%)
10.2
10.5
10.8
11.1
18.4
25.0
30.9
36.0
40.5
44.4
47.9
51.0
53.8
56.2
58.5
60.5
62.3
64.0
65.5
66.9
69.4
71.6
73.5
75.1
76.6
77.9
79.0
80.1
81.0
81.9
87.5
50
Telecom & Networks
%
&
2005-05-17
Antenna Systems and Troubleshooting
51
Telecom & Networks
'
5
"
" 6
" B
$
$
$
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$
$
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Waveguides look very different from coaxial transmission lines and the principle of
operation is fundamentally different. However, many of the same characteristics such
as velocity factor, characteristic impedance, standing waves have their equivalents.
The main difference between coaxial and waveguide transmission lines it that a wave of
current and voltage flows along coaxial cable, whereas it is an electromagnetic wave
that flows inside the waveguide. The conductive walls of the waveguide confine the
wave by reflection.
The equivalent to propagation velocity for a waveguide is Group Velocity. The more
reflections a wave makes (zig-zags), the longer it takes for the wave to propagate the
length of the waveguide and therefore the lower the group velocity. The cut-off
frequency is the minimum frequency which can propagate in a waveguide of certain
dimensions. The larger the waveguide, the lower the cut-off frequency.
2005-05-17
Antenna Systems and Troubleshooting
52
Telecom & Networks
*
"
'
)
$
:
):
"
)
$
$
)
$
02
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Waves can only travel in different specific modes in the waveguide, because of
reflective resonance. The electric field is constrained to be zero where the waved
reflects off a conductive wall that is parallel to the electric field. This means that there
must be an integer number of half-wavelengths between side-wall reflections for vertical
polarization.
The lowest order mode (TE10) is polarized along the shorter dimension (vertically as
shown above) and has 1/2 wavelength between side-wall reflections but no top/bottom
reflections. The most commonly used modes in rectangular waveguides are TEm0.
The cut-off frequency for the TE10 mode is fC=c / 2a where c is the speed of light and a
is the widest dimension of the waveguide.
Just like other types of transmission lines, waveguides have a propagation velocity
factor and can suffer from impedance mismatch, reflections and standing waves.
2005-05-17
Antenna Systems and Troubleshooting
53
Telecom & Networks
%!8 %
5
+
$
$
Electric Field in TE10 Mode
The electric and magnetic field distribution depends on the mode, but the electric field
for TE10 is relatively easy to visualize. The Electric field is constrained to be zero at the
side walls because the conductive walls effectively short-circuit the electric field that is
parallel to it. The electric field amplitude is largest in the centre of the waveguide. The
wave propagates forward at the group velocity.
2005-05-17
Antenna Systems and Troubleshooting
54
Telecom & Networks
'
.
Z0
TE modes
377
Ohms
TM modes
fc
f
Waveguides operating in Transverse Electric mode have a characteristic impedance
which is higher than that of free space (377 ohms) and approaches infinity when the
frequency reaches the cut-off frequency. In Transverse Magnetic mode, the impedance
approaches zero at the cut-off frequency.
2005-05-17
Antenna Systems and Troubleshooting
55
Telecom & Networks
'
Rectancular WG: 1" x 2"
20.0
Attenuation(dB/100m)
TE20
TM11
TE10
10.0
TE11
5.0
3.0
2.0
1.0
Singlemode
region
1.5
2
3
4
5 6 7 8 10
15
20
30 40
60
80
Frequency (GHz)
The attenuation of the waveguide also depends on the frequency and on the operating
mode. Different modes have different cut-off frequencies. The TE10 mode has the
lowest cut-off frequency and also the lowest attenuation curve. When a waveguide is
operated above the TE10 cut-off frequency but below the TE20 cut-off frequency (2 x fC
for TE10), only one mode is possible.
2005-05-17
Antenna Systems and Troubleshooting
56
Telecom & Networks
*
'
(
Frequence
TE10 Cutoff
Waveguide designation
Range, TE10
Frequency
JAN RG-/U
(GHz)
(GHz)
153-IEC RETMA
Brass
Alum.
1.14 - 1.73
0.908
R 14
WR 650
69
103
1.45 - 2.20
1.158
R 18
WR 510
1.72 - 2.61
1.375
R 22
WR 430
104
105
2.17 - 3.30
1.737
R 26
WR 340
112
113
2.60 - 3.95
2.080
R 32
WR 284
48
75
3.22 - 4.90
2.579
R 40
WR 229
3.94 - 5.99
3.155
R 48
WR 187
49
95
4.64 - 7.05
3.714
R 58
WR 159
5.38 - 8.17
4.285
R 70
WR 137
50
106
6.57 - 9.99
5.260
R 84
WR 112
51
68
7.00 - 11.00
5.790
WR 102
320
8.20 - 12.50
6.560
R 100
WR 90
52
67
9.84 - 15.00
7.873
R 120
WR 75
11.90 - 18,00
9.490
R 140
WR 62
91
107
14.50 - 22.00
11.578
R 180
WR 51
17.60 - 26.70
14.080
R 220
WR 42
53
121
21.70 - 33.00
17.368
R 260
WR 34
26.40 - 40.00
21.100
R 320
WR 28
96
32.90 - 50.10
26.350
R 400
WR 22
97
39.20 - 59.60
31.410
R 500
WR 19
49.80 - 75.80
39.900
R 620
WR 15
98
60.50 - 91.90
48.400
R 740
WR 12
99
73.80 - 112.00
59.050
R 900
WR 10
92.20 - 140.00
73.840
R 1200
WR 8
138
114.00 - 173.00
90.845
R 1400
WR 7
136
Band
L
D
S
A
G
C
J
H
T
X
M
P
Q
Width
(mm)
165.10
129.54
109.22
86.36
72.14
58.17
47.55
40.39
34.85
28.499
25.900
22.860
19.050
15.799
12.954
10.668
8.636
7.112
5.690
4.775
3.759
3.099
2.540
2.032
1.651
Height
(mm)
82.55
64.77
54.61
43.18
34.04
29.083
22.149
20.193
15.799
12.624
12.950
10.160
9.525
7.899
6.477
4.318
4.318
3.556
2.845
2.388
1.880
1.549
1.270
1.016
0.826
Tolerance
±
0.330
0.260
0.220
0.170
0.140
0.120
0.095
0.081
0.070
0.057
0.125
0.046
0.038
0.031
0.026
0.021
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
Since the cut-off frequency for a mode depends on the waveguide dimensions, it is
important to select an appropriate waveguide for the operating frequency. The above
chart shows the operating range for the TE10 mode for various rectangular waveguides.
2005-05-17
Antenna Systems and Troubleshooting
57
Telecom & Networks
"
6
2005-05-17
Antenna Systems and Troubleshooting
58
Telecom & Networks
6
2005-05-17
Antenna Systems and Troubleshooting
59
Telecom & Networks
;
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Gain – expressed in dBd or dBi
Radiation Pattern – usually polar plot for each plane
Beamwidth – angle between 3 dB points for each plane
Electrical Tilt – degrees available (usually not adjustable)
Side-lobe Suppression – maximum level of side lobes below main lobes
Front-Back Ratio – difference between forward (maximum) gain and gain in the
opposite direction
Frequency Range – range of frequencies over which the antenna is available (may not
indicate bandwidth)
Bandwidth – Range of frequencies over which the antenna will operate within
specifications
Return Loss (or VSWR)
Impedance – usually 50 Ohms nominal
Polarization – usually Vertical or diagonal (+/- 45 deg. with polarization diversity)
Polarization Discrimination – ability to reject cross-polarized signal (in dB)
Port-Port Isolation – isolation between two ports on a polarization diversity antenna
Maximum Power – rating of antenna
IM products – power of intermodulation products generated in Antenna under specific
conditions
2005-05-17
Antenna Systems and Troubleshooting
60
Telecom & Networks
*&2$<8 <88
)(
#
$ %
The SRL-480 from Sinclair Technologies is one of the most common 800 MHz omnidirection antennas in the Canadian cellular market.
It is a collinear array antenna, meaning that there is a series of elements one above the
other and all enclosed in a radome. These multiple vertical elements provide vertical
(elevation) directivity while maintaining horizontal omni-directionality and an overall gain
of 10 dBd. The vertical beam-width (angle between main lobe 3 dB points on the
elevation pattern) is 6 degrees.
Electrical down-tilt versions are also available which maintain the omni-directional
horizontal pattern but reduce the amount of power directed toward the horizon (primarily
used to control interference).
2005-05-17
Antenna Systems and Troubleshooting
61
Telecom & Networks
2005-05-17
Antenna Systems and Troubleshooting
62
Telecom & Networks
%$!=:
-
# $ &$ '
An enclosed array of 9 dipoles provides an offset (semi-directional) pattern. The
horizontal pattern depends whether the dipoles are mounted one quarter or one half
wavelength from the internal supporting mast.
This antenna is well suited for side-mounting on a tower since relatively little power is
radiated into the tower structure, while maintaining a wide radiation pattern.
2005-05-17
Antenna Systems and Troubleshooting
63
Telecom & Networks
2005-05-17
Antenna Systems and Troubleshooting
64
Telecom & Networks
-,<$$)<8
(
)
This is an 80 degree horizontal beam-width directed dipole array antenna for 800 MHz
sectored applications. The front-back ratio is particularly good with this model.
2005-05-17
Antenna Systems and Troubleshooting
65
Telecom & Networks
2005-05-17
Antenna Systems and Troubleshooting
66
Telecom & Networks
%
5:<82!!
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This is an 80 degree horizontal beam-width antenna for 800 MHz sectored applications.
2005-05-17
Antenna Systems and Troubleshooting
67
Telecom & Networks
2005-05-17
Antenna Systems and Troubleshooting
68
Telecom & Networks
6 +
!=>?!> "
*
"* )
This 65 degree horizontal beam-width antenna for 1900 MHz uses log periodic array
technology and provides a front-back ration of 40 dB.
2005-05-17
Antenna Systems and Troubleshooting
69
Telecom & Networks
2005-05-17
Antenna Systems and Troubleshooting
70
Telecom & Networks
%
*:2>?2!<8"- &"
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)
Another 65 degree beam-width, vertically polarized antenna for sectored PCS
applications. Note the intermodulation specification.
2005-05-17
Antenna Systems and Troubleshooting
71
Telecom & Networks
2005-05-17
Antenna Systems and Troubleshooting
72
Telecom & Networks
%
**2>?2!<8"- &?
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)
Dual Polarization antennas are used for polarization diversity. The radome includes two
antenna arrays, one array at 45 degrees polarization and the other with -45 degrees
polarization. Therefore, there are two feed-line connectors (ports). Since the ports are
connected to cross-polarized antennas, there is 30 dB of isolation between the ports.
Note that the terms H-Plane and E-Plane can be misleading for diagonally polarized
antennas such as this.
This is a dual (cross) polarized version but otherwise similar to the previous antenna.
Note the two antenna connectors.
2005-05-17
Antenna Systems and Troubleshooting
73
Telecom & Networks
2005-05-17
Antenna Systems and Troubleshooting
74
Telecom & Networks
2
2<8#
# $ &$ '
Sector antennas combine vertical (E-Plane) and horizontal (H-Plane) directivity. The
above antennas from Til-Tek is specifically designed to minimize side-lobes, which is
important in controlling interference.
The horizontal beam-width is adjustable. A narrower beam-width results in a higher
gain.
Electrical down-tilt is also available. Electrical down-tilt is preferable to mechanical
down-tilt because it remains constant for all directions within the sector. Mechanical
down-tilt has more effect directly in front of the antenna and less effect to the sides.
Front to Back Ratio is another important characteristic for sector antennas because
poor performance here can directly cause interference.
2005-05-17
Antenna Systems and Troubleshooting
75
Telecom & Networks
2005-05-17
Antenna Systems and Troubleshooting
76
Telecom & Networks
2
2<!!
# $ &$ '
Making an antenna taller, generally makes it more directional in the vertical plane,
without affecting the directionality in the horizontal plane.
The above antenna is similar to the TA-803 except that it is twice as tall. As expected,
the vertical (E-Plane) pattern is much narrower and the gain is 3 dB higher. Note that
the horizontal pattern (H-Plane).
2005-05-17
Antenna Systems and Troubleshooting
77
Telecom & Networks
2005-05-17
Antenna Systems and Troubleshooting
78
Telecom & Networks
@!<$$8 &
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This is a linear (vertically) polarized antenna from Allgon for 1900 MHz.
Note that this specification sheet includes intermodulation performance (3rd order and
product for two 10-Watt transmitters).
2005-05-17
Antenna Systems and Troubleshooting
79
Telecom & Networks
2005-05-17
Antenna Systems and Troubleshooting
80
Telecom & Networks
6
2005-05-17
Antenna Systems and Troubleshooting
81
Telecom & Networks
;
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• Characteristic Impedance – normal 50 Ohms for our applications
• Diameter – outside dimensions of cable
• Attenuation – usually expressed in dB/100 ft or dB/100m and depends on the
frequency
• Propagation Velocity – fraction or percentage of the speed of light (3 x 108 m/s)
• Maximum Frequency – recommended maximum
• Power Rating – maximum CW power rating. Multiple transmitters can produce
instantaneous power exceeding the sum of their powers
• Shield Type – solid or braided
• Jacket type – outdoor, indoor, plenum rated?
• Bending Radius – minimum radius of a bend without causing damage or VSWR
2005-05-17
Antenna Systems and Troubleshooting
82
Telecom & Networks
6
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The above chart compares key characteristics of cables offered by Andrew Corporation,
including Superflex, Extraflex and LDF Heliax of various sizes. The propagation
velocity is between 80% and 90% of the speed of light for all these cables. Note that
the attenuation increases by about a factor of 10 as the frequency goes from 150 MHz
to 10 GHz.
2005-05-17
Antenna Systems and Troubleshooting
83
Telecom & Networks
6
6
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)
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-./
Larger LDF Heliax are primarily used as transmission lines to the antenna. The
propagation velocity is between 85% and 90% of the speed of light for all these cables.
Note that the minimum bending radius of LDF6 is 5 times larger than that of LDF1
cable.
2005-05-17
Antenna Systems and Troubleshooting
84
Telecom & Networks
,
6
#
)
,
-./
Braided cables tend to be more flexible but suffer from two main disadvantages:
leakage (increases attenuation and reduces isolation) and oxidization of individual braid
strands, which can cause intermodulation due to the non-linear nature of the contact
between strands.
2005-05-17
Antenna Systems and Troubleshooting
85
Telecom & Networks
6
!
2005-05-17
Antenna Systems and Troubleshooting
86
Telecom & Networks
-
. 2,
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B
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Providing coverage inside buildings from distant cell-sites can be challenging. A more
reliable approach is to design a building-specific network of antennas, cables and
amplifiers to repeat the signals into and out of the building.
2005-05-17
Antenna Systems and Troubleshooting
87
Telecom & Networks
-
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"
"
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The above single-band system shows some key elements of an in-building antenna
system. A repeater is a high-gain bi-directional amplifier which takes the off-air signals
from the donor antenna and amplifies them for distribution withing the building. System
gain is limited by the isolation between its input and outputs for stability (Isolation – Gain
> 15 dB), amplifier power limits and noise considerations.
Power splitters are used to distribute the signal to multiple antennas and these splitters
are often non-symmetrical with more loss on a tap port and less loss on a through port.
Depending on the accumulated losses, it may be necessary to use a bi-directional line
amplifier. A BDA differs from a repeater primarily by its simplicity and lower gain. The
BDA maybe have a local power supply or be powered by 24V DC bias applied to the RF
cable. In this latter case, it is important that the DC be blocked or passed to different
sections of the network as required using jumpers in the equipment.
In-building networks are often designed for multiple bands (e.g. iDEN and PCS plus
sometimes WiFi) In these cases, a cross-band coupler is used to combine the signals
and multi-band amplifiers and antennas are used.
2005-05-17
Antenna Systems and Troubleshooting
88
Telecom & Networks
*
! $ $
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Single-band or multi-band repeaters are available. The above Allgon models are singleband for either CDMA 1900 or iDEN 800 MHz. Gain can be as high as 85 dB, provided
that the isolation between the input and output is at least 100 dB. Output power is + 33
dBm (2W) RMS composite power for all amplified channels.
2005-05-17
Antenna Systems and Troubleshooting
89
Telecom & Networks
,2
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This simple BDA has 20 dB gain with jumper-selectable attenuators for each band.
There are no external adjustments. It can be powered from a local 24VDC power
supply, or from a bias voltage on the coaxial cable depending on how the jumpers are
selected.
2005-05-17
Antenna Systems and Troubleshooting
90
Telecom & Networks
6
"
2005-05-17
!
Antenna Systems and Troubleshooting
91
Telecom & Networks
*
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RF Filtering is done for three reasons:
Combining :
To combine several transmitters and/or receivers in a same site (or antenna)
Interference Control :
Filtering must control three types of interference:
• Receiver Desensitization
• Transmitter Sideband Noise
• Intermodulation
Minimize Losses :
Perform the above task with the lowest possible insertion loss
2005-05-17
Antenna Systems and Troubleshooting
92
Telecom & Networks
%
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Bandpass Filter (BP)
Passes one frequency band while attenuating all others
Band Reject Filter (BR)
Attenuate one frequency band while passing all others
Q Filter (QF)
Combines a Bandpass and a Band Reject filter
Isolator (circulator)
Allows power to flow in one direction but not the other
Hybrid Coupler
Combines transmitters regardless of their frequencies
Rx Power Splitters and Pre-amplifiers
2005-05-17
Antenna Systems and Troubleshooting
93
Telecom & Networks
*
6
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Size depends on wavelength (frequency)
A VHF filter can be 1m in length but an 800 MHz filter can be only 10 or 15 cm
Flexible
Resonant frequency and insertion loss adjustable
Low Loss
As low as 0.5 dB per cavity
High Power Handling
100 to 500 W
Applications :
• Duplexers
• Pre-selectors
• Multi-couplers
• Transmit combiners
2005-05-17
Antenna Systems and Troubleshooting
94
Telecom & Networks
*
6
" 8
" -
" 8
Cylindrical or rectangular cavity
The performance (selectivity for given loss) depends on the cavity volume
Invar tuning rod
• The (adjustable) length of the rod in the cavity determines the resonant frequency
• Invar is an alloy with a thermal expansion coefficient close to zero
Coupling loops
• Input and output of filter are via connectors attached to coupling loops
• The size and orientation of the loops determines the selectivity (and insertion loss)
2005-05-17
Antenna Systems and Troubleshooting
95
Telecom & Networks
.
"
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A circulator is a ferromagnetic device in which the input power on one port leaves by the
next port in one direction of rotation.
To make an insulator, it is enough to place a resistive load at one of the ports (example:
port 3). The transmitter would be connected to port 1 and port 2 would carry out towards
the antenna.
The insulator prevents the considered antenna or transmission line power to turn over
towards the transmitter. The considered wave which arrives at port 2 can take two
ways, move towards port 1 or port 3. Indeed, the considered wave must overcome an
attenuation of approximately 0.5 dB between port 2 and port 3, while it should overcome
an attenuation between 20 and 30 dB to move towards port 1. If the resistive load is
optimal, almost all energy coming from port 2 is absorbed by port 3.
2005-05-17
Antenna Systems and Troubleshooting
96
Telecom & Networks
)
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The power entering the two input ports are summed together, then divide between the
two output ports.
In combining applications, one of the output ports is usually terminated with a 50 ohm
load. Since the power is divided between the two output ports, the insertion loss to one
port is at least 3 dB and typically about 3.5 dB.
The primary advantage of Hybrid couplers is that they are not frequency-selective. This
means that it is possible to combine transmitters which are too close in frequency to be
combined with cavity filters, or to allow the frequency to be changed without needing to
re-tune the combiner.
2005-05-17
Antenna Systems and Troubleshooting
97
Telecom & Networks
.
6
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Any antenna system filtering system must provide sufficient Transmitter to Receiver
isolation and Transmitter to Transmitter isolation to prevent interference caused by :
• The transmitter sideband noise which falls on the receiver frequency
• Excessive power on the transmit frequency desensitizing the receiver
• Mixing of two or more frequencies in a non-linear device, producing new
intermodulation frequencies which can interfere with a receiver
2005-05-17
Antenna Systems and Troubleshooting
98
Telecom & Networks
A
"
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D
In addition to its centre frequency, all transmitters produce sideband energy which can
be a source of interference to receivers located at the same site (and especially on the
same antenna). This interference is directly on the same frequency as the received
signal with which it interferes, so it must be removed by filtering at the source
(transmitter). It is impossible to remove sideband noise interference with filtering at the
affected receiver because the interference is on the same frequency as the desired
receive signal.
2005-05-17
Antenna Systems and Troubleshooting
99
Telecom & Networks
*
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Receivers are designed to respond to only one frequency, but this selectivity is not
perfect. If an off-frequency signal is strong enough, it can still reduce the receivers
ability to detect the desired receive frequency, especially if it is weak.
When a transmitter is located at the same site, or worse, on the same antenna, the
power of a 100W transmitter can be more than 150 dB (1,000,000,000,000,000 times)
stronger than the receiver maximum sensitivity. The receiver selectivity is not sufficient
to block out such a strong signal and receiver desensitization can occur unless the
transmit frequency is block using filters at the receiver. Since the interference is on the
same frequency as the interfering transmitter, it is impossible to remove receiver
desensitization using filters on the interfering transmitter.
2005-05-17
Antenna Systems and Troubleshooting
100
Telecom & Networks
B"
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To combine a single transmitter with a single receiver on the same antenna, a duplexer
based on Q-Filters would be a good choice. There are two filters, one on the receiver
and the other on the transmitter side of the duplexer.
The role of the transmitter filter is to pass the transmit frequency while notching out the
receive frequency to prevent Sideband Noise interference. The role of the receive side
filter is to pass the receive frequency while notching out the transmit frequency to
prevent desensitization interference.
The transmit and receive filters are combined using 1/4 wavelength cables. The
impedance of the filter is 50 ohm at the pass frequency but very low impedance at the
notch frequency. The quarter-wavelength cable section transforms this low impedance
to a very high impedance at the junction point. This maintains an impedance of 50 ohms
at both the transmit and receive frequencies and ensures that transmit power flows to
the antenna and receive power flows to the receiver.
This configuration is very effective to protect one receiver from one transmitter.
However, if the site contains many transmitters and receivers, this type of duplexer may
not provide sufficient interference protection.
2005-05-17
Antenna Systems and Troubleshooting
101
Telecom & Networks
,
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A bandpass duplexer works in a similar way, except that it protects a block of receive
frequencies from a block of (previously combined) transmit frequencies. An entire band
(15 MHz in the above example) of transmit frequencies can pass through the transmit
side of the duplexer, and likewise for the receive side. This type of configuration is used
in cellular or trunking application together with a transmitter combiner and receiver
multicoupler.
2005-05-17
Antenna Systems and Troubleshooting
102
Telecom & Networks
)
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When multiple transmitters must share a common antenna, they must be combined in a
way that prevents the power from one transmitter from entering the Power amplifier of
the another transmitter, where it could mix and produce intermodulation interference.
The isolation between transmitters is improved by using isolators at on each transmitter.
Hybrid couplers can be cascaded to combine several transmitters, although the
insertion loss increases quickly. A 2-way combiner has about 3.6 dB insertion loss, a 4way has 7.4 dB and an 8-way would have about 11 dB of insertion loss. In the case of
iDEN RFDS, three-way combiners are most often used to provide up to 9 BR on 3
antennas.
2005-05-17
Antenna Systems and Troubleshooting
103
Telecom & Networks
6
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A cavity combiner has lower insertion loss than the equivalent hybrid combiner. The
insertion loss depends on the frequency separation. If any of the transmitters are closer
than the minimum frequency separation specification (e.g. 250 kHz), then hybrid
combining or separate antennas should be used. A cavity combiner usually requires retuning should any of the frequencies change.
Transmitter-transmitter isolation is improved with isolators on each input. The outputs
of the cavity filters are combined using a rigid coupler or a 1/4 wavelength cable
harness which maintains 50 ohm impedance to each of the transmitters.
2005-05-17
Antenna Systems and Troubleshooting
104
Telecom & Networks
2
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While cavity combiners are more efficient that hybrid combiners, they require tuning
each time the frequencies change. To do this manually requires a service-affecting
outage and can be difficult to coordinate during a major frequency retune. Auto-tune
combiners have a servo-motor controlled tuning mechanism which automatically adjusts
the band-pass filter for each transmitter in order to minimise the insertion loss and
maximize the return loss within a few seconds of applying RF power to an input port.
Due to the QAM modulation used in iDEN, auto-tune combiners should be designed
specifically for this type of signal. The Allgon ATC unit above can also combine iDEN
Quad BRs.
2005-05-17
Antenna Systems and Troubleshooting
105
Telecom & Networks
2
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2005-05-17
Antenna Systems and Troubleshooting
(
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Telecom & Networks
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Many receivers (within a frequency band) can share a common antenna using a
receiver multicoupler. A pre-selector bandpass filter passes the desired frequency band
and the combined receive frequencies are amplified before being divided between the
receivers.
2005-05-17
Antenna Systems and Troubleshooting
107
Telecom & Networks
+ 2
88-
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Receiver sensitivity can be significantly improved by using tower-top receiver preamplifiers. This configuration can affectively eliminate the affects of receive
transmission line loss. Different configurations are possible but the above duplex TTA
has the advantage of using a standard duplexed EBTS configuration. Sweeping the
antenna becomes difficult in this situation due to the filters and amplifier.
2005-05-17
Antenna Systems and Troubleshooting
108
Telecom & Networks
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An antenna filtering system may include a duplexer, receiver multicoupler and
transmitter combiner.
Here the transmitters are combined in a cavity combiner (hybrid may also be used)
before being duplexed. The receive port of the duplexer is split between multiple
receivers using a receiver multi-coupler.
2005-05-17
Antenna Systems and Troubleshooting
109
Telecom & Networks
#
2005-05-17
Antenna Systems and Troubleshooting
110
Telecom & Networks
!
2005-05-17
Antenna Systems and Troubleshooting
111
Telecom & Networks
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There are many things which can go wrong in an antenna system and it is impossible to
consider all potential problems in detail.
In general, however, most problems can be grouped into one or more of the above
categories. In fact, many problems will manifest themselves with more than one
symptom (example, an impedance mismatch can also cause excessive loss and
possibly even intermodulation).
2005-05-17
Antenna Systems and Troubleshooting
112
Telecom & Networks
.
/
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2005-05-17
! $
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Antenna Systems and Troubleshooting
113
Telecom & Networks
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2005-05-17
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Antenna Systems and Troubleshooting
114
Telecom & Networks
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2005-05-17
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Antenna Systems and Troubleshooting
115
Telecom & Networks
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2005-05-17
N26
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Antenna Systems and Troubleshooting
116
Telecom & Networks
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2005-05-17
Antenna Systems and Troubleshooting
EM
117
Telecom & Networks
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2005-05-17
Antenna Systems and Troubleshooting
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Telecom & Networks
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2005-05-17
Antenna Systems and Troubleshooting
EM
119
Telecom & Networks
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The best way to control intermodulation is to reduce the possibility that multiple signals
of different frequencies (especially strong signals) are simultaneously present in any
non-linear element. When faced with an intermodulation problem, one challenge is to
determine where the mixing occurs.
Very often, frequencies mix directly in the receiver front-end or pre-amplifier. To
confirm if this is the case, insert a 3 dB attenuator at the receiver or pre-amplifier input.
If the interference does not change, the products are likely generated outside the
receiver. If interference improves my 6 dB or more, the products are definitely
generated in the receiver or pre-amplifier.
2005-05-17
Antenna Systems and Troubleshooting
120
Telecom & Networks
A
"
$
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In addition to its centre frequency, all transmitters produce sideband energy which can
be a source of interference to receivers located at the same site (and especially on the
same antenna). This interference is directly on the same frequency as the received
signal with which it interferes, so it must be removed by filtering at the source
(transmitter). It is impossible to remove sideband noise interference with filtering at the
affected receiver because the interference is on the same frequency as the desired
receive signal.
2005-05-17
Antenna Systems and Troubleshooting
121
Telecom & Networks
*
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Receivers are designed to respond to only one frequency, but this selectivity is not
perfect. If an off-frequency signal is strong enough, it can still reduce the receivers
ability to detect the desired receive frequency, especially if it is weak.
When a transmitter is located at the same site, or worse, on the same antenna, the
power of a 100W transmitter can be more than 150 dB (1,000,000,000,000,000 times)
stronger than the receiver maximum sensitivity. The receiver selectivity is not sufficient
to block out such a strong signal and receiver desensitization can occur unless the
transmit frequency is block using filters at the receiver. Since the interference is on the
same frequency as the interfering transmitter, it is impossible to remove receiver
desensitization using filters on the interfering transmitter.
2005-05-17
Antenna Systems and Troubleshooting
122
Telecom & Networks
!
2005-05-17
#
Antenna Systems and Troubleshooting
123
Telecom & Networks
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2005-05-17
#
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Antenna Systems and Troubleshooting
124
Telecom & Networks
: '*
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Try the above exercise. If the forward power is 100W and the reflected power is 2W,
what is the Return Loss in dB as measured at the transmitter?
2005-05-17
Antenna Systems and Troubleshooting
125
Telecom & Networks
: '*
*
/
&
0226
<2Ω
.6
1" + 0
= 02
1" − 0
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Using the Return Loss formula gives an answer of 17 dB, which corresponds to a
VSWR of 1.33:1
Is this acceptable?
2005-05-17
Antenna Systems and Troubleshooting
126
Telecom & Networks
: '*
*
/
&
1 7
0226
<2Ω
1" =
F6
.6
1" + 0
= 02
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In order to determine whether the measured RL of 17 dB is acceptable, one must
consider the line loss.
If the line loss is 3 dB, the forward power is attenuated by half, resulting in 50W arriving
at the antenna. If we measure a reflected power of 2 W at the bottom of the line, then
this reflection would have started at 4 W (assuming that the reflection is from the
antenna) because the line loss is in both directions.
Using 50W forward and 4W reflected, the return loss at the antenna would be 11 dB,
clearly indicating a problem.
Generally, to convert a RL measured at the bottom of the line to the equivalent RL from
the antenna, one must subtract 2 times the line loss.
RL (ant) = RL (bottom) – 2 x Line Loss
Or conversely, to convert an antenna RL specification to the minimum reading at the
bottom,
RL(bottom) = RL (ant) + 2 x Line Loss
2005-05-17
Antenna Systems and Troubleshooting
127
Telecom & Networks
5-*
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B
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Frequency Domain Reflectometry is currently the most commonly used and flexible
analysis method. There are two analysis mode :
Return Loss Sweep measures the return-loss (intensity of reflection) as a function of
frequency, as the RF frequency is swept over a programmable range. The entire
operating frequency range of an antenna can be measured in such a sweep and
problems can be detected which would have been hidden on a single-frequency returnloss measurement.
The return-loss should be within the specification at every point within the operating
frequency range. Since micro-reflections are caused by every element in the
transmission line system, the return-loss sweep provides a unique signature for the
antenna system and depends on the exact configuration including lengths between
junctions.
Therefore it is important to keep the initial commissioning sweep as a record of the asinstalled signature. Any major changes in the Return Loss Sweep signature indicate a
problem.
2005-05-17
Antenna Systems and Troubleshooting
128
Telecom & Networks
Distance to Fault provides a graph of return-loss versus distance along the
transmission line. Here, it is possible to see which components in the transmission line
contribute most to reflection. The antenna typically operates over a limited frequency
range, so the indicated return loss for the antenna will be large if the frequency sweeps
outside its range.
The Distance to Fault also provides a unique signature of the antenna system and
should be kept as a record of the as-installed performance. An increase in the returnloss of one component in the system, indicates a problem.
2005-05-17
Antenna Systems and Troubleshooting
129
Telecom & Networks
5 0
+
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λ0
.
λ.
.
As we have seen, an impedance mismatch causes a reflection, which in turn sets up a
standing wave in the transmission line. The nulls of the standing wave are ½
wavelength apart.
If the frequency increases slightly (by Df), the wavelength decreases and the nulls
become closer together. The null closest to the reflection point will move slightly (say
distance d) towards the reflection point. Since each standing wave compresses by the
same amount, the next farthest null will move twice as much and the third null will move
three times as much as the first null.
The antenna analyser connects to the feed-point of the transmission line and cannot
directly measure all the nulls in the transmission line. However, if we measure how
many nulls go by the feed-point as we shift the frequency by Df, we can easily calculate
how many wavelengths we are away from the reflection point.
2005-05-17
Antenna Systems and Troubleshooting
130
Telecom & Networks
5
5 0
+
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Unlike Time Domain Reflectometry, FDR uses the frequency-domain return-loss sweep
results to mathematically calculate the time-domain distance to fault graph. As we
have seen in transmission line theory, a reflection causes a standing wave if a fixed
frequency is used, the nulls of which occur every half-wavelength in the line. If the
frequency is changed, the position of those nulls change.
By sweeping the frequency over a range, the analyser can measure the position and
intensity of the peaks and nulls as they reach the measurement point, thus producing
the SWR or return-loss graph.
The farther away the reflection, the more nulls will occur in a given frequency sweep
range. It is possible to estimate the distance to the source of a reflection by counting
the number of nulls or ripples in the return-loss signature and using the above formula.
This works if there is one or two dominant sources of reflection but becomes
progressively more difficult as the complexity of the transmission system increases.
Note that the velocity factor is key to obtaining an accurate distance measurement.
We have also seen that it is mathematically possible to relate any time-domain signal to
an equivalent frequency domain signal an vice-versa.
2005-05-17
Antenna Systems and Troubleshooting
131
Telecom & Networks
The mathematical tool to do this is called the Fourier Transform. Modern antenna
system analysers use a technique called Inverse Fast Fourier Transform to produce a
time-domain graph based solely on samples of the frequency-domain return-loss
sweep. Time is related to distance by the propagation (or group) velocity.
2005-05-17
Antenna Systems and Troubleshooting
132
Telecom & Networks
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There are limits to the resolution and range of time-domain measurements using FDR
which are analogous to the pulse-width trade-off for time-domain reflectometry. In FDR,
a wider frequency sweep provides better resolution. This means that you will be able to
distinguish faults that are closer together on the line.
But just when you thought that wider sweep was better, a conflicting limitation emerges.
Since FDR units typically use a fixed (or selectable) number of data points (128 in the
case of Anritsu Site Master S820A) the maximum distance decreases as the sweep
width increases.
The above graphs show the relationship of maximum distance (range) and resolution as
a function of sweep width for a velocity factor of 88%. For different velocity factors, the
above formula may be used.
Note that the same sweep width need not be used for evaluating return-loss
performance and isolating faults. When measuring return-loss, the sweep width should
be the RF operating range of the the antenna system. When attempting to isolate a
fault, you can increase or decrease the sweep width in order to obtain the required timedomain resolution and range.
2005-05-17
Antenna Systems and Troubleshooting
133
Telecom & Networks
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2005-05-17
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To verify the performance of the transmission feed line system and analyze these
problems, three types of line sweeps are required :
Return Loss Measurement :
Measures the reflected power of the system in decibels (dB). This measurement can
also be taken in the Voltage Standing Wave Ratio (VSWR) mode, which is the ratio of
the transmitted power to the reflected power. However, the return loss measurement is
typically used for most field applications.
Insertion Loss Measurement :
Measures the energy absorbed, or lost, by the transmission line in dB/ft or dB/meter.
Different transmission lines have different losses, and the loss is frequency and
distance specific. The higher the frequency or longer the distance, the greater the loss.
Distance-To-Fault (DTF) Measurement :
Reveals the precise fault location of components in the transmission line system. This
test helps to identify specific problems in the system, such as connector transitions,
jumpers, kinks in the cable or moisture intrusion.
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Return Loss - System Sweep :
A measurement made when the antenna is connected at the end of the transmission
line. This measurement provides an analysis of how the various components of the
system are interacting and provides an aggregate return loss of the entire system.
Distance To Fault - Load Sweep :
A measurement made with the antenna disconnected and replaced with a 50 precision
load at the end of the transmission line. This measurement allows analysis of the
various components of the transmission feed line system in the DTF mode.
Cable Loss - Insertion Loss Sweep :
A measurement made when a short is connected at the end of the transmission line.
This condition allows analysis of the signal loss through the transmission line and
identifies the problems in the system. High insertion loss in the feed line or jumpers can
contribute to poor system performance and loss of coverage.
This whole process of measurements and testing the transmission line system is called
Line Sweeping.
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Procedure :
1. Press the MODE key.
2. Select FREQ-RETURN LOSS using the Up/Down arrow key and press
ENTER.
3. Set the start and stop frequencies, F1 and F2, as described on page 3-2.
4. Calibrate the Site Master as described on page 3-2.
5. Connect the Device Under Test to the Site Master. A trace will be displayed on
the screen when the Site Master is in the sweep mode.
6. Press SAVE DISPLAY (page 3-5) name the trace, and press ENTER.
NOTE : The antenna must be connected at the end of the transmission feed line when
conducting a System Return Loss measurement.
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Procedure - Cable Loss Mode :
1.
2.
3.
4.
5.
6.
7.
8.
9.
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11.
Press the MODE key.
Select FREQ-CABLE LOSS using the Up/Down arrow key and press ENTER.
Set the start and stop frequencies, F1 and F2, as described on page 3-2.
Connect the Test Port Extension cable to the RF Out port and calibrate the Site
Master as described on page 3-2.
Save the calibration set up (page 3-5).
Connect the Device Under Test to the Site Master phase stable Test Port Extension
cable. A trace will be displayed on the screen as long as the Site Master is in sweep
mode.
Press the AMPLITUDE key and set the TOP and BOTTOM values of the display.
Press the MARKER key.
Set M1 to MARKER TO PEAK.
Set M2 to MARKER TO VALLEY.
Calculate the measured insertion loss by averaging M1 (marker to peak) and M2
(marker to valley) as follows: Insertion Loss = (M1+ M2) / 2
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Procedure - DTF-Return Loss Mode :
1.
2.
3.
4.
5.
6.
7.
8.
9.
Press the MODE key.
Select DTF-RETURN LOSS using the Up/Down arrow key and press ENTER.
Set the start and stop frequencies, F1 and F2, as described on page 3-2.
Connect the Test Port Extension cable to the RF Out port and calibrate the Site
Master as described on page 3-2.
Save the calibration set up (page 3-5).
Connect the Device Under Test to the Site Master phase stable Test Port
Extension cable. A trace will be displayed on the screen as long as the Site
Master is in sweep mode.
Press the FREQ/DIST key.
Set the D1 and D2 values. The Site Master default for D1 is zero.
Press the DTF AID soft key and select the appropriate CABLE TYPE to set the
correct propagation velocity and attenuation factor.
NOTE: Selecting the right propagation velocity, attenuation factor and distance is very
important for accurate measurements, otherwise the faults can not be identified
accurately and insertion loss will be incorrect.
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Interpretation if the RTS and DTF signatures is made much easier if a base-line or
reference signature is available for the antenna system. Since the signature is different
for every system, it is difficult to see subtle changes. When comparing signatures, it is
critical to use the same frequency sweep width, otherwise the results will be quite
different.
In the above example, the RTS indicates a slight degradation in Return Loss over all
frequencies in the sweep, although it is not possible to identify a source of the problem.
In DTF mode, we can see that there are two dominant sources of reflection and that the
return loss has degraded by about 5dB on one of them. This indicates a possible loose
or corroded connector or coupler located at about 48 feet from the test point.
Without the baseline signature, this subtle change would have been difficult to detect.
Minor differences in the position and depth of lower nulls is normal and can result from
temperature changes or system aging.
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The above plot shows the return-loss signature after installation of 1900 MHz antenna.
It meets the specification of 20 dB return-loss.
Note that there are two different frequencies of ripple in the return-loss signature,
indicating two reflection points in the system. As we have seen previously, the low
frequency ripple is caused by a reflection very close to the test point, since few nulls go
by the test point for a frequency change of Df. The high frequency ripple is caused by a
reflection farther down the line, because it is many more wavelengths away and
therefore more nulls go by the test point for a frequency change of Df.
This type of return-loss sweep is normal since there is almost always some component
of reflection from each end of the transmission line.
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The same antenna measured later shows severe return-loss degradation indicating a
problem somewhere in the antenna system.
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The signature DTF plot at installation shows that the return loss is predominately from
the antenna located at 32 meters up the transmission line.
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The DTF plot taken after the problem appeared shows that the return loss is still
predominately from the antenna. It appears from this plot that the problem is at the
antenna itself or possibly the connector or jumper immediately next to the antenna.
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Calibration is key to accuracy since the analyser measures very small changes in
reflected power. The Site Master kit includes 3 terminations that are used in the
automatic calibration procedure.
To initiate calibration, select the type of test (e.g. RLS or DTF), the frequency range, the
transmission line type, then follow the instructions when the software asks you to apply
various reference terminations (open, short or load).
Always ensure that the Calibrated indication is On before interpreting any
measurement.
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