Dipole
Isotropic
Typical Wireless
Omni Antenna
RF100 (c) 1998 Scott Baxter July, 1998 5 - 1

July, 1998 RF100 (c) 1998 Scott Baxter 5 - 2

TX
July, 1998
Zero current at each end each tiny imaginary “slice” of the antenna does its share of radiating
Maximum current at the middle
RX
Current induced in receiving antenna is vector sum of contribution of every tiny “slice” of radiating antenna
Width of band denotes current magnitude
 An antenna is just a passive conductor carrying RF current
• RF power causes the current flow
• Current flowing radiates electromagnetic fields
• Electromagnetic fields cause current in receiving antennas
 The effect of the total antenna is the sum of what every tiny “slice” of the antenna is doing
• Radiation of a tiny “slice” is proportional to its length times the current in it
• remember, the current has a magnitude and a phase!
RF100 (c) 1998 Scott Baxter 5 - 3

TX
Minimum
Radiation: contributions out of phase, cancel
Minimum
Radiation: contributions out of phase, cancel
Maximum
Radiation: contributions in phase, reinforce
 Each “slice” of the antenna produces a definite amount of radiation at a specific phase angle
 Strength of signal received varies, depending on direction of departure from radiating antenna
• In some directions, the components add up in phase to a strong signal level
• In other directions, due to the different distances the various components must travel to reach the receiver, they are out of phase and cancel, leaving a much weaker signal
 An antenna’s directivity is the same for transmission & reception
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 4

Antenna 1
Vertically
Polarized
TX current
Antenna 2
Horizontally
Polarized
Electromagnetic
Field almost no current
RX
RF current in a conductor causes electromagnetic fields that seek to induce current flowing in the same direction in other conductors.
The orientation of the antenna is called its polarization.
Coupling between two antennas is proportional to the cosine of the angle of their relative orientation
 To intercept significant energy, a receiving antenna must be oriented parallel to the transmitting antenna
• A receiving antenna oriented at right angles to the transmitting antenna is “cross-polarized”; will have very little current induced
•
Vertical polarization is the default convention in wireless telephony
• In the cluttered urban environment, energy becomes scattered and
“de-polarized” during propagation, so polarization is not as critical
• Handset users hold the antennas at seemingly random angles…..
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 5

 Antennas are passive devices: they do not produce power
• Can only receive power in one form and pass it on in another, minus incidental losses
• Cannot generate power or “amplify”
 However, an antenna can appear to have “gain” compared against another antenna or condition. This gain can be expressed in dB or as a power ratio. It applies both to radiating and receiving
 A directional antenna, in its direction of maximum radiation, appears to have “gain” compared against a non-directional antenna
 Gain in one direction comes at the expense of less radiation in other directions
 Antenna Gain is RELATIVE , not ABSOLUTE
• When describing antenna “gain”, the comparison condition must be stated or implied
July, 1998 RF100 (c) 1998 Scott Baxter
Omni-directional
Antenna
Directional
Antenna
5 - 6

 Isotropic Radiator
• Truly non-directional -- in 3 dimensions
• Difficult to build or approximate physically, but mathematically very simple to describe
• A popular reference: 1000 MHz and above
– PCS, microwave, etc.
 Dipole Antenna
• Non-directional in 2-dimensional plane only
• Can be easily constructed, physically practical
• A popular reference: below 1000 MHz
– 800 MHz. cellular, land mobile, TV & FM
Isotropic
Antenna
Dipole Antenna
Quantity Units
Gain above Isotropic radiator
Gain above Dipole reference
Effective Radiated Power Vs. Isotropic
Effective Radiated Power Vs. Dipole dBi dBd
(watts or dBm) EIRP
(watts or dBm) ERP
Notice that a dipole has 2.15 dB gain compared to an isotropic antenna.
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 7

 An antenna radiates all power fed to it from the transmitter, minus any incidental losses.
Every direction gets some amount of power
 Effective Radiated Power (ERP) is the apparent power in a particular direction
• Equal to actual transmitter power times antenna gain in that direction
 Effective Radiated Power is expressed in comparison to a standard radiator
• ERP : compared with dipole antenna
• EIRP : compared with I sotropic antenna
Directional
Antenna
Reference
Antenna
A
B
ERP B A (ref)
TX
100 W
TX
100 W
A
B
Example : Antennas A and B each radiate 100 watts from their own transmitters. Antenna A is our reference, it happens to be isotropic.
Antenna B is directional. In its maximum direction, its signal seems 2.75 stronger than the signal from antenna
A. Antenna B’s EIRP in this case is 275 watts.
275w 100w
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 8

 Many wireless systems at 1900 & 800 MHz use omni antennas like the one shown in this figure
 These patterns are drawn to scale in E-field radiation units, based on equal power to each antenna
 Notice the typical wireless omni antenna concentrates most of its radiation toward the horizon, where users are, at the expense of sending less radiation sharply upward or downward
 The wireless antenna’s maximum radiation is 12.1 dB stronger than the isotropic (thus 12.1 dBi gain), and
10 dB stronger than the dipole (so 10 dBd gain).
Isotropic
Dipole
Gain Comparison
12.1 dBi
10dBd
Isotropic
Dipole
Omni
July, 1998
Typical Wireless
Omni Antenna
Gain 12.1 dBi or 10 dBd
RF100 (c) 1998 Scott Baxter 5 - 9

An antenna’s directivity is expressed as a series of patterns
 The Horizontal Plane Pattern graphs the radiation as a function of azimuth
(i.e..,direction N-E-S-W)
 The Vertical Plane Pattern graphs the radiation as a function of elevation (i.e.., up, down, horizontal)
 Antennas are often compared by noting specific landmark points on their patterns:
• 3 dB (“HPBW”), -6 dB, -10 dB points
• Front-to-back ratio
• Angles of nulls, minor lobes, etc.
270
(W)
Typical Example
Horizontal Plane Pattern
Notice -3 dB points
0 (N)
0
-10
-20
10 dB points
-30 dB nulls or minima a Minor
Lobe
Front-to-back Ratio
Main
Lobe
90
(E)
180 (S)
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 10

Quasi-Optical Techniques (reflection, focusing )
• Reflectors can be used to concentrate radiation
– technique works best at microwave frequencies, where reflectors are small
• Examples:
– corner reflector used at cellular or higher frequencies
– parabolic reflector used at microwave frequencies
– grid or single pipe reflector for cellular
Array techniques (discrete elements)
• Power is fed or coupled to multiple antenna elements; each element radiates
• Elements’ radiation in phase in some directions
• In other directions, a phase delay for each element creates pattern lobes and nulls
July, 1998 RF100 (c) 1998 Scott Baxter
In phase
Out of phase
5 - 11

 Collinear vertical arrays
• Essentially omnidirectional in horizontal plane
• Power gain approximately equal to the number of elements
• Nulls exist in vertical pattern, unless deliberately filled
 Arrays in horizontal plane
• Directional in horizontal plane: useful for sectorization
• Yagi
– one driven element, parasitic coupling to others
• Log-periodic
– all elements driven
– wide bandwidth
 All of these types of antennas are used in wireless
RF power
Collinear
Vertical
Array
Yagi
RF power
Log-Periodic
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 12

The family of omni-directional wireless antennas:
 Number of elements determines
• Physical size
• Gain
• Beamwidth, first null angle
 Models with many elements have very narrow beamwidths
• Require stable mounting and careful alignment
• Watch out: be sure nulls do not fall in important coverage areas
 Rod and grid reflectors are sometimes added for mild directivity
Examples: 800 MHz.: dB803, PD10017, BCR-
10O, Kathrein 740-198
1900 MHz.: dB-910, ASPP2933
-3 d
B
Typical Collinear Arrays
Number of
Elements
9
10
11
12
13
14
7
8
5
6
3
4
1
2
Power
Gain
9
10
11
12
13
14
7
8
5
6
3
4
1
2
Gain, dB
0.00
3.01
4.77
6.02
6.99
7.78
8.45
9.03
9.54
10.00
10.41
10.79
11.14
11.46
Angle q n/a
26.57
°
18.43
°
14.04
°
11.31
°
9.46
°
8.13
°
7.13
°
6.34
°
5.71
°
5.19
°
4.76
°
4.40
°
4.09
°
Vertical Plane Pattern beamwidth q
Angle of first null
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 13

Vertical Plane Pattern
Up
 Typical commercial sector antennas are vertical combinations of dipoles, yagis, or log-periodic elements with reflector (panel or grid) backing
• Vertical plane pattern is determined by number of vertically-separated elements
– varies from 1 to 8, affecting mainly gain and vertical plane beamwidth
• Horizontal plane pattern is determined by:
– number of horizontally-spaced elements
– shape of reflectors (is reflector folded?)
Down
Horizontal Plane Pattern
N
W
S
E
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 14

Antenna Model
Frequency Range, MHz.
Gain - dBd/dBi
VSWR
Beamwidth (3 dB from maximum)
Polarization
Maximum power input - Watts
Input Impedance - Ohms
Lightning Protection
Termination - Standard
Jumper Cable
Electrical Data
ASPP2933
1850-1990
3/5.1
<1.5:1
32

Vertical
400
50
Direct Ground
N-Female
Order Sep.
ASPP2936
1850-1990
6/8.1
<1.5:1
15

Vertical
400
50
Direct Ground
N-Female
Order Sep.
dB910C-M
1850-1970
10/12.1
<1.5:1
5

Vertical
400
50
Direct Ground
N-Female
Order Sep.
Antenna Model
Overall length - in (mm)
Radome OD - in (mm)
Wind area - ft2 (m2)
Wind load @ 125 mph/201 kph lb-f (n)
Maximum wind speed - mph (kph)
Mechanical Data
ASPP2933
24 (610)
1.1 (25.4)
.17 (.0155)
4 (17)
140 (225)
Weight - lbs (kg)
Shipping Weight - lbs (kg)
Clamps (steel)
4 (1.8)
11 (4.9)
ASPA320
ASPP2936
36 (915)
1.0 (25.4)
.25 (.0233)
6 (26)
140 (225)
6 (2.7)
13 (5.9)
ASPA320 dB910C-M
77 (1955)
1.5 (38)
.54 (.05)
14 (61)
125 (201)
5.2 (2.4)
9 (4.1)
Integral
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 15

 Vertical Plane Pattern
• E-Plane (elevation plane)
• Gain: 10 dBd
• Dipole pattern is superimposed at scale for comparison (not often shown in commercial catalogs)
• Frequency is shown
• Pattern values shown in dBd
• Note 1-degree indices through region of main lobe for most accurate reading
• Notice minor lobe and null detail!
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 16

July, 1998 RF100 (c) 1998 Scott Baxter 5 - 17

Antenna
Jumper
Transmission Line
Directional
Coupler
Jumpers
F R e x e r
D u p l
Combiner
BPF
TX
TX
RX
 Antenna systems include more than just antennas
 Transmission Lines
• Necessary to connect transmitting and receiving equipment
 Other Components necessary to achieve desired system function
• Filters, Combiners, Duplexers - to achieve desired connections
• Directional Couplers, wattmeters - for measurement of performance
 Manufacturer’s system may include some or all of these items
• Remaining items are added individually as needed by system operator
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 18

Physical Characteristics
 Type of line
• Coaxial, stripline, openwire
• Balanced, unbalanced
 Physical configuration
• Dielectric:
– air
– foam
• Outside surface
– unjacketed
– jacketed
 Size (nominal outer diameter)
• 1/4”,1/2”, 7/8”, 1-1/4”,
15/8”, 2-1/4”, 3”
Typical coaxial cables
Used as feeders in wireless applications
Foam
Dielectric
Air
Dielectric
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 19

 Transmission lines practical considerations
• Periodicity of inner conductor supporting structure can cause
VSWR peaks at some frequencies, so specify the frequency band when ordering
• Air dielectric lines
– lower loss than foam-dielectric; dry air is excellent insulator
– shipped pressurized; do not accept delivery if pressure leak
• Foam dielectric lines
– simple, low maintenance; despite slightly higher loss
– small pinholes and leaks can allow water penetration and gradual attenuation increases
Air
Dielectric
Foam
Dielectric
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 20

Electrical Characteristics
 Attenuation
• Varies with frequency, size, dielectric characteristics of insulation
• Usually specified in dB/100 ft and/or dB/100 m
 Characteristic impedance Z
0
(50 ohms is the usual standard; 75 ohms is sometimes used)
• Value set by inner/outer diameter ratio and dielectric characteristics of insulation
• Connectors must preserve constant impedance (see figure at right)
 Velocity factor
• Determined by dielectric characteristics of insulation.
 Power-handling capability
• Varies with size, conductor materials, dielectric characteristics
D d
Characteristic Impedance of a Coaxial Line
Z o
= ( 138 / ( e
1/2 ) ) Log ( D / d ) e
= Dielectric Constant
10
= 1 for vacuum or dry air
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 21

 Transmission lines have impedancetransforming properties
• When terminated with same impedance as Z o
, input to line appears as impedance Z o
• When terminated with impedance different from Z o
, input to line is a complex function of frequency and line length. Use Smith Chart or formulae to compute
 Special case of interest: Line section one-quarter wavelength long has convenient properties useful in matching networks
• Z
IN
= (Z o
2 )/(Z
LOAD
)
July, 1998
Matched condition
Z
IN
= 50
W
Z o
=50
W
Z
LOAD
50
W
=
Z
IN
Mismatched condition
=
Z o
=50
W
Z
LOAD
=
83
-j22
W
Z
Deliberate mismatch for impedance transformation l
/4
IN
=25
W
Z o
=50
W
Z
LOAD
=
100
W
Z
IN
= Z
O
2
/
Z
LOAD
RF100 (c) 1998 Scott Baxter 5 - 22

 Respect specified minimum bending radius!
• Inner conductor must remain concentric, otherwise Z o changes
• Dents, kinks in outer conductor change Z o
 Don’t bend large, stiff lines (1-
5/8” or larger) to make direct connection with antennas
 Use appropriate jumpers, weatherproofed properly.
 Secure jumpers against wind vibration.
Observe
Minimum
Bending
Radius!
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 23

 During hoisting
• Allow line to support its own weight only for distances approved by manufacturer
• Deformation and stretching may result, changing the Z o
• Use hoisting grips, messenger cable
 After mounting
• Support the line with proper mounting clamps at manufacturer’s recommended spacing intervals
• Strong winds will set up damaging metal-fatigueinducing vibrations
July, 1998 RF100 (c) 1998 Scott Baxter
200 ft
~60 m
Max.
3-6 ft
5 - 24

 Types of Filters
• Single-pole:
– pass
– reject (notch )
• Multi-pole:
– band-pass
– band-reject
 Key electrical characteristics
• Insertion loss
• Passband ripple
• Passband width
– upper, lower cutoff frequencies
• Attenuation slope at band edge
• Ultimate out-of-band attenuation
Typical RF bandpass filter insertion
0 loss passband ripple
-3 dB passband width
Frequency, megaHertz
Typical bandpass filters have insertion loss of 1-3 dB. and passband ripple of 2-6 dB.
Bandwidth is typically 1-20% of center frequency, depending on application. Attenuation slope and out-of-band attenuation depend on # of poles & design
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 25

 Filters are the basic building blocks of duplexers and more complex devices
 Most manufacturers’ network equipment includes internal bandpass filters at receiver input and transmitter output
 Filters are also available for special applications
 Number of poles (filter elements) and other design variables determine filter’s electrical characteristics
• Bandwidth rejection
• Insertion loss
• Slopes
• Ripple, etc.
July, 1998
Typical RF Bandpass Filter
~l
/4
Notice construction: RF input excites one quarter-wave element and electromagnet fields propagate from element to element, finally exciting the last element which is directly coupled to the output.
Each element is individually set and forms a pole in the filter’s overall response curve.
RF100 (c) 1998 Scott Baxter 5 - 26

 Allows multiple transmitters to feed single antenna, providing
• Minimum power loss from transmitter to antenna
• Maximum isolation between transmitters
 Combiner types
• Tuned
– low insertion loss ~1-3 dB
– transmitter frequencies must be significantly separated
• Hybrid
– insertion loss -3 dB per stage
– no restriction on transmitter frequencies
• Linear amplifier
– linearity and intermodulation are major design and operation issues
Typical tuned combiner application
Antenna
TX TX TX TX TX TX TX TX
Typical hybrid combiner application
Antenna
~-3 dB
~-3 dB
~-3 dB
TX TX TX TX TX TX TX TX
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 27

 Duplexer allows simultaneous transmitting and receiving on one antenna
• Nortel 1900 MHz BTS RFFEs include internal duplexer
• Nortel 800 MHz BTS does not include duplexer but commercial units can be used if desired
 Important duplexer specifications
• TX pass-through insertion loss
• RX pass-through insertion loss
• TX-to-RX isolation at TX frequency (RX intermodulation issue)
• TX-to-RX isolation at RX frequency (TX noise floor issue)
• Internally-generated IMP limit specification
July, 1998 RF100 (c) 1998 Scott Baxter
f
R
Antenna f
T
RX TX
Principle of operation
Duplexer is composed of individual bandpass filters to isolate TX from
RX while allowing access to antenna for both. Filter design determines actual isolation between TX and RX, and insertion loss TX-to-Antenna and RX-to-Antenna.
5 - 28

 Couplers are used to measure forward and reflected energy in a transmission line; it has 4 ports:
• Input (from TX),
Output (to load)
• Forward and Reverse Samples
 Sensing loops probe E& I in line
• Equal sensitivity to E & H fields
• Terminations absorb induced current in one direction, leaving only sample of other direction
 Typical performance specifications
• Coupling factor ~20, ~30,
~40 dB ., order as appropriate for application
• Directivity ~30-~40 dB., f($)
– defined as relative attenuation of unwanted direction in each sample
Input
Typical directional coupler
Principle of operation
R
T
Forward Sample
Reverse Sample
R
T
Z
LOAD
50
W
=
Main line’s E & I induce equal signals in sense loops. E is direction-independent, but I’s polarity depends on direction and cancels sample induced in one direction.
Thus sense loop signals are directional.
One end is used, the other terminated.
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 29

Directional coupler
Fwd
Antenna
RF
Power
Transmission line
Refl
A perfect antenna will absorb and radiate all the power fed to it
 Real antennas absorb most of the power, but reflect a portion back down the line
 A Directional Coupler or Directional Wattmeter can be used to measure the magnitude of the energy in both forward and reflected directions
 Antenna specs give maximum reflection over a specific frequency range
 Reflection magnitude can be expressed in the forms VSWR ,
Return Loss , or reflection coefficient
• VSWR = Voltage Standing Wave Ratio
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 30

50
40
30
20
10
0
1
VSWR vs. Return Loss
1.5
2
VSWR
2.5
3
Return
Loss, dB = 10 x Log
10
Reflected Power
Forward Power
Forward Power, Reflected Power,
Return Loss, and VSWR can be related by these equations and the graph.
• Typical antenna VSWR specifications are 1.5:1 maximum over a specified band.
• VSWR 1.5 : 1
= 14 db return loss
= 4.0% reflected power
VSWR =
1 +
1 -
Reflected Power
Forward Power
Reflected Power
Forward Power
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 31

-20
-30 f
1
Directional
Coupler
Network Analyzer
-10 f
2
Refl
Fwd
Antenna
Transmission
Line
A Network Analyzer can also display polar plots, Smith
Charts, phase response
A Spectrum Analyzer and tracking generator can be used if Network Analyzer not available
 It’s a good idea to take swept or TDR return loss measurements of a new antenna at installation and to recheck periodically
• maintain a printed or electronically stored copy of the analyzer output for comparison
• most types of antenna or transmission line failures are easily detectable by comparison with stored data
What is the maximum acceptable value of return loss as seen in sketch above?
Given:

Antenna VSWR max spec is 1.5 : 1 between f1 and f2

Transmission line loss = 3 dB.
Consideration & Solution:
 From chart, VSWR of 1.5 : 1 is a return loss of -14 dB, measured at the antenna
 Power goes through the line loss of -3 db to reach the antenna, and -3 db to return
 Therefore, maximum acceptable observation on the ground is -14 -3 -3 = - 20 dB.
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 32

July, 1998 RF100 (c) 1998 Scott Baxter 5 - 33

 Antenna behavior is very different close-in and far out
 Near-field region: the area within about 10 times the spacing between antenna’s internal elements
• Inside this region, the signal behaves as independent fields from each element of the antenna, with their individual directivity
 Far-field region: the area beyond roughly 10 times the spacing between the antenna’s internal elements
• In this region, the antenna seems to be a point-source and the contributions of the individual elements are indistinguishable
• The pattern is the composite of the array
 Obstructions in the near-field can dramatically alter the antenna performance Far-field
Near-field
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 34

 Obstructions near the site are sometimes unavoidable
 Near-field obstructions can seriously alter pattern shape
 More distant local obstructions can cause severe blockage, as for example roof edge in the figure at right
• Knife-edge diffraction analysis can help estimate diffraction loss in these situations
• Explore other antenna mounting positions
July, 1998 RF100 (c) 1998 Scott Baxter
Local obstruction example
Diffraction over obstructing edge
5 - 35

Often multiple antennas are needed at a site and interaction is troublesome
 Electrical isolation between antennas
• Coupling loss between isotropic antennas one wavelength apart is
22 dB
• 6 dB additional coupling loss with each doubling of separation
• Add gain or loss referenced from horizontal plane patterns
• Measure vertical separation between centers of the antennas
– vertical separation usually is very effective
 One antenna should not be mounted in main lobe and near-field of another
• Typically within 10 feet @ 800 MHz
• Typically 5-10 feet @ 1900 MHz
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 36

 Before considering downtilt, beamwidths, and depression angles, do some personal experimentation at a high site to gain a sense of the angles involved
 Visible width of fingers, etc. can be useful approximate benchmark for visual evaluation
 Measure and remember width of your own chosen references
 Standing at a site, correlate your sightings of objects you want to cover with angles in degrees and the antenna pattern
July, 1998
Visually estimating angles with tools always at hand distance width angle = arctangent (width / distance)
Typical Angles
Thumb width
Nail of forefinger
~2 degrees
~1 degree
All knuckles ~10 degrees
“Calibrate” yourself using the formula!
RF100 (c) 1998 Scott Baxter 5 - 37

Cell A
Scenario 1
Scenario 2
Cell B
Downtilt is commonly used for two reasons
 1. Reduce Interference
• Reduce radiation toward a distant co-channel cell
• Concentrate radiation within the serving cell
 2. Prevent “Overshoot”
• Improve coverage of nearby targets far below the antenna
– otherwise within “null” of antenna pattern
 Are these good strategies?
 How is downtilt applied?
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 38

 Basic principle: important to match vertical pattern against intended coverage targets
• Compare the angles toward objects against the antenna vertical pattern -what’s radiating toward the target?
• Don’t position a null of the antenna toward an important coverage target!
 Sketch and formula
• Notice the height and horizontal distance must be expressed in the same units before dividing
(both in feet, both in miles, etc.) q
Depression angle Vertical distance
Horizontal distance q
= ArcTAN ( Vertical distance / Horizontal distance )
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 39

 Mechanical downtilt
• Physically tilt the antenna
• The pattern in front goes down, and behind goes up
• Popular for sectorization and special omni applications
 Electrical downtilt
• Incremental phase shift is applied in the feed network
• The pattern “droops” all around, like an inverted saucer
• Common technique when downtilting omni cells
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 40

Cell A weak
Cell B
The Concept:
 Radiate a strong signal toward everything within the serving cell, but significantly reduce the radiation toward the area of Cell B height difference
150 ft q
1
q
2
4
12 miles q
1 = ArcTAN ( 150 / ( 4 * 5280 ) )
= -0.4 degrees q
2 = ArcTAN ( 150 / ( 12 * 5280 ) )
= -0.1 degrees
July, 1998 RF100 (c) 1998 Scott Baxter
The Reality:
 When actually calculated, it’s surprising how small the difference in angle is between the far edge of cell A and the near edge of Cell B
• Delta in the example is only 0.3 degrees !!
• Let’s look at antenna patterns
5 - 41

q
1 = -0.4 degrees q
2 = -0.1 degrees
 It’s an attractive idea, but usually the angle between edge of serving cell and nearest edge of distant cell is
-0.1
-0.4
just too small to exploit
• Downtilt or not, can’t get much difference in antenna radiation between q
1 and q
2
• Even if the pattern were sharp enough, alignment accuracy and wind-flexing would be problems
– delta q in this example is less than one degree!
• Also, if downtilting -- watch out for excessive RSSI and IM involving mobiles near cell!
 Soft handoff and good CDMA power control is more important
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 42

July, 1998
Scenario 2
 Application concern: too little radiation toward low, close-in coverage targets
 The solution is common-sense matching of the antenna vertical pattern to the angles where radiation is needed
• Calculate vertical angles to targets!!
• Watch the pattern nulls -- where do they fall on the ground?
• Choose a low-gain antenna with a fat vertical pattern if you have a wide range of vertical angles to “hit”
• Downtilt if appropriate
• If needed, investigate special “nullfilled” antennas with smooth patterns
RF100 (c) 1998 Scott Baxter 5 - 43

Before choosing an antenna for widespread deployment, investigate:
 Manufacturer’s measured patterns
• Observe pattern at low end of band, mid-band, and high end of band
• Any troublesome back lobes or minor lobes in H or V patterns?
• Watch out for nulls which would fall toward populated areas
• Be suspicious of extremely symmetrical, “clean” measured patterns
• Obtain Intermod Specifications and test results (-130 or better)
• Inspect return loss measurements across the band
 Inspect a sample unit
• Physical integrity? weatherproof?
• Dissimilar metals in contact anywhere?
• Collinear vertical antennas: feed method?
• End (compromise) or center-fed (best)?
• Complete your own return loss measurements, if possible
• Ideally, do your own limited pattern verification
 Check with other users for their experiences
July, 1998 RF100 (c) 1998 Scott Baxter 5 - 44