THREE DEE…
THREE DEE…
THREE DEE…
THREE DEE…
THREE DEE…
Basic Antenna Theory and
Concepts
In 1913, the Eiffel Tower was used an antenna. Back when
Communication was at very low frequencies, the antennas had
 to be very large to get any sort of radiation. The Eiffel Tower
 fit this bill well, and was used to communicate with
 the United States Naval Observatory in Arlington, Virginia.
JOKE
· TWO ANTENNAS MET ON A
ROOF, FELL IN LOVE AND GOT
MARRIED. THE WEDDING
WASN'T GREAT BUT THE
RECEPTION WAS EXCELLENT.
Introduction

An antenna is an electrical conductor or
system of conductors



Transmission - radiates electromagnetic energy
into space
Reception - collects electromagnetic energy
from space
In two-way communication, the same
antenna can be used for transmission and
reception
One of the Fundamental
Secrets of the Universe
Figure 1. A simple waveform.
Figure 2. First fundamental frequency (left)
and original waveform (right) compared.
Figure 3. Second fundamental frequency (left) and original
waveform compared with the first two frequency components.
Figure 4. Third fundamental frequency (left) and original
waveform compared with the first three frequency components.
Figure 5. Fourth fundamental frequency (left) and original
waveform compared with the first four frequency components
(overlapped).

While this seems made up, it is true for all
waveforms. This goes for TV signals, cell
phone signals, the sound waves that travel
when you speak. In general, waveforms are
not made up of a discrete number of
frequencies, but rather a continuous range of
frequencies.
i e…
The Fourier Series
Antenna Definition

An antenna is a circuit element that
provides a transition for a guided wave
on a transmission line to a free space
wave and it provides for the collection
of electromagnetic energy.
Antenna research from
Miller & Beasley, 2002
Antenna Definition-cont’d


In transmit systems the RF signal is
generated, amplified, modulated and
applied to the antenna
In receive systems the antenna collects
electromagnetic
waves
that
are
“cutting” through the antenna and
induce alternating currents that are
used by the receiver
Reciprocity


Is An antenna ability to transfer energy
from the atmosphere to its receiver with the
same efficiency with which it transfers
energy from the transmitter into the
atmosphere
Antenna characteristics are essentially the
same regardless of whether an antenna is
sending or receiving electromagnetic energy
Antenna RadiationTheory
(how EM waves emerge from a
dipole)
Polarization

Polarization is the direction of the electric
field and is the same as the physical attitude
of the antenna


A vertical antenna will transmit a vertically
polarized wave
The receiving and transmitting antennas
need to possess the same polarization
Radiation & Induction Fields

The mechanics launching radio
frequencies from an antenna are not
fully understood. The RF fields that are
created around the antenna have
specific properties that affect the
signals transmission. The radiated field
field is known as the (surprisingly!)
radiation field
Radiation & Induction Fields-cont’d

There are two induction fields or areas
where signals collapse and radiate from
the antenna. They are known as the
near field and far field. The distance
that antenna inductance has on the
transmitted signal is directly
proportional to antenna height and the
dimensions of the wave
R  2D

2
Radiation & Induction Fields-cont’d
R  2D

R = the distance from the antenna
2
Where:
D = dimension of the antenna
 = wavelength of the transmitted
signal
Fd Regions of an Antenna
Fresnel Region
Fraunhofer Region
The Antenna Formula
  c  3 x 10 msec
frequency of the signal
8
•c is the speed of light
 is the wavelength of the signal
 use 3 x 108 when dealing in meters for the speed of light
The Antenna Formula - applied

If a half-wave dipole antenna needed to
be constructed for a 60 Hz signal, how
large would it need to be?
  c  3 x 10 msec
60
8
2 = 2500 km!
= 5000 km
Radiation Patterns

A radiation pattern defines the variation
of the power radiated by an antenna as
a function of the direction away from
the antenna. This power variation as a
function of the arrival angle is observed
in the far field.
Radiation Patterns



Radiation pattern
 Graphical representation of radiation properties
of an antenna
 Depicted as two-dimensional cross section
Beam width (or half-power beam width)
 Measure of directivity of antenna
Reception pattern
 Receiving antenna’s equivalent to radiation
pattern
Radiation Patterns
Radiation Pattern

Radiation pattern is an indication of
radiated field strength around the
antenna. Power radiated from a /2
dipole occurs at right angles to the
antenna with no power emitting from
the ends of the antenna. Optimum
signal strength occurs at right angles or
180° from opposite the antenna
Radiation Pattern

A pattern is "isotropic" if the radiation pattern
is the same in all directions. These antennas
don't exist in practice, but are sometimes
discussed as a means of comparison with real
antennas. Some antennas may also be
described as "omnidirectional", which for an
actual means that it is isotropic in a single
plane (as in Figure 1 above for the x-y plane).
The third category of antennas are
"directional", which do not have a symmetry
in the radiation pattern.
Directivity / Directive Gain (D)

Power density of an ae in a certain direction/
power density of an isotropic ae.
Efficiency

ε = P(radiated) / P(input).
Antenna Gain

Peak power in certain direction/
peak power of isotropic ae.
G = εx D
Directivity Figures
Antenna Type
Short Dipole
Typical Directivity
Typical Directivity
(dB)
1.5
1.76
Half Wave Dipole 1.64
2.15
Patch (Microstrip)
3.2-6.3
Antenna
5-8
Horn Antenna
10-100
10-20
Dish Antenna
10-10,000
10-40
Antena Gain


The term Gain describes how much power is
transmitted in the direction of peak radiation to
that of an isotropic source. Gain is more commonly
quoted in a real antenna's specification sheet
because it takes into account the actual losses that
occur.
A gain of 3 dB means that the power received far
from the antenna will be 3 dB (twice as much)
higher than what would be received from a
lossless isotropic antenna with the same input
power.
Beamwidth
3 D view
Polar coordinates
(measured off of z-axis)
Half Power Beamwidth(HPBW)

The Half Power Beamwidth (HPBW) is the
angular separation in which the magnitude of the
radiation pattern decrease by 50% (or -3 dB) from
the peak of the main beam. From a/m fig, the
pattern decreases to -3 dB at 77.7 and 102.3
degrees. Hence the HPBW is 102.3-77.7 = 24.6
degrees.
Null to Null Beamwidth

Another commonly quoted beamwidth is the Null
to Null Beamwidth. This is the angular
separation from which the magnitude of the
radiation pattern decreases to zero away from the
main beam. From Fig, the pattern goes to zero at
60 degrees and 120 degrees. Hence, the Null-Null
Beamwidth is 120-60=60 degrees.
Sidelobes & Sidelobe levels


The sidelobes are smaller beams that are away
from the main beam. These sidelobes are usually
radiation in undesired directions which can never
be completely eliminated. The sidelobes in Figure 2
occur at roughly 45 and 135 degrees.
Sidelobe Level is another important parameter
used to characterize radiation patterns. The
sidelobe level is the maximum value of the
sidelobes (away from the main beam). From Fig,
the Sidelobe Level (SLL) is -14.5 dB.
Impedance
LF
HF
Impedance


If the antenna is matched to the transmission line
(ZA=ZO), then the input impedance does not depend on
the length of the transmission line.
If the antenna is not matched, the input impedance will
vary widely with the length of the transmission line. And if
the input impedance isn't well matched to the source
impedance, not very much power will be delivered to the
antenna. This power ends up being reflected back to the
generator, which can be a problem in itself (especially if
high power is transmitted). Hence, we see that having a
tuned impedance for an antenna is extremely important.
Bandwidth


This describes the range of frequencies over which
the antenna can properly radiate or receive
energy. Often, the desired bandwidth is one of the
determining parameters used to decide upon an
antenna. For instance, many antenna types have
very narrow bandwidths and cannot be used for
wideband operation.
When the power drops to ½ (3db), the upper &
lower extremities of these frequencies have been
reached & the antenna no longer performs
satisfactorily.
Antenna Q


The Q of an antenna is a measure of the
bandwidth of an antenna relative to the center
frequency of the bandwidth. If the antenna
operates over a band between f1 and f2 with
center frequency fc=(f1+f2)/2, then the Q is given
by:
Antennas with a high Q are narrowband, antennas
with a low Q are wideband. The higher the value
of Q, the more sensitive the input impedance is to
small changes in frequency.
Polarisaion
Linear Polarisation
Polarisation
…also Linear Polarisation
Polarisation
Circular Polarisation
Polarisation
Criteria for Circular Polarization
 The E-field must have two orthogonal (perpendicular) components.
 The E-field's orthogonal components must have equal magnitude.
 The orthogonal components must be 90 degrees out of phase.
If the wave in Fig is travelling out of the screen, the field is
rotating in the counter-clockwise direction and is said to be
Right Hand Circularly Polarized (RHCP). If the fields
were rotating in the clockwise direction, the field would be
Left Hand Circularly Polarized (LHCP).
Polarisation
Elliptical Polarisation
If the E-field has two perpendicular components that are
out of phase by 90 degrees but are not equal in
magnitude, the field will end up Elliptically Polarized.
Antenna Polarisation


The polarization of an antenna is the polarization of the
radiated fields produced by an antenna, evaluated in the far
field. Hence, antennas are often classified as "Linearly
Polarized" or a "Right Hand Circularly Polarized Antenna".
This simple concept is important for antenna to antenna
communication. First, a horizontally polarized antenna will
not communicate with a vertically polarized antenna. Due to
the reciprocity theorem, antennas transmit and receive in
exactly the same manner. Hence, a vertically polarized
antenna transmits and receives vertically polarized fields.
Consequently, if a horizontally polarized antenna is trying to
communicate with a vertically polarized antenna, there will be
no reception.
Effective Area


A useful parameter calculating the receive power of an
antenna is the effective area or effective aperture. Assume
that a plane wave with the same polarization as the receive
antenna is incident upon the antenna. Further assume that the
wave is travelling towards the antenna in the antenna's
direction of maximum radiation (the direction from which the
most power would be received).
Then the effective aperture parameter describes how much
power is captured from a given plane wave. Let W be the
power density of the plane wave (in W/m^2). If P represents
the power at the antennas terminals available to the antenna's
receiver, then:
Effective Area


Hence, the effective area simply represents how
much power is captured from the plane wave and
delivered by the antenna. This area factors in the
losses intrinsic to the antenna (ohmic losses,
dielectric losses, etc.). This parameter can be
determined by measurement for real antennas.
A general relation for the effective aperture in
terms of the peak gain (G) of any antenna is given
by:
Frii’s Transmission Formula
……………………………………..
……………………………………..
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!
VSWR



Voltage Standing Wave Ratio (VSWR) or sometimes just
Standing Wave Ratio (SWR) is a measure of how well matched
an antenna is (in terms of impedance) to the transmission line it
connects to.
The smaller the VSWR is, the better the antenna is matched to
the transmission line and the more power is delivered to the
antenna. The minimum VSWR=1.0, in which case none of the
power is reflected, which is the ideal case.
Often antennas must satisfy a bandwidth requirement that is
given in terms of VSWR. For instance, an antenna might claim
to operate from 100-200 MHz with VSWR<3. This implies that
the VSWR is <3 over the specified frequency range
Radiation Resistance

Radiation Resistance is the portion of
the antenna’s impedance that
results in power radiated into space
(i.e., the effective resistance that is
related to the power radiated by the
antenna. Radiation resistance varies
with
antenna
length.
Resistance
increases as the  increases
Effective Radiated Power (ERP)

ERP is the power input value and the
gain of the antenna multiplied together


dBi = isotropic radiator gain
dBd = dipole antenna gain
Propagation Modes



Ground-wave propagation
Sky-wave propagation
Line-of-sight propagation
Ground Wave Propagation
Ground Wave Propagation




Follows contour of the earth
Can Propagate considerable distances
Frequencies up to 2 MHz
Example

AM radio
Sky Wave Propagation
Sky Wave Propagation




Signal reflected from ionized layer of atmosphere
back down to earth
Signal can travel a number of hops, back and forth
between ionosphere and earth’s surface
Reflection effect caused by refraction
Examples


Amateur radio
CB radio
Line-of-Sight Propagation
Line-of-Sight Propagation

Transmitting and receiving antennas must be
within line of sight


Satellite communication – signal above 30 MHz not
reflected by ionosphere
Ground communication – antennas within effective line
of site due to refraction
Types of Antenna
Types of Antenna











Wire Antennas
Short Dipole
Dipole Antenna
Half-Wave Dipole
Broadband Dipoles
Monopole
Folded Dipole
Small Loop
Microstrip Antennas
Rectangular Microstrip (Patch)
Antenna
Shorting Pins: Qr-Wavelength
Microstrips









Reflector Antennas
Corner Reflector
Parabolic Reflector (Dish Antenna)
Travelling Wave Antennas
Helical Antenna
Yagi-Uda Antenna
Aperture Antennas
Slot Antenna
Horn Antenna
Short Dipole
Half Wave Dipole
Directivity
HPBW
-
1.64 (2.15db)
78 degree
Broadband Dipole


An antenna can be made more broadband by
increasing the volume it occupies. Hence, a
dipole antenna can be made more broadband
by increasing the radius of the dipole.
The fatter the dipole gets the lower the
resonant frequency becomes. In other words,
if an antenna is to resonate at 100 MHz, the
resonant length decreases as the dipole gets
fatter.
Monopole Antenna


Monopoles are half the size of their dipole
counterparts, and hence are attractive when
a smaller antenna is needed.
Uses Image Theory.
Folded Dipole


The folded dipole is resonant and radiates
well at odd integer multiples of a halfwavelength (0.5 , 1.5 , ...).
The radiation pattern of half-wavelength
folded dipoles have the same form as that of
half-wavelength dipoles.
Small Loop Antenna

The small loop antenna is a closed loop.
These antennas have low radiation resistance
and high reactance, so that their impedance
is difficult to match to a transmitter. As a
result, these antennas are most often used as
receive antennas, where impedance
mismatch loss can be tolerated.
Microstrip Antenna

Microstrip or patch antennas are becoming
increasingly useful because they can be printed
directly onto a circuit board. They are becoming very
widespread within the mobile phone market. They
are low cost, have a low profile and are easily
fabricated.
Parabolic Reflector (Dish Antenna)



All rays emanating from the focal
point (the source or feed antenna)
will be reflected towards the same
direction.
The distance each ray travels from
the focal point to the reflector and
then to the focal plane is constant.
As a result of these observations, it follows the distribution of
the field on the focal plane will be in phase and travelling in the
same direction. This gives rise to the parabolic dish antenna,s
highly directional radiation pattern. This is why the shape of the
dish is parabolic.
Helical Antenna

The helix is a travelling wave antenna, which
means the current travels along the antenna and the
phase varies continuously.
Yagi – Uda Antenna


It is simple to construct and has a high gain, typically
greater than 10 dB. These antennas typically operate
in the HF to UHF bands (about 3 MHz to 3 GHz),
although their bandwidth is typically small, on the
order of a few percent of the center frequency.
Is an End-Fire Array.
Slot Antenna


Slot antennas are used typically at frequencies between 300
MHz and 24 GHz. These antennas are popular because they can
be cut out of whatever surface they are to be mounted on, and
have radiation patterns that are roughly omnidirectional (similar
to a linear wire antenna, as we'll see). The polarization is linear.
The slot size, shape and what is behind it (the cavity) offer
design variables that can be used to tune performance.
Ideal for Array.
Horn Antenna


Horn antennas are very popular at UHF (300 MHz-3 GHz) and
higher frequencies.They often have a directional radiation
pattern with a high gain , which can range up to 25 dB in some
cases, with 10-20 dB being typical.
The gain often increases (and the beamwidth decreases) as the
frequency of operation is increased. Horns have very little loss,
so the directivity of a horn is roughly equal to its gain.
YOU BETTER STOP NOW…