ANTENNAS

— An antenna is a metallic conductor system

capable of radiating and capturing
electromagnetic energy.
— At the transmit end of a free space radio
communications system, an antenna converts
electrical energy travelling along a
transmission line into electromagnetic waves
that are emitted into space.
— At the receive end, an antenna converts EM
waves in space into electrical energy on a
transmission line.

Introduction
— All electrical circuits that carry alternating
current radiate a certain amount of
electrical energy in the form of EM waves.
— The amount of energy radiated is
negligible unless the physical dimensions
of the circuit approach the dimensions of
a wavelength of the wave.

Introduction
— A relatively small antenna can efficiently
radiate high-frequency electromagnetic
waves, while low frequency waves require
relatively large antennas.
— Every antenna has directional
characteristics and radiate more energy in
certain directions relative to other
directions.

Basic Antenna Operation

— Electromagnetic wave reception occurs in
an antenna because the electromagnetic
flux of the wave cuts across the antenna
conductor, inducing a voltage into the
conductor that varies with time in exactly
the same manner as the current flowing
in the antenna that radiated the wave.

Basic Antenna Operation

— A transmission line terminated in an open
circuit can represent an abrupt
discontinuity to the incident voltage in the
form of phase reversal.
— The phase reversal results in some of the
incident voltage being radiated, not
reflected back toward the source.
— The radiated energy propagates away
from the antenna in the form of
transverse electromagnetic waves.

Basic Antenna Operation

— Radiation efficiency is the ratio of the
radiated to reflected energy.
— The radiation efficiency of an open
transmission line is extremely low.
— To radiate more energy, simply spread the
conductors farther apart. Such an antenna
is called a dipole.

Radiation Efficiency
— When the conductors are spread out in a
straight line to a total length of one-quarter
wavelength, it is called quarter-wave antenna
or vertical monopole (Marconi antenna).
— A half-wave dipole is called a Hertz antenna.

Basic Antenna
— An antenna is a passive reciprocal device.

Passive • It cannot amplify a signal

• The transmit and receive

characteristics and performance are
Reciprocal identical (gain directivity, frequency of
operation, bandwidth, radiation
resistance and efficiency)

Antenna Reciprocity
 Capable of Produces very In radio
Transmit Antenna

Antenna Reciprocity
Receive Antenna
handling high small voltages communications
powers and currents systems, the
Uses materials It can be same antenna
that can constructed is used for
withstand high from small transmitting
voltages and diameter wire. and receiving
currents such which is
as metal tubing constructed
from heavy-
duty materials

Antenna Reciprocity
— It is used to direct the transmit and
receive signals and provide necessary
isolation for high power transmit signals
from relatively sensitive receiver.

Diplexer
 It has no active
PASSIVE components and
ANTENNA reciprocal

It is a combination ACTIVE
of passive antenna ANTENNA
and LNA. They are
nonreciprocal as
well.

Passive and Active Antenna

— A radiation pattern is a polar diagram or
graph representing field strengths or
power densities at various angular
positions relative to an antenna.

Absolute Radiation Relative Radiation

Pattern Pattern
• It is plotted in terms of • It is plotted in terms of field
electric field strength or strength or power density
power density with respect to the value at
a reference point

Radiation Pattern
Radiation Pattern
 Major Lobe. It
propagates and receives
the most energy. (Front Minor Lobe. Secondary beam
Lobe) in a direction other than that
of the major lobe. Minor
lobes represent undesired
radiation or reception

Major and Minor Lobes

— The line bisecting the major lobe, or
pointing from the center of the antenna in
the direction of maximum radiation.

Line of Shoot / Point of Shoot

— An omnidirectional antenna radiates
energy equally in all directions; thus, the
radiation pattern is simply a circle.
— Omnidirectional antenna has no front,
back, or side lobes because radiation is
equal in all direction.

Omnidirectional Antenna
— The radiation field that is close to an
antenna is not the same as the radiation
field that is at a great distance.
— Near field refers to the field pattern that is
close to the antenna.
— Far Field refers to the field pattern that is
at great distance.

Near and Far Fields

— During one-half of a cycle, power is radiated
from an antenna where some of the power is
stored temporarily in the near field.
— During the second half of the cycle, power in
the near field is returned to the antenna
which is similar to an inductor which stores
and releases energy.
— The near field is sometimes called the
induction field.
— The near field is the area within a distance
(antenna diameter)^2/wavelength

Near Field
— Power that reaches the far field continues
to radiate outward and is never returned
to the antenna.
— Far field is sometimes called the radiation
field.

Far Field
— All the power supplied to an antenna is not
radiated. Some of it is converted to heat and
dissipated.
— Radiation resistance is an ac resistance and
is equal to the ratio of the power radiated by
the antenna to the square of the current at
its feedpoint.
Prad
Rr = 2
i
— Radiation resistance is the resistance that, if
it replaced the antenna, would dissipate
exactly the same amount of power that the
antenna radiates.

Radiation Resistance
— Antenna efficiency is the ratio of the power
dissipated by an antenna to the sum of the
power radiated and power dissipated or the
ratio of the power radiated by the antenna to
the total input power.
Prad Prad Rr
h= x100 = x100 = x100
Pin Prad + Pd Rr + Re

— Where: n = antenna efficiency (percentage)

Prad = radiated power (W)
Pin = input power = Prad + Pd

Antenna Efficiency
Directive Gain Power Gain

The ratio of the power

density radiated in a
particular direction to
the power density It is the same as
radiated to the same directive gain except
point by a reference that the total power
antenna, assuming fed to the antenna is
both antennas are used.
radiating the same
amount of power.

Antenna Gain
— The relative power density radiation pattern
for an antenna is usually a directive gain
pattern if the power density reference is
taken from a standard reference antenna,
which is generally an isotropic antenna.
P
D=
Pref
— Where: D = directive gain
P = power density at some point with a
given antenna
Pref = power density at the same point
with a reference antenna

Directive Gain
— Where: D = Directive gain
n = antenna efficiency
A p = Dh
For an isotropic reference, the power gain (dB)
of a half wave dipole is approximately 1.64
(2.15 dB)

Power Gain
— Effective isotropic radiated power (EIRP) is
defined as an equivalent transmit power
EIRP = Prad Dt EIRP = Pin At
— Where: Prad = total radiated power, W
Dt = transmit antenna directive
gain
Pin = input power
At = transmit antenna power gain

Effective Isotropic Radiated Power

 Pin At Prad Dt
P= =
4pR 2
4pR 2

¨ Where: P = Power density, W/m^2

Prad = total radiated power, W
Dt = transmit antenna directive gain
Pin = input power
At = transmit antenna power gain
R = distance from transmit antenna

Power Density at a given point

distance R
— For a transmit antenna with a power gain
At = 10, and an input power Pin = 10 W,
determine
A. EIRP in watts, dBm and dBW
B. Power density at a point 10 km from the
transmit antenna
C. Power density had an isotropic antenna been
used with the same input power and
efficiency.

Example #1
— For a transmit antenna with a radiation
resistance Rr = 72 ohms, an effective
antenna resistance Re = 8 ohms, a
directive gain D = 20, and an input power
Pin = 100 W, determine
A. Antenna efficiency
B. Antenna gain (absolute and dB)
C. Radiated power in watts, dBm and dBW
D. EIRP in watts, dBm and dBW

Example #2
— Given the free-space radio transmission system
shown below with the following transmission
characteristics

Transmitter Output Power 40 dBm

Transmission Line Loss (Lf) 3 dB
Free-space path loss (Lp) 50 dB

Prad P
Pin Lp

EIRP
Lf
Pout Rx

Earth Surface
Example #3
A. Determine the antenna input power (Pin),
radiated power (Prad), EIRP, and receive power
density (P) for an isotropic transmit antenna
with a directivity of unity Dt = 1 and an
efficiency n = 100% (a gain of 1).
B. Determine the antenna input power (Pin),
radiated power (Prad), EIRP, and receive power
density (P) for an isotropic transmit antenna
with a directivity of ten Dt = 10 and an
efficiency n = 50%.
C. Determine the antenna input power (Pin),
radiated power (Prad), EIRP, and receive power
density (P) for an isotropic transmit antenna
with a power gain of At = 5; n=50%.

Example #3
— Captured power density is the power density
(W/m^2) in space at the receiving antenna.
Pin At Ar
C=
4pR 2
where: C = captured power (W/m^2)
Pin = transmit antenna input power
(W)
At = transmit antenna power gain
Ar = receive antenna power gain
R = distance between Tx and Rx
antennas (m)

Captured Power Density

— Determine the power density captured by
a receiving antenna for the following
parameters: transmit antenna input, Pin
= 50 W; transmit antenna gain, At = 30
dB; distance between transmit and
receive antennas, d = 20 km; and receive
antenna directive gain, Ar = 26 dB.

Example 4
— Capture area is a natural parameter used
in describing the reception properties of
an antenna.
— A transmit antenna radiates an
electromagnetic wave that has a power
density at the receive antenna’s location
in W/m^2. This is not the power received
but rather the amount of power incident
on, or passing through each unit area.

Antenna Capture Area

— where: Ac = effective Ar l capture area
2

(m^2) Ac =
4p
λ = wavelength of receive signal (m)
Ar = receive antenna power gain

Antenna Capture Area

— The power available at the antenna’s output
terminals (in watts) is the captured power.
— For the captured power to appear at the
antenna’s output terminals, the antenna
must have captured power from a surface in
space immediately surrounding the antenna.
— Captured power is directly proportional to the
received power density and the effective
capture area of the receive antenna

Captured Power
— where: Pcap = captured power (watts)
Pcap = PA
P = power density
c (W/m^2)
Ac = capture area (m^2)

Captured Power
— For a receive power density of 10
uW/m^2 and a receive antenna with a
capture area of 0.2 m^2, determine
A. Captured power in watts
B. Captured power in dBm

Example 5
— The polarization of an antenna refers
simply to the orientation of the electric
field radiated from it.

Antenna Polarization
— Antenna beamwidth is the angular
separation between the two half-power (-
3 dB) points on the major lobe of an
antenna’s plane radiation pattern, usually
taken in one of the principal planes.
— Antenna beamwidth is sometimes called -
3 dB beamwidth or half-power beamwidth
— Antenna gain is inversely proportional to
beamwidth.

Antenna Beamwidth
— An omnodirectional (isotropic) antenna
radiates well in all direction. Thus, it has a
gain of unity and a beamwidth of 360°.
— Typical antennas have beamwidths
between 30° and 60°.
— Microwave antennas have beamwidth as
low as 1°.

Antenna Beamwidth
— Antenna bandwidth is vaguely defined as
the frequency range over which antenna
operation is satisfactory.
— Bandwidth is normally taken as difference
between the half-power frequencies but
sometimes refers to variations in the
antenna’s input impedance.
— Antenna bandwidth is often expressed as
a percentage of the antenna’s optimum
frequency of operation.

Antenna Bandwidth
— Determine the percent bandwidth for an
antenna with an optimum frequency of
operation of 400 MHz and -3 dB
frequencies of 380 MHz and 420 MHz

Example 6
— Radiation from an antenna is a direct result
of the flow of RF current.
— The current flows to the antenna through a
transmission line, which is connected to a
small gap between the conductors that make
up the antenna.
— The point on the antenna where the
transmission line is connected is called the
antenna input terminal or the feedpoint.
— The feedpoint presents an ac load to the
transmission line called the antenna input
impedance.

Antenna Input Impedance

— Antenna input impedance is generally
complex; however, if the feedpoint is at a
current maximum and there is no reactive
component, the input impedance is equal
to the sum of radiation resistance and the
effective resistance.

Antenna Input Impedance