Antennas & Propagations

Unit I
Outline
 Basic
 Retarded Potentials
 Radiations from alternating Current
 Monopoles and Dipoles
 Effective Length
 Radiation Resistance
 Basic
 What is an Antenna?
 Antenna is a source or radiator of Electromagnetic
waves or a sensor of Electromagnetic waves.
 It is a transition device or transducer between a
guided wave and a free space wave or vice versa.
 It is also an electrical conductor or system of
conductors that radiates EM energy into or collects
EM energy from free space.
 Antennas function by transmitting or receiving
electromagnetic (EM) waves.
 Some Antenna Types :
 Wire Antennas- dipoles, loops and Helical
 Aperture Antennas-Horns and reflectors Array
 Array Antennas-Yagi, Log periodic Patch
Antennas-
 Microstrips, PIFAs
 Reflector antennas - parabolic reflectors, corner
reflectors
 Lens antennas: convex-plane, convex-convex ,
convex-concave and concave-plane lenses
 Antenna Parameter
 A radio antenna may be defined as the structure associated with the
region of transition between a guided wave and a free space wave or
vice versa.
 Principle:
 Under time varying conditions, Maxwell’s equations predict the
radiation of EM energy from current source (or accelerated
charge).
 This happens at all frequencies, but is insignificant as long as the
size of the source region is not comparable to the wavelength.
 While transmission lines are designed to minimize this radiation
loss, radiation into free space becomes main purpose in case of
Antennas.
 The basic principle of radiation is produced by accelerated charge.
Transmission line opened out in a Tapered fashion as Antenna:
 As Transmitting Antenna :
 Here the Transmission Line is connected to source or generator
at one end.
 Along the uniform part of the line energy is guided as Plane TEM
wave with little loss.
 Spacing between line is a small fraction of λ. As the line is
opened out and the separation between the two lines becomes
comparable to λ, it acts like an antenna and launches a free
space wave since currents on the transmission line flow out on
the antenna but fields associated with them keep on going.
 From the circuit point of view the antennas appear to the
transmission lines as a resistance Rr, called Radiation resistance.
 As Receiving Antenna :
 Active radiation by other Antenna or Passive
radiation from distant objects raises the apparent
temperature of Rr .
 This has nothing to do with the physical temperature
of the antenna itself but is related to the temperature
of distant objects that the antenna is looking at.
 Rr may be thought of as virtual resistance that does
not exist physically but is a quantity coupling the
antenna to distant regions of space via a virtual
transmission line.

Reciprocity :
 An antenna exhibits identical impedance during
Transmission or Reception, same directional patterns
during Transmission or Reception, same effective height
while transmitting or receiving.
 Transmission and reception antennas can be used
interchangeably.
 Medium must be linear, passive and isotropic (physical
properties are the same in different directions).
 Antennas are usually optimized for reception or
transmission, not both.
 Retarded Potential

Propagation time is important for plane waves in
space and waves on transmission lines.

Its also important with antenna or radiating systems

The only difference is in radiating system, we deal
with two or three dimensions while plane waves
along transmission line deals with one dimensional
propagation.

The time-varying current I in radiating current
element is
I = I0 cos ωt
 This is implies instantaneous propagation of the effect of the current.
 Instead we take into account the time of propagation (retardation
time)

If the EM field is radiated at position vector r′ (within the source
charge distribution), and an observer at position r measures the EM
field at time t,
 The time delay for the field to travel from the charge distribution to
the observer is |r − r′|/c, so subtracting this delay from the observer's
time t gives the time when the field actually began to propagate - the
retarded time, t′
t′ = t - |r − r′|/c
 Hence retarded current can be written as
[I] = I0 cos ωt′
 Radiation from an
alternating current element
 An alternating current element or oscillating current
dipole possesses electromagnetic field.
 We will find these fields everywhere around using
the concept of Retard Vector Potential.
 Let the elemental length (dl) of the wire be placed
at the origin of the spherical coordinate and I be
current flowing through it as shown in the figure
 The length is so short that current is constant along
the length.
Monopoles & Dipoles
 A monopole antenna is one half of a dipole
antenna, almost always mounted above
some sort of ground plane.
 The case of a monopole antenna of length
L mounted above an infinite ground plane
 Using image theory, the fields above the ground
plane can be found by using the equivalent source
(antenna) in free space as shown in Figure 1(b).
 This is simply a dipole antenna of twice the length.
 The fields above the ground plane in Figure 1(a)
are identical to the fields in Figure which are
known and presented in the dipole antenna
section.
 The monopole antenna fields below the ground
plane in Figure 1(a) are zero.
 The dipole antenna with a very thin radius is
considered.
 The dipole antenna is similar to the short
dipole except it is not required to be small
compared to the wavelength (at the frequency
the antenna is operating at).
 For a dipole antenna of length L oriented
along the z-axis and centered at z=0, the
current flows in the z-direction with amplitude
which closely follows the following function:

This current is also oscillating in time
sinusoidally at frequency f.

The current distributions for the quarter-
wavelength (left) and full-wavelength (right)
dipole antennas are given in Figure 1.

Note that the peak value of the current I0 is
not reached along the dipole unless the
length is greater than half a wavelength.
 Effective Length

Effective height provides an indication as to how much of the
antenna is involved in radiating (or receiving).

When a receiving antenna intercepts incident electromagnetic
waves, a voltage is induced across the antenna terminals.

The effective length he of a receiving antenna is defined as the
ratio of the open circuit terminal voltage to the incident electric
field strength
h =
e
V m
oc
E
in the direction of antennas polarization.


where
Voc = open circuit voltage
E = electric field strength
 To demonstrate this, consider the current
distributions a dipole antenna for two different
lengths.
 If the current distribution of the dipole were
uniform, it’s effective height would be l Here
the current distribution is nearly sinusoidal
with average value 2/π=0.64(of the maximum)
so that it’s effective height is 0.64l .
 It is assumed that antenna is oriented for
maximum response.
 If the same dipole is used at longer
wavelength so that it is only 0.1λ long,
 The current tapers almost linearly from the
central feed point to zero at the ends in a
triangular distribution.
 The average current is now 0.5 & effective
height is 0.5l
Radiation resistance
 The radiation resistance of an antenna is
defined as the equivalent resistance that
would dissipate the same amount power as
is radiated by the antenna.
 Radiation Pattern
 The relative distribution of radiated power as a function of
direction in space is called the radiation pattern of the
antenna.
 E-plane and H-plane patterns give two most important
views.
 The E-plane pattern is a view obtained from a section
containing maximum value of the radiated field and electric
field lies in the plane of the section.
 Similarly when such a section is taken such that the plane
of the section contains H field and the direction of
maximum radiation.
 The main lobe contains the direction of maximum
radiation.

However in some antennas, more than one major
lobe may exist.

Lobe other than major lobe are called minor lobes.
 Minor lobes can be further represent radiation in
the considered direction and require to be
minimized.
 Directivity

The DIRECTIVITY of an antenna or array is a
measure of the antenna‘s ability to focus the
energy in one or more specific directions.

You can determine an antenna‘s directivity by
looking at its radiation pattern.

The elements in the array can be arranged so
they change the pattern and distribute the
energy more evenly in all directions.

The elements can be arranged so the radiated
energy is focused in one direction.
 It is defined as the ratio of maximum radiation
intensity of subject or test antenna to the radiation
intensity of an isotropic antenna.
(or)
 Directivity is defined as the ratio of maximum
radiation intensity to the average radiation
intensity.
 Directivity (D) in terms of total power radiated is,
D = 4π x Maximum radiation intensity/ Total
power radiated
 Gain
 Gain is a parameter which measures the degree of
directivity of the antenna's radiation pattern.
 A high-gain antenna will preferentially radiate in a
particular direction.
 The antenna gain, or power gain of an antenna is
defined as the ratio of the intensity (power per unit
surface) radiated by the antenna in the direction of
its maximum output, at an arbitrary distance,
divided by the intensity radiated at the same
distance by a hypothetical isotropic antenna.

Some antennas are highly directional.

That is, they propagate more energy in certain
directions than in others.

The ratio between the amount of energy
propagated in these directions and the energy
that would be propagated if the antenna were
not directional is known as antenna GAIN.

The gain of an antenna is constant. whether
the antenna is used for transmitting or
receiving.
 The maximum of directivity function is called the
directivity.
 In defining directivity function total radiated power
is taken as the reference.
 Another parameter called the gain of an antenna is
defined in the similar manner which takes into
account the total input power rather than the total
radiated power is used as the reference.
 The maximum gain function is termed as gain of
the antenna.
Antenna Equivalent
Circuit
 To a generator feeding a transmitting
antenna, the antenna appears as a lead.
 The receiver circuitry connected to a
receiving antenna's output terminal will
appear as load impedance.
 Both transmitting and receiving antennas
can be represented by equivalent circuits

Vg = open circuit voltage of the generator

Zg = antenna impedance

Z0 = Characteristics impedance of the transmission line connecting
generator to the antenna.

Pinc = Incident power to the antenna terminal

Prefl = Power reflected from the antenna terminal.

Pin = Input power to the antenna

XA = Antenna reactance

Rl = Loss resistance of the antenna

Rr = Radiation resistance

ZA = ( Rl + Rr ) + jXA = RA + jXA antenna impedance.

he = effective length
 E = incident field strength

Voc = h0 E open circuit voltage

Zload = Input impedance of the receiver.

Re, Rr and XA as defined earlier.