UNIT II
WIRE ANTENNAS AND ANTENNA ARRAYS
Dr.T.V.Padmavathy Professor/ECE
RMKCET
Presentation Outline
Introduction Dipole antenna Half-Wave Dipole
Quarter-Wave Monopole Loop Antenna
Turnstile Antenna Antenna Arrays
Two Element Arrays Broad Side Array End- Fire Array
Introduction
The antenna is the interface between the transmission line and
space
Antennas are passive devices; the power radiated cannot be
greater than the power entering from the transmitter
When speaking of gain in an antenna, gain refers to the idea that
certain directions are radiated better than others
Antennas are reciprocal - the same design works for receiving
systems as for transmitting
Antenna is a transducer that converts radio frequency electric
current to electromagnetic waves that are radiated into space
:2 wires each with length l = /4
Attach ends to terminals of a high frequency AC generator at time t, the generator’s right side = ‘+’ and the left side = ‘−’
Electrons flow away from the ‘−’ terminal and towards the ‘+’ terminal most current flows in the center and none flows at the ends
i(t) at any point will vary directly with v(t) current distribution at time t
− +
i(t) l = /4
A B − +
++++ +++++++ +++++++++++ +++++++++++++++ +++++++++++++++
--- ---
---
--- ---
---voltage distribution at time t
A B
Unit -II Wire Antennas and Antenna Arrays
Sinusoidal distribution of charge exists on the antenna that reverses
polarity every ½ cycle
Sinusoidal variation in charge magnitude lags the sinusoidal variation
in
current by ¼ cycle.
Electic field E and magnetic field H 90 out of phase with each other
fields add and produce a single EM field
Total energy in the radiated wave is constant, except for some
absorption as the wave advances, the energy density decreases
Unit -II Wire Antennas and Antenna Arrays
Half-Wave Dipole
2. For Hertzian dipole of length l, E field is: 1. Current in half-wave dipole
3. Each length element dz of half-wave dipole is like a Hertzian dipole, radiating a field
Half-Wave Dipole
A more practical antenna is the
half-wave dipole
Dipole simply means it is in two
parts
A dipole does not have to be
one-half wavelength, but that length is
handy for impedance matching
A half-wave dipole is sometimes
referred to as a Hertz antenna
Typically, the length of a half-wave
dipole is 95% of one-half the
wavelength measured in free space
Unit -II Wire Antennas and Antenna Arrays
c
Half-Wave Dipole
2. For Hertzian dipole of length l, E field is: 1. Current in half-wave dipole
3. Each length element dz of half-wave dipole is like a Hertzian dipole,
radiating a field
4. For the entire dipole, the total radiated field is
4 / 4 / cos ) ( cos Re cos cos ) ( 0 0 0 z kz I z I is phasor whose kze I kz t I ti j t
m V R e lk jI E jkR / sin 4 0 0 s jks s e dz z I jk z dE
( ) sin
Half-Wave Dipole
Integration leads to
0 0 sin cos 2 / cos 60 E H R e I j E jkR Radiation pattern resembles that of the Hertzian dipole.
Its beamwidth is slightly narrower, 78 degrees compared with 90
degrees for the Hertzian dipole.
Radiation resistance
2 2 362.6 73
Directivity
Quarter-Wave Monopole
Quarter-wave monopole antenna
consists of one-half of a half-wave dipole
For low- and medium-frequency
transmissions, it is necessary to use
vertical polarization to take advantage of
ground-wave propagation
A vertical dipole would be possible, but
similar results are available from a
quarter-wavelength monopole antenna
Fed at one end with an unbalanced
feedline, with the ground conductor of
the feedline taken to earth ground
Loop Antenna
It is a radio antenna consisting of a loop of wire with its ends connected to a balanced transmission line
It is a single turn coil carrying RF current through it.
The dimensions of coil are smaller than the wavelength hence current flowing through the coil has same phase.
Small loops have a poor efficiency and are mainly used as receiving antennas at low frequencies.
A technically small loop, also known as a magnetic loop, should
have a circumference of one tenth of a wavelength or less.
As the frequency or the size are increased, a standing wave starts to
develop in the current.
The antenna starts to have some of the characteristics of a folded
dipole antenna or a self-resonant loop.
Self-resonant loop antennas are larger.
The radiation efficiency is also high and similar to that of a dipole
Unit -II Wire Antennas and Antenna Arrays
Radiation pattern of loop antenna is a
doughnut pattern.
Can be circular or square loop
No radiation is received normal to the
plane of loop and null is obtained in this direction.
Application: Used for direction finding applications
Unit -II Wire Antennas and Antenna Arrays
Turnstile Antenna
It is a set of two dipole antennas aligned
at right angles to each other and fed 90 degrees out-of-phase.
The name reflects that the antenna looks
like a turnstile when mounted
horizontally.
When mounted horizontally the antenna
is nearly omnidirectional on the horizontal plane.
Turnstile Antenna
When mounted vertically the
antenna is directional to a right angle to its plane and is
circularly polarized.
The turnstile antenna is often
used for communication satellites
Antenna Arrays
Antenna arrays are becoming increasingly important in wireless
communications
Advantages
They can provide the capability of a steerable beam as in smart
antennas.
They can provide a high gain by using simple antenna elements.
They provide a diversity gain in multipath signal reception.
They enable array signal processing.
Two Element Arrays
Two Hertzian dipoles of length
dℓ separated by a distance d
and excited by currents with an equal a.
E1 is far-zone electric field
produced by antenna 1 =
E2 is far-zone electric field
produced by antenna 2 =
Unit -II Wire Antennas and Antenna Arrays
1 E a
2
The total E field is:
Unit -II Wire Antennas and Antenna Arrays
Two Element Arrays
Unit -II Wire Antennas and Antenna Arrays
The magnitude of the total E field is
Driven Arrays: Broadside Antenna
A broadside array is a stacked collinear antenna consisting of
half-wave dipoles spaced from one another by one-half half-wavelengths.
This antenna produces a highly directional radiation pattern that is
broadside or perpendicular to the plane of the array.
The broadside antenna is bidirectional in radiation, but the radiation
pattern has a very narrow beam width and high gain.
Broadside Array
Bidirectional Array
Uses Dipoles fed in phase and separated by 1/2 wavelength
Broadside arrays may also b e formed from other types of elements,
O. Kilic EE 542 23
Radiation Pattern for Arrays
Depends on:
The type of the individual elements
Their orientation
Their position in space
The amplitude and phase of the current feeding them
Broadside Arrays
Driven Arrays: End-Fire Antenna
The end-fire array uses two half-wave dipoles spaced one-half
wavelength apart.
The end-fire array has a bidirectional radiation pattern, but with
narrower beam widths and lower gain.
The radiation is in the plane of the driven elements.
A highly unidirectional antenna can be created by careful selection of
the optimal number of elements with the appropriately related spacing.
Similar to broadside array except dipoles are fed 180 degrees out of
phase
Radiation max. off the ends
End-fire Arrays
Higher directivity.
Provide increased directivity in elevation and azimuth planes.
Generally used for reception.
Impedance match difficulty in high power transmissions.
Variants are:
Horizontal Array of Dipoles
RCA Fish borne Antenna
Binomial array antenna
Advantages
To reduce secondary lobes
To optimize them.
As the array length increases the directivity increases and side lobes
Pattern multiplication
Multiplication pattern is the total field pattern of an array of
non-isotropic but similar source
It is the multiplication of the individual source pattern and the pattern
of an array of isotropic point source each located at the phase centre of individual sources having the relative amplitude and phase T
The total phase pattern is the addition of the phase pattern of ‘the
