5. Popular Antennas
The University of Liverpool 
 
popular antennas using relevant antenna
theories, to see why they have become
popular, what their major features and
properties (including advantages and
be designed.
Dipoles orn and open waveguide
!onopoles "eflector antennas
'lement antennas ntenna array
arrow*band #road*band
5.# Wire !"pe Antennas
Evolution of a dipole of total length 2l  and diameter d 
$ipole Antennas
&urrent distribution of dipoles
dipole
"adiation pattern is E θ(θ) % sinθ 
The directivity is D 2 0.3 (0.45d#i)
 
 6 "adiation resistance7 4;
 6 Directivity7 0.5< (9.03 d#i)
 6 The input impedance is not sensitive to the radius
and is about 4; = which is well matched with a
standard transmission line of characteristic
impedance 43 = or 31 = (with a >$-" 9).
 6 +ts si?e and radiation pattern are suitable for many
applications
'xample 3.0
  dipole of the length 9l  2 ; cm and diameter '  2 9 mm is
made of copper wire (σ 2 3.4 × 014 $/m) for mobile
communications. +f the operational fre@uency is 0 A?,
a). obtain its radiation pattern and directivityB
b). calculate its input impedance, radiation resistance and
radiation efficiencyB
c). if this antenna is also used as a field probe at 011 !?
for '!& applications, find its radiation efficiency again,
and express it in d#.
$olution on pages 135 % 13
 
 
the earth or a ground plane
ormally one*@uarter wavelength long
 6 almost the same feature as a dipole,
except the ;4 radiation resistance, higher gain, a
shorter length, and easier to feedC
#ased on the Image Theory 
l  θ
+ts si?e and material property of can change the radiation
pattern (hence the directivity) and input impedance.
 
 
which are closely lined. +n antennas, the duality theory  
means that it is possible to write the fields of one
antenna from the field expressions of the other antenna
by interchanging parameters7
 
 
"adiation pattern of a one wavelength loop 6 this is
very different from that of a short loopC
 
 
Helical Antennas
+t may be viewed as a derivative of the dipole or monopole,
 
Nor'al 'o(e Heli)
 6 (t may )e treate' as the s"perposition of n elements*
each consisting of a small loop of 'iameter D an' a
short 'ipole of length s* thus the far fields are
 6 They are orthogonal and F1 degrees out of phaseB
 6 The combination of them gives a circularly or
elliptically polarised wave.
the axial ratio becomes unity and the radiation is
circularly polarised. 
circumference of the helix is comparable with the
wavelength (+ % π D G λ) and the total length is much
greater than the wavelength.
circ"larly-polarise' )roa')an' antenna at the >E
and HE band fre@uencies
 6 The recommended parameters for an optimum
design to achieve circular polarisation are7
 
*alf po)er beam)idth:
1st null beam)idth:
Design a circularly polarised helix antenna of an end*
fire radiation pattern with a directivity of 0; d#i. Eind
out its radiation resistance, !./0* 12  and
radiation pattern.
 
35
The 'riven element  (feeder) is the very heart of the antenna. +t
determines the polarisation and centre fre@uency. Eor a
dipole, the recommended length is about 1.<4λ to ensuring a
good input impedance to a 31 = feed line.
The reflector is longer than the feeder to force the radiated
energy towards the front. The optimum spacing between the
reflector and the feeder is between 1.03 to 1.93 wavelengths.
The 'irectors are usually 01 to 91I shorter than the feeder
and appear to direct the radiation towards the front. The
director to director spacing is typically 1.93 to 1.;3
wavelengths,
directivity and gain.
3
 6 The antenna is divided into the so called active region 
and inactive regions.
 6 The role of a specific dipole element is lined to the
operating fre@uency7 if its length, L, is around half of
the wavelength, it is an active dipole and within the
active regionB Jtherwise it is in an inactive region and
acts as a director or reflector as in Kagi*Hda antenna
 6 The driven element shifts with the fre@uency 6 this is
why this antenna can offer a much wider bandwidth
than the Kagi*Hda. travelling wave can also be
formed in the antenna.
shortest dipole length while the lowest fre@uency is
determined by the longest dipole length (L).
 
This seems to have too many variables. +n fact, there
are only three in'epen'ent varia)les for log-perio'ic
antenna 'esign.
4#
+n practice, the most liely scenario is that the fre@uency
range is given from f min to f ma, , the following e@uations
may be employed for design 
-nother parameter .such as the directivit' or the length of
the antenna/ is re0uired to produce an optimised design
 
'xample 3.;
Design a log*periodic dipole antenna to cover all HE T>
channels, which is from <41 !? for channel 0< to LF1
!? for channel L;. 'ach channel has a bandwidth of 5
!?. The desired directivity is L d#i. 
$olution on page 16#
 
ow to lin the aperture ' field to the radiated field
irectivit':
 
 n open waveguide aperture of dimensions a long , and ) 
along y  located in the 4  2 1 plane. The field in the
aperture is T'01 mode and given by
Eind
i). the radiated far field and plot the radiation pattern in both
the ' and planesB
ii). the directivity.
 
Horn Antennas
  orn antennas are the simplest and one of the most widely
used microwave antennas 6 the antenna is nicely integrated
with the feed line (waveguide) and the performance can be
easily controlled.   They are mainly used for standard antenna gain and field
measurements, feed element for reflector antennas, and
microwave communications.
ie
 
'xample 3.3
Design a standard gain horn with a directivity of 91 d#i
at 01 A?. -"*F1 waveguide will be used to feed the
horn.
 
horn antennas and are easy to design and construct.
The most widely used antennas for high fre@uency and
high gain applications in radio astronomy, radar,
microwave and millimetre wave communications, and
satellite tracing and communications.
The most popular shape is the paraboloid 6 because of
its excellent ability to produce a pencil beam (high gain)
with low sidelobes and good cross*polarisation
characteristics
 
matching the fee' antenna pattern to the reflector The
"s"al goal is to have the fee' pattern a)o"t 5 '/ 'on
in the 'irection of the rim* that is the e'ge taper  2 (the
field at the edge)/(the field at the centre) G01 d#.
Directivity7
structure rigidity 
 
lots Antennas
They are very low*profile and can be conformed to basically
any configuration, thus they have found many
applications, for example, on aircraft and missiles.
 
-ntenna E0uivalent circuit
'@uivalence Principle7 for field analysis
The radiated field by the slot is the same as the field
radiated by its e@uivalent surface electric current and
magnetic current which were given by 
where  E and  H  are the electric and magnetic fields within the
slot, and   n  is the unit vector normal to the slot surface S 
or a half%)avelength slot its e0uivalent electric surface current  J  S  
8 9n  H 8 # the remaining source at the slot is its e0uivalent
magnetic current M  S  8 ;9n  E .it )ould be 2 M S  if the conducting
ground plane )ere removed using the imaging theor'/
 
#abinetNs Principle
The fiel' at any point )ehin' a plane having a screen* if
a''e' to the fiel' at the same point hen the
complementary screen is s")stit"te'* is e7"al to the
fiel' at the point hen no screen is present
 pply this to antennas7
$ince the impedance for a half%)avelength dipole is about 3
ohms the corresponding slot has an impedance of 
 
This type of antenna has a constant
impedance of
compact low profile configuration and good flexibility
Typical applications for 0 * 91 A?
 
To be a resonant antenna, the length L should be
around half of the wavelength. +n this case, the antenna
can be considered as a λ  8 transmission line resonant
cavity ith to open en's here the fringing fiel's from
the patch to the gro"n' are e,pose' to the "pper half
space 4 5; an' are responsi)le for the ra'iation.
This radiation mechanism is the same as the slot line,
thus there are two radiating slots on a patch antenna.
 s a resonant cavity, there are many possible modes (as
waveguides), thus a patch antenna is multi*mode and
may have many resonant fre@uencies.
 
"T/Duroid 3LL1 substrate ( and '  2 0.3LL mm) is to be
used to mae a resonant rectangular patch antenna of
linear polarisationB
a). Design such an antenna to wor at 9.<3 A? for
#luetooth applicationsB
b). 'stimate its directivityB
c). +f it is to be connected to a 31 ohms microstrip using the
same P&# board, design the feed to this antennaB
d). Eind the fractional bandwidth for >$-" 9.
$olution on pages 1!" % 1"1
 
#
5.0 Antenna Arra"s
!otivations7 to achieve desired high gain or radiation pattern, and the ability to provide an electrically scanned beam.
+t consists of more than one antenna element and these radiating elements are strategically placed in space to form an array with desired characteristics which are achieved by varying the feed (amplitude and phase) and relative position of each radiating elementB
 
 
Pattern !ultiplication Principle
$ince the total radiated field for an array is the summation of
the fields from each element
where  An is the amplitude,  n is the relative phase, E e is the
radiated field of the antenna element and AF  is called array
 factor.
 Th"s the ra'iation pattern of an array is the pro'"ct of the
 pattern of in'ivi'"al element antenna ith the isotropic
so"rce; array pattern 
4
or a uniform array of a constant d  and identical amplitude .sa' 1/
Thus
 
 
 

 
!
 
"
The maximum of the radiation occurs at ψ  2 1
That is:
 >ormall' the spacing d is fied for an arra' )e can control
the maimum radiation .or scan the beam/ b' changing the
 phase  # and the wavelength (frequency) – this is the
 principle of phase/frequency scanned array 
 
!#
 n array is called a broadside array  if the maximum
radiation of the array is directed normal to its axis (θ  2 11)B while it is called an end-fire array  if the maximum
 
source distributions of eight in%phase isotropic sources spaced b'
λ@2A there are trade-offs!
well as the radiation pattern
The voltage generated at each element can be
expressed as
The differences between transmitting and receiving antennas
 6 Erom the reciprocity theorem, the field patterns are the same for transmitting or receiving.
 ntenna feeding and matching
 6 #alun (a device to connect a balanced antenna to an unbalanced transmission line) may be re@uired.
Polarisation
 6 Polarisation has to be matched from Tx to "x.
"adomes, housings and supporting structures.
 6  ffecting the antenna performance (impedance, pattern, O)