Chapter 3
Antenna Types
Part 1
3.2 Helical Antennas
Geometry of Helical Antennas
Diameter of ground
plane at least 3λ/4
Helical Antennas (Cont’d..)
Modes of Operation:
Normal
Axial
(Broadside)
(End-fire) – Most practical
Circular
polarization can be achieved over a wider
bandwidth (usually 2:1)
More
efficient
Helical Antennas (Cont’d..)
Helical Modes
Normal Mode
End-fire Mode
Helical Antennas (Cont’d..)
Important Parameters
  tan
1 
S 
1  S 

  tan  
 D 
C 
α = 0o (flat loop)
α = 90o (linear wire)
L0 
S 2  C 2 = single turn
Ln  NL0  N S 2  C 2
(10.24)
Helical Antennas (Cont’d..)
Normal Mode (NL0 << λ)
Dipole:
k o I 0 Se  jko r
E  j
sin 
4r
Loop:
E  
k o2
 2 I e
D
2
0
 jko r
sin 
4r
Δφ = j = 90o
AR 
E
E

4S
k o D
2

20 S
D 2
(4-26a)
(10.25)
(5-27b)
(10.26)
Helical Antennas (Cont’d..)
For this special case,
AR 
1)
2 0 S
D 
C
2
1
D  C  20 S
(10.28a)
2 0 S
S

2) tan  
D
(10.28)
S
20 S

S
D

20
20
(10.29)
The radiated field is circularly polarized in all directions other
than θ = 00
Helical Antennas (Cont’d..)
End-fire Mode
1)
12 o    14 o
2)
3
4
0  C   0
4
3
3)
N 3
(C ≈ λ0 near optimum)
Parameters for End-fire Mode
C
R  140 
 0




Accuracy (± 20%)
(10.30)
3
HPBW 
52 0 2
C NS
(10.31)
Helical Antennas (Cont’d..)
Parameters for End-fire Mode (cont)
3
FNBW (deg) 
115 0 2
(10.32)
C NS
Do  15 N
C 2S
30
2N  1
AR 
2N
(Dimensionless)
(10.33)
(10.34)
Helical Antennas (Cont’d..)
Feed Design for Helical Antennas

The nominal impedances of ordinary helices is 100-200 Ω.
However,
for many practical Tlines, it is desired to make it 50
Ω, and can be accomplished in many ways.
One way is to properly design the first ¼ turn of the helix
next to the feed.

This
is done by flattening the wire in the form of a strip
width, w, and nearly touching the ground plane which is
covered by a dielectric slab of height (h):
Helical Antennas (Cont’d..)
Feed Design for Helical Antenna (cont)
h
w
377
 0 Z0
2
(10.41)
where
w – width of the strip starting at feed
εr – dielectric constant of the dielectric slab
Z0 – characteristic impedance of the input Tline
The helix transitions from the strip to the regular wire
gradually during the ¼ to ½ turns.
Helical Antennas (Cont’d..)
3.3 Microstrip Patch Antenna
Other shapes such as circles, triangles and annular rings also
been used. It can be excited by an edge or probe fed, where its
location is chosen for impedance match between cable and
antenna.
Microstrip Patch AntennaRectangular
Design Steps
Calculate W,
1
2
W
2 f (  o o )  r  1
14
Microstrip Patch AntennaRectangular
Calculate L,
1
L
2f(

eff

0


 0.824h

0)





W


0
.
3

0
.
264


eff
h


W

 0.258   0.8  
eff
h


where,

eff

  r  1    r  1 
  
 1  12 h
 
 

W
 2   2 

0.5



15
Microstrip Patch AntennaRectangular
Calculate Edge Resistance of the patch
1
Rin 
2Ge 
Antenna Impedance
where,
Ge  0.00836
w

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3.4 Horn Antenna
• Horn antennas are the simplest and one of the most widely
used microwave antennas – 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.
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Horn Antenna
Characteristics
Horn antennas often have a directional radiation pattern with a
high antenna gain, which can range up to 25 dB in some cases, with 1020 dB being typical.
Horn antennas have a wide impedance bandwidth.
The gain of horn antennas often increases (and the beamwidth decreases)
as the frequency of operation is increased.
Horn antennas have very little loss, so the directivity of a horn is roughly
equal to its gain
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3.5 Reflector Antennas
• Reflector antennas can offer much higher gains than
horn antennas and are easy to design and construct.
• The most widely used antennas for high frequency and
high gain applications in radio astronomy, radar,
microwave and millimetre wave communications, and
satellite tracking and communications.
• The most popular shape is the paraboloid – because of
its excellent ability to produce a pencil beam (high gain)
with low sidelobes and good cross-polarisation
characteristics
19
Reflector Antennas
Parabolic reflector antenna
Parabolic reflectors typically have a very
high gain (30-40 dB is common) and
low cross polarization.
They also have a reasonable bandwidth.
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3.5 Emerging Antenna Technologies
•
•
•
•
Wearable Antenna
Reconfigurable Antenna
Smart Antenna
MIMO
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3.5.1 Wearable Antenna
• A wearable antenna is meant to be a part of the
clothing used for communication purposes,
which includes tracking and navigation, mobile
computing and public safety.
• Commonly, wearable antenna requirements for
all modern application require light weight, low
cost, almost maintenance-free and no
installation.
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Wearable Antenna
23
Wearable Antenna
• Apart from S11, gain and etc. another important
measurement to be conducted for wearable
antennas is SAR.
• Specific absorption rate (SAR) is a measure of
the rate at which energy is absorbed by the
human body when exposed to a radio
frequency (RF) electromagnetic field
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3.5.2 Reconfigurable Antenna
Reconfigurable Antenna Property
Frequency
Multiple Operating Frequencies
Beam steering
Pattern
Beam shaping
Linear to Circular
(RHCP/LHCP)
Polarization
V–H/H-V
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Reconfigurable Antenna
• Control Mechanisms
26
Reconfigurable Antenna
• Pattern Reconfigurable Antenna
y
z
x
RF Switch
L1
VDC
L2
L1
VDC
L2
(a)
Inductors
Metal pads
Capacitors
y
L3
L4
x
-z
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(b)
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3.5.3 Smart Antennas
What is a smart
antenna system?
• Let’s imagine that you are in a classroom.
– Lecturer is teaching
– Your friend is talking
• And you are GOOD student …..
Your Lecturer
YOU
Your Friend
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Smart Antennas
• Many refer to smart-antenna systems as smart
antennas, but in reality, antennas are not smart:
………………........it is the digital signal processing,
along with the antennas, which make the system
smart.
Ear smart?
Or the brain?
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Smart Antennas
Digital signal processing /
Beamforming unit
Antenna Arrays
Antenna-1
x1(n)
d(n)
Antenna-2
x2(n)
y(n) +
Antenna-3
x3(n)
e(n)
Antenna-k
xk(n)
W1 W2 W3
30
Wk
Adaptive
Algorithm
Smart Antenna System
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3.5.4 MIMO
• Multiple Input Multiple Output technology is uses multiple antennas
to make use of reflected signals to provide gains in channel
robustness and throughput.
Standard wireless transceiver
MIMO transceiver
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MIMO
The two main formats for MIMO are given below:
• Spatial diversity: Spatial diversity used in this narrower
sense often refers to transmit and receive diversity.
These two methodologies are used to provide
improvements in the signal to noise ratio and they are
characterised by improving the reliability of the system
with respect to the various forms of fading.
• Spatial multiplexing : This form of MIMO is used to
provide additional data capacity by utilising the different
paths to carry additional traffic, i.e. increasing the data
throughput capability.
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