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Antenna Types
Part 1
Chapter 3Chapter 3
3.2 Helical Antennas3.2 Helical Antennas
Diameter of ground plane at least 3λ/4
Geometry of Helical Antennas
Helical Antennas (Cont’d..)Helical Antennas (Cont’d..)
Modes of Operation:
Normal (Broadside)
Axial (End-fire) – Most practical
Circular polarization can be achieved over a wider bandwidth (usually 2:1)
More efficient
Helical Antennas (Cont’d..)Helical Antennas (Cont’d..)
Helical Modes Normal Mode End-fire Mode
Helical Antennas (Cont’d..)Helical Antennas (Cont’d..)
Important Parameters
C
S
D
S 11 tantan
(10.24)
α = 0o (flat loop)
α = 90o (linear wire)
220 CSL = single turn
220 CSNNLLn
Helical Antennas (Cont’d..)Helical Antennas (Cont’d..)
Normal Mode (NL0 << λ)
Dipole:
Loop:
sin4
0
r
SeIkjE
rjko
o
(4-26a) (10.25)
sin
42 0
22
r
eIDkE
rjko
o
(5-27b) (10.26)
Δφ = j = 90o
20
2
24
D
S
Dk
S
E
EAR
o
Helical Antennas (Cont’d..)Helical Antennas (Cont’d..)
For this special case,
The radiated field is circularly polarized in all directions other than θ = 00
1
22
0 D
SAR
SCD 02 (10.28)
SC 02
000 222tan
DS
S
S
D
S
(10.28a)
(10.29)
1)
2)
Helical Antennas (Cont’d..)Helical Antennas (Cont’d..)
End-fire Modeoo 1412
00 3
4
4
3 C
1)
2)
3N3)
Parameters for End-fire Mode
(C ≈ λ0 near optimum)
0
140C
R Accuracy (± 20%)
NSCHPBW
23
052
(10.30)
(10.31)
Helical Antennas (Cont’d..)Helical Antennas (Cont’d..)
Parameters for End-fire Mode (cont)
NSCFNBW
23
0115(deg)
30
2
15SC
NDo
N
NAR
2
12
(Dimensionless)
(10.32)
(10.33)
(10.34)
Helical Antennas (Cont’d..)Helical Antennas (Cont’d..)
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):
Feed Design for Helical Antennas
Helical Antennas (Cont’d..)Helical Antennas (Cont’d..)
2377
00
Z
wh
(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
Feed Design for Helical Antenna (cont)
The helix transitions from the strip to the regular wire gradually during the ¼ to ½ turns.
Helical Antennas (Cont’d..)Helical Antennas (Cont’d..)
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.
3.3 Microstrip Patch Antenna3.3 Microstrip Patch Antenna
14
1
2
)(2
1
roofW
Microstrip Patch Antenna-RectangularMicrostrip Patch Antenna-Rectangular
Design Steps
Calculate W,
15
8.0258.0
264.03.0824.0
)(2
1
00h
Wh
W
hf
L
eff
eff
eff
W
hrreff
1212
1
2
1 5.0
Microstrip Patch Antenna-RectangularMicrostrip Patch Antenna-RectangularCalculate L,
where,
16
Microstrip Patch Antenna-RectangularMicrostrip Patch Antenna-Rectangular
Calculate Edge Resistance of the patch
Antenna Impedance
ein GR
2
1
w
Ge 00836.0
where,
17
3.4 Horn Antenna3.4 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.
18
Horn AntennaHorn 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 10-20 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
19
• 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
3.5 Reflector Antennas3.5 Reflector Antennas
20
Reflector AntennasReflector 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.
• Wearable Antenna• Reconfigurable Antenna• Smart Antenna• MIMO
21
3.5 Emerging Antenna Technologies3.5 Emerging Antenna Technologies
3.5.1 Wearable Antenna3.5.1 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.
22
Wearable AntennaWearable Antenna
23
Wearable AntennaWearable 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
24
3.5.2 Reconfigurable Antenna3.5.2 Reconfigurable Antenna
25
Reconfigurable Antenna Property
Reconfigurable Antenna Property
Frequency
Pattern
Polarization
Multiple Operating Frequencies
Beam steering
Beam shaping
V – H / H - VLinear to Circular (RHCP/LHCP)
Reconfigurable AntennaReconfigurable Antenna• Control Mechanisms
26
Reconfigurable AntennaReconfigurable Antenna• Pattern Reconfigurable Antenna
27
Inductors
Capacitors
Metal pads(a)
(b)
VDCVDC L1 L2
L3 L4
z
y
x
-z
y
x
L1 L2
RF Switch
27
3.5.3 Smart Antennas3.5.3 Smart Antennas
28
What is a smart antenna system?
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 …..
28
YOU
Your Lecturer
Your Friend
Smart AntennasSmart Antennas
29
• 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.
29
Ear smart?
Or the brain?
Smart AntennasSmart Antennas
3030
Adaptive AlgorithmW1 W2 W3 Wk
d(n)
y(n)
e(n)
+-
x1(n)
x2(n)
x3(n)
xk(n)
Antenna-1
Antenna-2
Antenna-3
Antenna-k
Antenna ArraysDigital signal processing / Beamforming unit
Smart Antenna System
3.5.4 MIMO3.5.4 MIMO
31
• 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
MIMOMIMOThe 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.
32