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

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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,

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Microstrip Patch Antenna-RectangularMicrostrip Patch Antenna-Rectangular

Calculate Edge Resistance of the patch

Antenna Impedance

ein GR

2

1

w

Ge 00836.0

where,

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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.

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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

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• 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

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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

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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.

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Wearable AntennaWearable Antenna

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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

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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

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Reconfigurable AntennaReconfigurable Antenna• Pattern Reconfigurable Antenna

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Inductors

Capacitors

Metal pads(a)

(b)

VDCVDC L1 L2

L3 L4

z

y

x

-z

y

x

L1 L2

RF Switch

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3.5.3 Smart Antennas3.5.3 Smart Antennas

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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 …..

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YOU

Your Lecturer

Your Friend

Smart AntennasSmart Antennas

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• 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.

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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

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• 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.

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