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BROADBAND ANTENNAS

Sumber buku:

Antenna Theory and Design

by Warren L. Stutzman dkk.

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Broadband Antennas:

Traveling wave antennas

Long terminated wires

Helical Antennas

Gambar-gambar antena

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There are many applications that it is extremely importantto use antenna with broad bandwidth

Definition:

2Lu

c fff

1

23

Lf Freq.

VSWR

cf uf

Lower

center

upper

Bandwidth as a percent of the

center frequency

Bandwidith as a ratio

%100

c

Lup

fffB

L

ur

f

fB

usually used for small

bandwidth antennas

usually used for large bandwidth antennas

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Classification of broadband antennas:

In practice, antennas whose impedance and patterndo not change significantly over about an octave

( fu/fl = 2) or more are considered to be broadband

Generally speaking, resonant antennas are not

broadband. Their bandwidth can vary from

0.5% to 30%

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Macam-macam broadband antena:

Traveling wave antennas

Helical antennas

Biconical antennas

Spiral antennas

Log periodic antennas etc.

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Question: What makes the dipole antenna not to be broadband ?

Answer: The establishment of the standing wave current

What causes

standing wavecurrent?

Reflection from the end points of the antenna

Recall :

)

2(sin)( zL

IzI m

Rewrite : zjzjLjm eeejIzI 2/

2)(

(only 10% to 20%)

Outward traveling wave Reflected wave

Current reflectioncoefficient at anopen circuit

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z

mI2/l

2/l

2sin

LI

VZ

m

A

strong function of

frequency

standing

wave

current

goes to zero

Observation: Suppression of the reflected wave

will enhance bandwidth. How?

If the reflected wave is not strongly present on an antenna, it is referredto as a traveling-wave antenna

The simplest traveling-wave wire antenna is a straight wire carrying apure traveling-wave, referred to as the traveling-wave long wire

antenna.

A long wire is one that is greater than

one- half wavelength long.

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Input impedance are

traveling wave antennas

are predominately real. In

this case, it is around 200

or 300 W.

For no reflection RL ischosen to be 200 or 300

W.

When this antenna is positioned near real

earth, it is called Beverage Antenna orWave antenna

Pattern:

)cos1(2/)cos1(2/sin

sin)(

L

LF

Example :

LR

zj

meII

L z

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Long Vee antenna:

o

L16

6

The directivity of a vee dipole canbe greater than that of astraight dipole

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Rhombic antenna:

o

L

166

The efficiency of the rhombic antennais decreased significantly because of thematched termination.

Small back lobe, better pattern

Produces wider impedance bandwidth

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If a conductor is wound into a helicalshape and fed properly, it is referred to asa helical antenna or simply as a helix.

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Geometry Parameters

Depending on the helix

dimensions, this antenna can

operate in normal or axial

mode.

antennaloop)0(0 oS

antennalinear)90(0 oD

Note:

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For normalmode :

LD ; Radiation characteristics can beviewed as the combination of an

ideal dipole and small loop.

Ideal

dipole:

Small

loop:

sin

4 r

eISjE

rj

D

sin44

22

r

eIDE

rj

L

Note that their fields areorthogonal to each other and

90o out of phase. Depending on

their relative amplitudes various

polarization can occur.

rE

E

I

I

Approximation

of one turn of

normal mode

helix

ideal dipole

of length S

along Z axis

small loop of area

in x-y plane

2

4D

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Polarization:

222

22)(

22

4

D

S

D

S

ID

IS

E

E

or2

)(

2

C

S

E

EAR ncecircumfere

DC

In general, the polarization is elliptical.

Special cases:

polarizedlyhorizontalantennaloopsmall0 S

polarizedyverticallantennadipoleideal0 C

)(polarizedCircularly2 EESDC

Note they

were 90o

out of

phase

Radiation pattern

2

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z

Axial modePatternKey feature of

axial mode:

DC

C ~CAxial mode helical antennas have very

complex radiation mechanism. Kraus was

the first to fully explain their characteristics.

Based on many experimental studies, it

has been suggested that the helix carries a

pure traveling wave that travels outward

from the feed.

Operates well for

3

4

4

3 C

78.14/33/4

//

L

u

L

ur

cc

ffB

Almost circularly polarized

15 dB gain with small

cross-section

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The phase of the axial mode helix

current shifts along the helix path.

Assuming that the element pattern of

the non uniform current loop is

and applying the uniform array theory

cos

One obtains,

hSN

NKF

cos)2/sin(

sincos)(

z

Since the beam is along z-axis,

i.e. and the peaks

happens for , the initial

thinking suggests that ,

however, more is happening!

o0

0

Sh

D

S

1tan

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What else is

happening?

Going around of C=1 loop additional 2

phase is required, then . 2 Sh

What else? However, since this antenna demonstrate very

small back lobe, it must operate near Hansen-Woodyard condition; therefore, a very realistic

phase progression among the loops after each

turn will be

)2( NSh

Axial phase Phase due to

C=1 turnSmall back lobe

condition

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Let us now assume that along the helical conductor thecurrent travels with phase constant , then

h

)2(12N

SLN

SL hh

total length of one turn

Now, we can find currents phase velocity along the helix.

Phase velocity:h

v

Speed of light:

c

)2/()12(//

NNSL

cvp

h

Velocity factor For p

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Based on the previous

expressions, the

normalized pattern is :

)2/sin(

)2/sin(cos2

sin)1()( 1

N

NF N

NS

2)1(cos

For practical condition of , the following

design parameters have been established

ooC 1512;3

4

4

3

SNc

HPo65

SN

c

HPG

2

2 2.6000,26

N

NAR

12

W 140

cRA

78.1rB

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Infinite Biconical Antena

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Finite Biconical Antenna

Di A

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Discone Antenna

H~0.7

B~0.6

D~0.4

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Planar Equiangular Spiral Antenna for theself-complementary case with = 90o

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Archimedean spiral antenna

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Log-periodic Wire Antenna

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Spiral Antenna for GroundPenetrating Radar (GPR)

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Pictures of Antennas