Matching Techniquesand Baluns

INEL 5305Dr. Sandra Cruz-Pol

ECE, UPRM

Ref. Balanis sect. 9.8, 14.8

Matching Techniques

Matching over the desired frequency range

when connecting the T.L. to the antenna

Types Stub-matching /4 transformer: single or multiple

Zo (real)

ZL (complex)

Stub-matching

Zo

s

l

/4 transformer-matching: Single Section First move so that ZL is

completely real

Zo

so

4

oinx ZZZ

Zx

/4 transformer-matching: Multiple- Section- provides broader BW First move so that ZL is completely real

Zo

so

4

444

Could be designed using Dolph-Tchevy or Taylor coefficients.

Multiple sections: Quarter-wave Xmer N-sections

Can be implemented several ways Binomial Tschebyscheff

Nn

nn

nnn

N

n

njn

NjN

jjoin

CZZ

ZZ

where

e

eeef

1

1

0

2

242

21 ...)(

o

o

f

flk

24

2

Multiple sections: Quarter-wave Xmer For symmetrical Xmer,

Binomial Coefficients

n=0,1,2,…N

...)2cos(cos(2)( 12 NNef o

Njin

N

n

njNn

oL

oLN eCZR

ZR

0

22

Nn

oL

oLNn C

ZR

ZR

2

!)!(

!

nnN

NC N

n

Binomial Design for Multiple sections Quarter-wave Xmer Fractional Bandwidth:

where m= maximum value of reflection coefficient which can be tolerated within the bandwidth.

f

for

Z

m

n

N

oL

oLm

o

mo

o ZR

ZR

f

ff

f

f/1

1cos4

22

design

for

NRZ

fm

Lo

,,

Use Radians!!!

Implementing with Microstrip For Microstrip lines:

tw

hZ

r

c 8.0

98.5ln

47.1

87

Tschebyscheff Design for Multiple sections Quarter-wave Xmer

mN

mN

oL

oLNj

oNj

in

T

T

ZZ

ZZe

NNef

sec

cossec

...)2cos(cos(2)(

2

12

mNoL

oLm TZZ

ZZ

sec

1

Comparison of frequency response for matching quarter-wave network

in

of

f

0.5 1.0 1.5

Binomial

(Maximally flat)

Tschebyscheff

Matching-techniques There are other techniques for matching a

line to the antenna; T-Match Gamma Match Baluns

BalunsBALance to UNbalance Converters a passive electronic device that converts

between balanced and unbalanced electrical signals, such as between coaxial cable and dipole antenna. Present in some radars, transmitters, satellites, in

every telephone network, and probably in most wireless network modem/routers used in homes.

A 75- to 300 ohm balun:

Baluns- current distribution

Coaxial Line(unbalanced)

Currentleak|| wire

transmission line(balanced)

Balanced load

Currentdistribution

Coaxial Line(unbalanced)

Balun

Some antennas like the monopole or horn are unbalance loads, and it’s ok to use a coaxial cable to feed them.

Balanced source The source is balanced if the voltage

potential of one level is equally above ground as the other is below ground at all instants of time. Example: +50V and -50V is balanced, +30V

and +12V or +20V and 0V are NOT balanced.

Some types of baluns

Shorted to coaxial outer conductor

/4 above looks like an open circuit, therefore

the leak current is zero.

Bazooka

Shorted to coaxial outer conductor

/4 above looks like an open circuit, therefore

the leak current is zero.

/4 coax/2 coax

Ferrite Core

(toroidal)

a) Transforms Z, 4:1 or 1:4

b) Increased bandwidth but may not behave well at very hi frequencies. There are losses in iron core. Can be made 1:1 or impedance transforming such as 4:1

Feed Networks for Arrays Series feed Parallel (Corporate) Space feed

Usually one source feeds all elements in the array. Have to be careful not to affect the amplitude and phase of the feed to each element. Also the mutual impedance of neighboring elements.

Corporate Feed

b) quarter-wave Xmers

a) Tapered lines