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Antenna

Bridge

GET THE

MOST

OUT

OF YOUR

SWL

ANTENNA

SYSTEM

THE

SWL

who

wants to put

up a home-

made

resonant

antenna

has

two strikes

against

him

to

start

with. More

than

likely,

he

won't have

the fancy

test

equipment

that is

needed

to

do

a respect-

able

job.

As

a result,

the

antenna

goes

up,

and

by

cutting

and pruning,

it might

just

accidentally

be tuned

to

the proper

frequencies.

However

9 out

of 10

SWL

antennas

are badly

mistuned

and

are

nothing

more

than

so much

wire

strung

up in

the

air.

In

a fraction

of the

time you've

spent

digging

out

some of

the

weaker

stations

you

could have

tuned

that antenna

and

possibly gained

anything from

3 to 10

dB

signal

strength

on that

S- meter.

All

you

need

is

a grid

-dip

oscillator

(

which

you can

maybe

borrow

from

a friendly

ham)

or

a r.f.

signal generator

covering

J2(

JI

RI

1.5K

A

R2

47.11

R4

esn

B

MI

R3

DI*

J3

CI

.00IyF

*C3

*SEE TEXT

7.7

With

system completely

set up, reactive

imbalances

between

A

and B cause

up -scale

meter

deflection.

BY

JIM

ASHE,

W2DXH

the

frequencies

you

want

the antenna

to

tune.

Use

this

signal

source in

conjunc-

tion

with

a

simple little

Wheatstone

bridge

(described

below)

and you

are

in

the

semi -professional

antenna

test-

ing

business.

How

It

Works.

Resistor

RI

and capacitor

Cl (

see schematic

diagram)

isolate

the

actual

bridge

circuit

from

meter M1

and

prevent

stray

r.f.

from

getting

into

the

bridge.

In

the

bridge

itself,

C2,

C3, R2,

and R3

function

as

a

voltage

divider

that

splits

in

half

the incoming

signal

from

J2. The

capacitor

values

( typically

0.01

FLF

below

30

MHz

and

0.001 µF

above

30

MHz)

should

present

low

reactance

at

the operating

frequency.

The

two

voltage

dividers

in

the

bridge

must

balance

if

a null

is

to be produced

and

prevent

deflection

of Ml's

pointer.

It

is

evident,

therefore,

that

the load

resistance

at J3

must

be exactly

the

same

as

the resistance

of R4

in

the second

voltage

divider

to preserve

the null

con-

dition.

A

68 -ohm

value

was

selected

for

R4,

but

you

could as

easily

substitute

one

of

the more

common

52- or

75 -ohm val-

ues if your

antenna is

designed

for

either

of these impedances.

Diode D1

rectifies

r.f.

only when

a dif-

ference

of potential

or

a difference

in

signal phase

exists

between points

A

and

B in

the

schematic. This

rectified

volt-

age

is

then fed

to the

meter

through J1.

66

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