Radio Shack SSM-1750 User Manual download pdf (Page 38)

Communications

Project

An

Experimenter's

Aviation

-Band

Receiver

Low

-cost

project

lets you tune

in on airport

and

airborne

radio

activity

By Dan Becker

f you live within 20 miles of a

busy airport, you

have an op-

portunity to tune in on some ex-

citing

radio activity.

With

the

Exper-

imenter's Aviation Band Receiver to

be described,

you can

listen

to air-

craft

and tower communications

in

the 118 -to-

135.95 -MHz band as-

signed to aviation

activities, includ-

ing

the

121.5 -MHz

world

-wide emer-

gency channel.

Channels

are divided

into incre-

ments of 50 kHz each,

with

each

as-

signed to a

different type of service.

Some channels

are used by air

traffic

controllers

who

continuously

give

course and

altitude instructions

to

pilots

aloft. Other channels

are used

while an airplane

is

on the ground.

These service channels carry

main-

tenance, fuel status,

weather (includ-

ing wind speed and direction and

temperature

aloft) and personal

re-

quests by pilots

enroute. Much of

what

you can

hear is routine, but

in

times of emergency, you

can

get

first -hand

information before the

news media report

it.

About

the Circuit

Shown

in Fig. 1 is the complete

sche-

matic diagram of the

Experimenter's

Aviation

Band

Receiver. It is built

around

a single transistor,

identified

as Q1,

which operates as

both and

r -f

amplifier and a

detector.

The output

of this transistor

is coupled

to an ex-

ternal audio amplifier

via

a coaxial

(shielded) audio cable.

Almost

any

epitaxial

transistor rated for

vhf

ser-

vice will work in

this circuit.

Because aviation

communication

is via amplitude -modulated (AM)

signals, the

value

of the dc collector

current

of QI

can

be made to

vary in

direct proportion to these signals.

This requires an input signal ampli-

tude of

greater

than

10 millivolts. In

addition, any

variation in the

dc

col-

lector current must be extracted

and

amplified.

Two

functions

are performed

by

the Aviation

Band Receiver. One

is

its operation as

a Hartley oscillator

in which the

approximately

120

-

MHz operating

signal frequency is

governed

by

Ti ,

C2 and C3.

This

greatly

amplifies the microvolt -level

signals that appear at the antenna

and feeds them back to the

base of

QI with an amplitude greater

than

the 10 -mV minimum level needed

by

the circuit for proper operation.

In

the second

function,

any

varia-

tion in the dc collector current will

appear

as a variation in the

emitter

-

collector

voltage, measured

at the

collector side of

R4.

It

is

here that the

audio information is extracted and

sent off to the audio amplifier.

Capacitors Cl, C4, C6 and C7 by-

I

44 / MODERN

ELECTRONICS

/

September

1986

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You

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