Boss Audio Systems BV8.5GA User Manual download pdf (Page 74)

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Listening

"SECRET" SW

STATIONS

winter

approaches,

static on

the

2 -3 -MHz

band

declines,

al-

lowing

hear some

well -known

broadcasters.

Low -powered

stations

in Latin America, Africa and the

Pacific inhabit the

120

-meter tropical

band (2300 -2495 kHz); high -powered

outlets

in China and both

Koreas

spread

beyond this range.

Usually closer

but offering

no less

challenge to DX

are

the

secret short-

wave

stations! So

secret are

they that

even

the

people operating them

are in

the dark (or if they

are

aware,

they

hope no

one

will

notice).

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BY MAIL -FROM

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Department 217S

East Delaware

Chicago, Illinois

60611

312-664-0020

CIRCLE

NO. 20

ON READERS

SERVICE

CARD

By Glenn

Hauser

We're

talking

about

harmonics

mediumwave

(standard

broad-

cast)

stations.

combining

a good

antenna and

receiver

system with lots

diligent delving

into the residual

noise level, you too

can intercept

these

secret flea -powered

broadcasts.

Even in the central

USA, the great

majority

of harmonics

heard originate

in Latin America; this

speaks highly

for the harmonic -suppression

stan-

dards of

the FCC, probably

the most

rigid in the world.

harmonic is

not

just

any

off -

frequency reception

of a

station.

must

be an exact integral multiple

the fundamental.

Second

harmonics

(twice

the fundamental)

are by far the

most

common,

since progressively

smaller amounts

of power

are

radiated

harmonic numbers

go up.

The way

harmonics

success-

fully is to

know

the

1.6

-3.2 -MHz

band

-so you can

quickly

eliminate

every-

thing that

isn't a

harmonic. If

you

live

near a nighttime MW

station, you

probably

can't help but hear

its sec-

ond and /or third

harmonic.

Wheth-

er transmitted,

the result

of receiv-

er overload on

the fundamental, local

harmonics are of

no DX

interest.

If you live

near two

more night-

time

stations,

their mixing prod-

ucts

may

be audible.

You can predict

where

most

of them

will

show by

summing each pair of fundamental

frequencies,

and by doubling one,

and

then

subtracting another, in all possi-

ble combinations.

You

can

also

eliminate

any trans-

mission not

consisting

program-

ming,

such as hams, LORAN, radiolo-

cation beeps, time

signals, ship and

shore,

military

nets,

aircraft, RTTY,

etc., etc.

This

leaves the legit

120

-m

broad-

casters.

the evenings,

you'll hear

mostly Venezuela, Brazil

and

Haiti;

and

Mexico

and

Guatemala

fight it out

only on

2390 kHz. Any

other pro-

gramming

just

about has

to be har-

monic!

When

you

suspect a harmonic, di-

vide

the measured frequency

see

if its fundamental

matches

with

known

station.

Then

tune

to the

fundamental

frequency;

chances

are

you won't hear it there,

but if you do,

should be under

entirely different

conditions

interference,

fading

and

strength. Such a check will prove that

you are receiving a transmitted har-

monic,

propagated on the frequency

where you find it.

Don't

give up if

first you

don't

succeed;

the

selection of harmonics

continually changing

different

sta-

tions tweak

their

traps.

report

your

harmonic DX

to other enthusiasts,

but

not to the

station! Once they

learn

their

harmonic

is getting out, they

just

might

moved

eliminate it perma-

nently.

Let's keep

the

secret.

Monographs.

The National

Radio Club

has become

quite a pub-

lisher, not

only for its members

but for

a wider readership.

Over

the past

few

years,

NRC

has published many

tech-

nical

articles about

antennas

suitable

for mediumwave

reception.

Now,

they're

compiled in

the

NRC Antenna

Book,

the first

of many reprint

books

issued. You'll

find everything

from

the portable

ferrite rod to

the

mile

-long

Beverage

wave

antenna, in-

cluding much

ado about

loops:

direct

coupled,

degenerate,

balanced,

un-

balanced

and box.

The

60 -page

book

$2.25;

and

many

other individual

ar-

ticle reprints

are available too. For

list,

send an SASE to NRC, Box 127,

Boonton, NJ

07005.

Courtesy. Back

the

heyday

radio, many MW

stations went

on the

air

in the

wee hours

with

special pro-

grams for DX

listeners.

The

practice

still

continues, thanks

a small band

of enthusiasts in the NRC, Interna-

tional Radio

Club of America,

and

Newark

News Radio

Club

-the clubs

most

involved in MW DX. Each

has its

own

"Courtesy

Programs

Committee"

(CPC),

but

they

coordinate their

ef-

forts to

avoid

duplication. Most

CPC

broadcasts occur on

Monday

morn-

ings during the winter

season,

when

24 -hour

stations take

few hours

off.

But the tests

are scheduled for the

most

open

"window,"

determined

from the

NRC

Log's

schedule

section

and continually revised "condition

frequencies" lists. Each

club pub-

lishes

calendars of

upcoming

spe-

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