Evening star., 1924-08-03
WH|STL|NG TYPES
Of RADIO TROUBLE.
Two Forms of Interruption to
All Broadcast Receivers
Explained.
BY JOHN v. l.. HOGAN.

Generally speaking. there are two
types of whistling interference heard
in broadcast receivers. One kind is
caused when two broadcasting sta
tions send simultaneously at wave
frequencies (wave lengths) that are
too close together Their waves re-
act on each other and produce a more
or less uniform whistling note, often
of very high pitch, in all the radio
receivers within range. This sort of
interference is not very bothersome,
except occasionally when listening
to S33 kilocycle (36O meter) waves.
1t is daily growing of less import
ance, and for the moment we need
not consider 1t further, although it
will be discussed in future article
in this series.
The second kind of whistling inter-
ference is caused by radiating receiv-
crs, or. in other words. by receiving &
stations that are so designed and SO
operated that they act as small radio
iransmitters. This kind of inter-
terence is now exceedingly trouu.e-
some and breaks up great deal of
broadcast reception. 1t is a partic-
ularly severe nuisance in localities
where many radio. receivers are set
up close together. as in the cities;
hut even in the country this squeal-
ing and whistling interference often
completely prevents satisfactory re
ceiving.
"Chirp" Frequently Heard.
1t is safe to say that nearly every
broadcaSting listener has often heard
the chfrp or whistle of rapidly vary
ing pitch that is the mark Of this
Kind OF interference. Many listeners,
hny Il
the troublesome whistles and many
do not know that their own receivers
may be adding nightly to the general
din or squawks and squeals
The reason why these chirps and
whistles occur is equally a mystery
to many radio listeners, although the
scientific basis of the action is not at
all complicated. 1t is merely another
manifestation of the common phe-
nomenon of beats that is frequently
noted in acoustics. You may have
observed that when two musical tones
OF neighboring pitch are sounded
Simuita neously, the combined tone
nutters in intensity. This happens
because the two sound waves interact
or beat together, and the rapidly
of the flutter is always equal to the
difference in frequency Of the two
sounds. Thus. If two organ pipes of
spcctively, are blown at the same
time, the sound heard will grow
strong and weak Tor fiutter in strength
four times per second.
In the same way, if two radio
waves or two radio frequency cur
rents Of somewhat different fire.
quencies are allowed to interact upon
each other they will produce beats.
Thus a carrier wave from station
WEAF. at the frequency of $4,000
cycles per second, might interact with
the carrier wave from another trans
yfsitter at GIL. 900 cycles per second to
produce 10000 beats per second When
picked up and rectified such beating
waves would produce, in the listen
ing telephones or loud speaker note
of 1000 per second pitch, correspond
ing approximately to the second c
above middle G on the musical scale.
Any change in frequency of either
beating wave would produce a change
In the pitch Of the beat note. since
this must always equal the difference
in the two wave frequencies.
Carrier Wave Conflict.
In the same way, carrier wave
from any station will beat with waves
or currents produced by any self-os
cillating receiving set. The fire.
quency of the oscillations in the fire.
ceiver and of the waves that those
oscillations will send out if they are
allowed to get into the receiving
antenna. depends upon the tuning ad-
u
the tuning knobs are turned the fire.
auency changes. Consequently the
pitch of the beat-note produced also
changes and this is what gives rise
to the bird-like' chirps and whistles
that are so often heard.
If YOU have q radio receiver of any
of the types that can be made to cause
oscillations in the antenna circuit,
your set is one that may interfere
with YOUR radio neighbor's reception.
Nh i,,inu ha ha
to generate antenna circuit oscilla-
tions. and thus to interfere with re-
ceiving throughout the neighborhood,
are probably made and used in larger
numbers than the non-oscillating and
hence nom interfering sets.
Nearly. II not absolutely, all of the
interference producing receivers are
of the simple regenerative type,
though more complicated outfits such
as the super-heterodyne when used
with an antenna, will cause this trou
BILE They may be of single circuit,
double circuit, triple circuit, or OF
any other design, or they may be or
unneutralized radio-frequency ampli-
fer or reflex layout: many of these
will generate oscillations in the an
,enna circuit and produce interfer-
cnce if not specifically designed other
wise, particularly when they are not
correctly handled by the user.
Two Ways to Remedy Trouble.
Tnere are only TWO ways W sup
this whistling interference produced
by oscillating receivers. The first and
simplest way is simply to tune and
manipulate your own receivers prop
crly, and to teach your radio friends
to do the same with theirs. A little
work among your nearby radio listen
erS will produce wonderful results in
this way, for no one wants to trouble
his friends. and ordinarily neigh
borhood can be cleaned up of the
strongest receiver whistles
quite easily. The second and more
difficult method is to arrange your
receiver SO that it cannot produce os-
citations or SO that when it does os-
cillate the currents will not reach
the antenna. By preventing the gen.
Grafton of oscillations in your an
tenna you prevent the radiation of
interfering waves.
Many Of you are perhaps now won-
dering how you can tell whether or
not your own receiver is ever a
source of nefghborhood interference
There is one simple rule that answers
that question: If, when turning your
wave,iength or tuning-control knob
you hear a whistling note in your
tel><hone or loud-speaker, and if,
also. you can change the pitch of that
note by turning the tuning Knob. Won
are making interference for ali the
listeners who live near you unless
your receiver is one Of the latest de-
signs that keep the oscillations out
of the antenna circuit whistles
whose pitch you cannot control do
not come from your set, and You need
not blame yourself for causing them.
On the other hand, IL you have no
blocking tube in your set, whenever
you hear whistle and find that you
can vary its pitch by moving your
tuning control. you may be sure that
all your nefghbors who are listening