Evening star., 1925-09-20
No. 80: Function of Wave Traps.

In complaining about interference
with the new local station, Smith had
come to hear much about wave traps,
but began to feel as If he were being
trapped himself. He was told a wave
trap was simple to construct and
equally simple to operate
AS It's name implies, a wave trap,
INFORMED him, is designed to trap out
the wave that is not wanted. This
electrical device originally came into
vogue with early radio fans as q
means OF tuning out interference from
spark transmitters but since then the
situation has changed somewhat
Nowadays radio fan often wants a
wave trap to assist him in greater
selectivity where, for instance, there
may be two stations broadcasting on
very nearly the same wave length.
You really do not need wave
trap" l explained. But it would be
well to have an idea of it clearly fixed
in your mind. You must remember
that many of these people who talk
about wave trap as a necessity are
thinking only in terms of their own
very limited receiving sets. When
our local station is on they find the
wave trap Indispensable, whereas
with a selective set like your own you
can easily tune out the local. Perhaps
you cannot get things very near the
locals wave length without interfer-
ence, but you have such a wide range
of wave lengths to select from you
hardly think Of complaining
That's interesting" Smith replied.
But / guess I'll have to buy a wave
trap or make one. You're simply tell
ing me the function of wave trap.
l want to know what it is
Explains Wave Tran.
A wave trap, 1 explained, is an ele-
mentary device connected in series
with the regular tuner of the set.
The electrical energy picked up from
the aerial issled first through the wave
trap device, and Is then fed to the
tuner of the set. Smith recalled for
mer explanations of the meaning of
connecting electrical circuits in series,
and quickly vfsualized the wave trap
as device that was interposed be-
tween the aerial and the first unit of
the receiving set, the tuner.
THE wave trap itself is just about
as simple as its location in the hook
up, / went on. 1T is composed of an
inductance and a condenser in parallel
with each other An inductance is a
length of wire. coiled for convenience
to provide an induction of current to
other coils placed in its field. The
condenser takes care of the capacity
When / say the two units are con-
nected in parallel / mean that each is
connected separately to the aerial,
and each is connected separately to
the wire leading to the tuner.
About the simplest wave trap is
composed of a 23-plate variable con-
denser and coil made from winding
25 turns of No. % D. C. c. double
cotton-covered) wire on cardboard or
bakelite tube about three inches in di-
ameter. About 80 turns of No. IS
bell wire would be about right if you
haven't the other handy. You can
even use a honeycomb or duo-lateral
coil of % turns if you prefer.
You can see that the efficiency of
the wave trap depends somewhat upon
the provisions you make for varying
1t. If you want to tap the coil at vari-
Gus turns. as you would in construct
ing a simple tuner, you can naturally
vary the results Of the device. Where
a device known as a five-slide tuner is
used to get theswave trap effect you
get ultra -selectivity, but since this ar
rangement would mean changing the
regular tuning of your set l should