The Ely miner., 1926-05-07

signal must be impeded so as to de-
crease its strength. The most COM
mon piece of apparatus for use in this
connection is the so-called wave trap
which is actually filter circuit tuned
to absorb the interfering signals.
These wave traps are very easily con-
structed, and cost little. They consist
merely of an ordinary coil and con-
denser. The traps are connected In
the antenna circuit and are adjusted
so that, at the frequency of the inter-
fering signal, they have a very high
impedance, the exact value depending
on whether the unit is connected in
This high impedance prevents the sig
nal from passing down through the
primary of the antenna coupling and
so into the receiver. At the same
time, these wave traps offer very low
impedance to all other frequencies.
Methods of Operation.
''There are several methods by
which these traps might be operated.
Fig. 1 shows the most common method.
In the drawing L, G, constitutes the
trap circuit and L. c. the antenna
coupler and tuning condenser of the
receiving set 1t will be seen that the
trap is connected between the antenna
post of the receiver and the antenna
lead-in. Fig. 2 represents a slightly
n -
trap in the circuit. In this latter
method, the wave trap is inductively
coupled to the antenna. This induc-
tive coupling is obtained by winding
few turns of wire about one end of the
coil L.. One end of this new winding
connects to the antenna and the other
to the antenna post of the receiver.
This circuit is practically equivalent
to that of Fig 1 with the difference
that somewhat sharper tuning is OBJ
tanned.
whether the trap circuit of Fig. 1
or that Of Fig. 2 is used, depends to
considerable extent upon the charac
teristics of the antenna system. A
well insulated antenna with a short
lead-in and short ground wire to
good ground, such as the cold-water
mains, gives best results with the in
ductively coupled circuit. Every effort
should be made to have as good a
ground and antenna as is possible, but
if it is necessary to use long ground
lead to rather poor ground system,
the direct coupled wave trap will prob
ably give better results.
"When using the latter method of
direct coupling, as illustrated in Fig.
1, some small change in the tuning Of
the antenna condenser will be noticed,
whereas with inductive coupling prac-
tically no change in tuning takes place.
This represents a slight advantage in
favor of inductive coupling since it
will be possible to continue to use the
same old dial readings after the trap
has been installed.
IF the trap is poorly made" says
Mr.. Rhodes in conclusion, "it may not
only eliminate the interfering signals
but will also decrease the strength of
those signals we desire to receive. The
most common cause OF poor results
with wave traps is due to the use Of
high resistance coils and lowagrade
condensers. 1t is therefore essential
that the trap operate efficiently, and
in order to obtain such operation, it
is generally best to use fairly small
coil and large condenser. With a
large coil the elimination is usually
more complete but the neutralized
band is larger so that the trap inter-
feres with reception on wave lengths
adjacent to that wave on which the
interfering station is operating. Most
satisfactory results are obtained when
low loss coils are used, such as space
wound solenoids, while toroid may
be used in the inductively coupled cir
cult"