Worcester Democrat and the ledger-enterprise. (Pocomoke City, Md.) 1921-1953, 1926-10-30

Many radio fans complain Of their
sets lacking selectivity. The sets have
not the power to separate stations
sufficiently and their operators are at
loss to know what to do. A wave
trap Is the only solution if one does
not want to pull his set apart again,
so here Is some data that will be of
interest to fans troubled In that re-
spect:
Wave traps and blocking devices are
becoming more and more essential to
operators of none too selective re-
ceivers. especially In congested dis
tricts where not only does one have
to contend with 25 or more stations
separated by the smallest possible
margin, but with annoying proximity
to many which are broadcasting on
high power.
The better-deslgned receivers are
not affected but great many Of the
sets built in the days before so many
stations were on the air, as well as
many Of the less selective creations Of
more recent months. find 1t expedient
10 use auxiliary apparatus in the form
Of wave traps
Here Is commercial trap and 9th.
ers just as effective that can be con-
structed very cheaply by the listener
who finds need for one in reducing to
a minimum the interference Of sta.
tons operating on waves too nearly
the same for the unselective receiver
to separate. 1t Is sometimes necessary
to use more than one trap to bring to
an unObjectionable point the interior
Ing stations. This Is especially true
where high power is the source of an
nOyance.
Series and shunt Traps.
The simplest form Of traps which
are effective are series and shunt traps
as well as few variations Of Absorb
ton and rejection traps.
The circuit is that given In Fig. 1
and consists Of two 13-plate variable
condensers. one of which Is connected
In series with on inductance Of 65
turns Of No. 20 single cotton-covered
copper wire, wound on a 2% Inch
cardboard tube. This Is the left-hand
dial In the commercial unit and tunes
the reflector circuit. The other con-
denser, adjusted by the right-hand
dial, Is shunted across a coil Of SINH.
lar dimensions, 65 turns. over which Is
wound primary Of Fl turns Of No. "a
gauge wire, and forms the absorption
side Of the circuit.
The theory Of operation Of this cir
cult Is very simple. The undesirable
station Is tuned to maximum intensity
on the receiver. The reflector or left
dial Is then turned until the signal is
reduced to the lowest audibility pos.
sible. The absorption trap Is then ad-
justed until all perceptible reception
Of the undesirable station Is absolute
ly eliminated or reduced to negligible
ity. The receiver proper can then be
tuned to any other station that Is
wanted without further interference
from the heretofore offending station.
Where more than one station causes
trouble It would be well for the opera
tor to log the interfering stations and
the proper dial settings for their elm
ination so as to facilitate the proper
tuning Of the receiver. This trap, as
we'll as the other traps to be described.
necessitates NO change Of wiring in
the sets to which they are attached,
further than connecting the antenna
and ground to the proper binding posts
on the trap and connecting the output
of the trap or traps to the ground and
onrial roSto of Tho Naaolvor
Connected In Series.
Fig. 2 shows a simple trap that may
be connected In series with the an
tenna or ground as shown. 1t can be
made with 90 turns Of No. "a DEG
wire on a 2h-inch cardboard tubing
tapped at the 9Th, 10th. 18th and 30th
turns and shunted by .005 MID. vari-
able condenser.
Fig. 3 Is a diagram Of q trap In
shunt that can be made by winding a
coll Of the same dimensions and val.
ues as that in Fig. 2, leaving out the
tapping arrangement and shuntlng It
with q variable condenser Of .O005
MID. capacity. In addition fixed
condenser Of .O001 MID capacity Is In.
serted in the connection that goes to
the antenna binding post Of the set
as shown In the diagram.
Fig. gives another arrangement Of
a shunt trap that consists of an Undue.
lance coil Of No. "a DEC. wire wound
on tube PA inches in diameter until
there is coil of wire Az inches long
In series with this variable con.