New-York tribune., 1922-05-21
Double Circuit
Set Reduces
Interference
Can Be Constructed Either
With Loose Coupler Or
Vario-Couplcr; Windings
Make the Two Circuits
Vario.Coupler Is the Best
Its Secondary Rotor Gives
Loose Coupling Without
Any Imductive Losses

Any Inductive Losses
How can l eliminate interference
from other stations? That - the
question which bothers most Of the
radio fans, and the answer to it is
bound up in the construction Of the
receiving set itself. A large number
of sets now on sale, and in use, enl-
ploy what is known as a single cir
cult receiver. They have been SO de-
signed because of the desire to give
to the novice an instrument that will
be as simple as possible to operate
and 70 adjust. For the purpose Of se
lectivity, however, such a set is not
nearly as efficient as a double circuit
receiver.
In this article, propose to outline
the functioning of the double circuit
set and in doing SO it will be necessary
to describe the vario-coupier and the
in which both Of these two instruments
operate. It will be helpful to those
who axe making or assembling their
own equipment, as we'll as to those
who have installed sets that are manu-
factured complete. In the first place,
with double circuit receiver, We have
two complete and independent cir
cults, as the name naturally implies.
The filso is the aerial or antenna cir-
cult, which consists Of the aerial and
an aerial tuning inductance and
ground connection. In some cases, the
addition Of condenser in series with
the aerial and the tuning irductance is
included. This description Of the
aerial circuit sounds imposing, but
in reality it is very simple and mere
! ly consists of the aerial wire and then
the primary winding of the vario-
coupler, or the loose coupler-which-
ever is used.
Circuits for High Frequency
The next circuit is called the secon-
Diary circuit and consists of an in
ductance and condenser. Now these
two circuits taken together form what
is technically known as oscillating cir
cults, or in palmer language the cir
cults which handle the high frequency
currents picked up out OF the ether.
The object of using two circuits is
as follows: By carefully adjusting
the aerial circuit we set it into reson-
ance with the station We desire to
listen to, just in the same manner as
two tuning forks pitched to the same
i tone are in resonance with each other.
In the natural course of events, how
ever, every electro magnetic disturb
ance in the ether passes over the aerial
to the ground. If the aerial circuit IS
not in resonance with any particular
wave, it will not oscillate to it-that
is, theoretically it should not do so.
In practice, however, We find that it
does, and this is the cause of inter
ference which of course will be re-
corded into telephone receivers where
only single circuit is being used.
Now, with the addition OF secondary
I circuit, which is also carefully ad-
justed we eliminate still further the
possibility Of Interference by increas-
ing the number Of circuits that have to
! be in oscillating resonance with each
other.
There are two main methods OF IN
staling a double circuit receiver-ons
! is by the use Of loose coupler, which
! is now more or less discarded. and
the other is By means Of the vario-
coupler.
The loose coupler consists of pri-
4 mary inductance, which Is nothing
more or less than a wire wound
around an insulated tube in such a
way that we have a certain number
Of turns on the tube. The more turns
of wire there are upon the tube, the
longer is the wave length that it will
respond to.
The secondary OF a loose coupler
consists Of winding on another tube,
which is so arranged that it will slide
in and out Of the primary tube. In
TACT, it will come completely out OF
the tube. and car be pulled away quite
a few inches clear Of the primary tube.
In this manner we have three possible
adjustments first, the number Of
turns on the primary that we can
bring into play In the primary circuit;
are bringing into the secondary cir
cult, and finally the inductive relation
ship between the two coils themselves
which is technically known as "close'
or "loose" coupling. The object Of this
loose coupling is to eliminate the ef-
feet of interference by loosening the
coupling between the two coils. It has
been found in practice that when the
primary and secondary circuits are ir
resonance with each other the two
coils can be separated quite a distance
and signals still be recorded In the
telephone receivers, whereas signal.
that are not in resonance with the cir.
cults will be eliminated by widening
the coupling between the two coils.
. vario-Couplcr More Efficient
Experience. however, showed tina,
there was considerable inductiye
loss when loose coupling was used ane
this led to the development OF ths
vario-coupler, which il a much NOR,
, efficient instrument. II has this draw.
back, however, tina. the secondxr
, winding is not variable, but this /,
, overcome By the addition Of variabl.
condenser placed across the ends Q.
. the secondary coil.
Now the varioAcoupler consists o. ,
primary exactly the same as the pri
mary of the loose coupler, although i
is invariably wound on a tube o
larger diameter. The secondary 4
wound on ball, or rotor, which is q
arranged inside Of the primary tub
that it can turn around in a complet
circle. When the windings Of the sec
ondary of the vario-coupler arc exac.
: ~ parable, with the winding on tn