Belington progressive. [volume] (Belington, W. Va.) 1912-1927, 1926-12-09
A short-wavs superheterodyns.

| Receivers used for the reception of
| short-wave signals generally make use
OF regenerative detector followed by
one or two stages Of audio frequency
amplification. In the majority of cases
r. (. application is not resorted to,
| stages becomes very difficult for the
| high frequeneies involved In the
Radio Broadcast Magazine. however.
| is described by George j. Eltz. Jr.,
superheterodyne short-wave receiver,
efficient, and easy to handle. This re-
ago and has since been developed to
that stage where 1t may be duplicated
by the average set constructor with
out fear of any snags cropping up. Il
employs two intermediate stages of
r. {. amplification.
The superheterodyne used for the
reception of short waves differs some
what from that used fol the recep-
ton of broadcasting, although. of
course. the general theory is identical.
The superheterolyne method of re-
ception consists of tuning to the in
coming frequency. beating with it an
other frequency. and then simplifying
and detecting the beat note. The ac-
tual signal listened TO has in it none
of the original frequency or the fire.
quency which caused the beat note.
In the reception of broadcast programs
or other signals between 800 and $50
kc. GOO and 500 meters) the beat note
selected is a frequency somewhere
between 80 and so kilocycles. This
relatively hig. frequency is selected
TO prevent the Introduction of distor-
tion by elimination of the side-band
frequencies in the intermediate am
plif1er and filter.
Reception of Short Waves.
In the reception of short waves. par
ticularly the reception of c. W., this
element of distortion may be disre-
garded, and such has been the case in
this receiver, the assumption being
that most of the signals received will
be C. W. The ordinary super used
for broadcast reception has two tun-
ings: first, the loop or antenna circuit
second, the oscillator circuit. This
short-wave "super" has only one tun-
ing arrangement, in which is COM
bined both the tuning operations indi-
Gated above. This method of tuning
was selected because Of its simplicity
and because 1t makes possible the
construction of what is practically
Single-control set.
The intermediate frequency chosen
is % kilocycles which, while too low
frequency for good telephone recep-
ton, when simple tuned circuits are
used, is satisfactory for C. W. or tele
graph signals. The selection OF this
frequency necessitates detuning the
set % kiloeycles from the incoming
signal, but at the frequencies CORE
spending to wave lengths of 100
meters or under, this detuning is of
no Importance in decreasing signal
strength.
The reader will recognize the de-
scription above as applying to the
'autodyne" or "selfheterodyne' type
of super" The beat note of % kc. Is
created in the same manner as in the
broadcast set, but at a lower fire.
quency. For the reception of short
wave telephone signals, the amplifca-
ton and detection of the 22-kc. beat
note Is In the usual manner When
c. w. signals are to be received, an
other beat note must be created
Nv of
tube or by a selfheterodyne beat note
III the second detector tube The lat
ter method has been selected. beat
note of 1000 cycles being chosen as
the most satisfactory. This detuning
of the second detector circuit while
1t may appear to be inefficient because
of the low intermediate frequency, is
not so bad as it seems since the am
plification in the intermediate circuit
is very great and there is plenty of
energy to spare.
Two Variable condensers.
In the accompanying diagram two
variable condensers are shown. but all
the tuning is done with the one in the
grid circuit The condenser in the
plate circuit must be set for each band
of frequencies covered, for instance
from 1905 kc. to 6,663 Ike. (40 to As
meters), 6,663 kc to 1896 kc. (45 to so
meters), etc. This setting Is not crit
ical, the only requirement is that the
tube oscillate strongly. but not so vio-
lently that II blocks. The variable
condenser across the secondary in the
sitter circuit Is not considered major
control.
The choke coil consists OF 100 turns
wound CT d wooden form l inch in
diameter and 2 inches long. honey
comb or similar coil IF $50 or $50
turns will also serve very nicely. The
! intermediate transformers must be
capable Of simplifying the rather low
| frequency of % Kc In this set, those
! manufactured by the General Radio
| company were used. but there are
| probably any Number OF others which