December 1936 Radio-Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
My first thought when seeing the cover for this edition of Radio-Craft
magazine was that it was an April Fools gag, but it turns out
the 'hat' being worn by the radio receiver's designer is a loop
antenna for AM reception. In a way it is the opposite of a tinfoil
hat in that this headgear invites electromagnetic energy around
the wearer's head rather than shielding it. Back in 1936, being
seen in public donning a contraption like this radio would have
been akin to Google Glass today - you'd be a superhero to fellow
nerds, and just be confirming your otherworldly nerd status
How to Make the World's Smallest 3-Tube Radio Set
This little receiver - a typical experimenter's set - will
attract the interest of radio men everywhere. Tested inside
Radio-Craft offices (the 11th floor of an all-steel building
in lower Manhattan) the local stations were received without
the least difficulty!
Fig. A. The tiny chassis removed from its
cabinet. Note the size.
Once again Radio-Craft presents a "world-beater" in small-space
radio equipment. In September, 1935, Radio-Craft was described
the world's smallest 1-tube set that first introduced midget
tubes to the American radio man; the article aroused international
comment! By a clever combination of midget parts, including
batteries, tubes, condensers, etc., the author has made a practical,
proportionately-small, 3-tube "world-beater" radio receiving
Arthur C. Miller
When using this "Belt-Radio" the wearer is quite unmindful
that the latest news or dance music is coming from an ultra-midget
receiver which is actually being worn on the belt! And it takes
only a minute to put the whole equipment on-and less to take
it off! Technical men will better appreciate the amazing sensitivity
of this tiny set, with its "hat" antenna, on being told that
the writer has had no difficulty in receiving WCAU (Philadelphia)
and WHAM (Rochester), on the 5th floor of a 14-story all-steel
building in mid-Manhattan! The circuit is fundamentally a regenerative
detector followed by 2 stages of audio amplification. A closer
inspection, though, will reveal that the gridleak-and-condenser
combination is arranged in an unusual manner-across the tuning
coil and in series with the variable condenser (C1). This was
found to give far better results with a closed antenna circuit
of the type used than the more conventional method, and also
enabled absolute stability to be obtained with the minimum of
bypassing and shielding. In fact no shielding at all was used.
Finally, there is not the slightest trace of hand-capacity effect
to upset the tuning on the 200 to 550 meter range (broadcast
Fig. B. The complete set, ready for operation.
The "A" battery is a storage cell; the 45 V. "B" batteries are
the new midget size. Note the fuse.
There are one or two important points to remember. The loop
aerial was designed to fit 'round the head because that was
found to be the only position in which it gave satisfactory
results. On the back it was too close to the body and when the
receiver was in its most sensitive state (just before circuit
oscillation) every movement of the body upset tuning and regeneration.
But, unfortunately, there is one disadvantage in having the
aerial placed that way. The 4-ft. cable connecting it with the
receiver acts as a capacity and restricts the tuning range of
the set. What the author did was to choose the most powerful
local (New York) transmitter (which happened to be WOR, Carteret,
N. J.) and wind the number of turns on the loop which enabled
him to tune to that frequency (710 kc.) . Besides WOR he could
receive WLW (700 kc.) in Cincinnati, WEAF (660 kc.) in New York,
and in the other direction WJZ (760 kc.) in Bound Brook, N.
J. Of course tapping the coil will help considerably to broaden
the tuning range.
Fig. C. The positions of the parts can be
seen here. The tube sockets had to be filed down to fit on sub-panel.
The special tuning condenser is seen behind the sockets.
Fig. D. Here is the picture wiring diagram
for those experimenters who prefer this system.
Building the set is quite simple. A thin Bakelite panel is
used (3/16-in. thick, and 3 1/2 x 2 1/2 ins. wide) on which
are mounted the midget tuning condenser and the 15,000-ohm,
wire-wound, regeneration and volume control. The tiny chassis
is also made from Bakelite of the same thickness, on which are
mounted the 3 sockets to hold the tiny tubes (distributed, like
the sockets, in the U.S. by Wholesale Radio Service Co.). The
sockets will have to be cut down to fit the chassis which measures
only 2 3/8 x 3/4-in. wide. It may be well to mention here the
importance of using only the specified parts. They are the smallest
known and any larger parts will, of course, make it impossible
for the reader to keep the set at its present small dimensions.
A good suggestion, too, would be o advise the use of a small
soldering iron! (The kind the author recommends is not larger
than an electric stencil pencil.)
The loop aerial is wound on a cardboard disc 13 ins. in dia.
Litz wire is used and 22 turns are interlaced around the 9 ribs.
(Note that an uneven number must be used.) A tap is made at
the 17th turn from the start, This is the" A-" lead to the set.
A 3-way mike plug and connector separates the loop from the
receiver so as to facilitate putting the equipment on and taking
it off. The completed set and the two 45-V. batteries are mounted
with black elastic bands (3/4-in. wide) onto an ordinary leather
belt. Tap one of the 45-V. units at 22 1/2 V. Due to the extremely
low "B" drain, about 2 1/2 ma., the two 45-V. batteries should
give at least 100 hours of service. This is calculated on the
daily use of the set for about 3 continuous hours.
A liquid unspillable storage cell supplies the 2 V. for the
filaments. This tiny "battery" (as most people prefer to call
it) should last from 7 to 10 hours before needing to be recharged.
It is sold with an oiled silk bag and fits in the hip pocket.
Note: the grid prong of the tiny tubes is larger than the
remaining prongs. As L, in Fig. 2, you may use either the "hat"
loop, or a small, experimental coil when using a ground and
outside antenna (for DX). The writer imported several of the
ultra-small condensers detailed, for constructors who wish to
make them, in Fig. 3.
Fig. E. The author "harnessed up" - and enjoying
a musical selection.
List of Parts
Fig. 1. The schematic circuit including values
of all parts. Note the grid leak and condenser.
Fig. 2. Alternative wiring of the grid leak.
Fig. 3. The special tuning condenser is made
according to these specifications. Two separators are used between
The "secret," if you want to call it that, of success with
this tiny set is in the high degree of A.F. and R.F. amplification
Fig. 4. The "hat" details.
Posted January 12, 2016