April 1945 QST
articles are scanned and OCRed from old editions of the ARRL's QST magazine. Here is a list
of the QST articles I have already posted. All copyrights are hereby acknowledged.
first recorded successful
was made from a hot air balloon in 1797 by Frenchman
. His performance elicited "a scream of
terror" from the crowd below and even caused women to faint at the
sight of his basket swinging wildly beneath the canopy. Many brave
souls met with less success before and after that jump. By World
War II the state of the art for parachutes had managed to cure
the wild swing problem, but reliability still left a lot to be desired,
especially when the 'chutes were packed by paratroopers with only
a few weeks of experience. Control during the descent was rather
limited due to the original round configuration with directional
commands effected by pulling on the shroud lines on one side or
the other. Getting to the ground safely with a field pack and a
rifle strapped to you involved a lot of luck. It required even more
luck to land with tube-powered radio gear of any sort. Bulky and
heavy was the norm, particularly if any amount of battle ruggedness
was built in. Massive high voltage transformers and lead-acid batteries
pushed the limit of the definition of 'portable.' The 'Para-Talkie'
was designed to address those problems. Three vacuum tubes, a single
transformer, and associated circuitry fit in a 3x4x5-inch metal
case that looked like a Bud project box, with an A-size battery
cell and a couple B-size cells strapped to the bottom. A 1/4-wave
braided wire antenna ran down to the soldier's boot allowing him
to communicate during the descent and while on the move on the ground.
It was quite an accomplishment for the time.
A Parachutist's Transceiver By
L.T. Arthur H. Copland, CAP
Communications Officer. Parachute Group 639, 18419 Santa Rosa Dr.,
Detroit, 21, Mich.
The para-talkie is given a test in the field by Lt. Ralph Berkhausen,
CAP, using the special parachutist's antenna attached to his
leg. (for some reason this guy reminds
me of Nicholas Cage)
The" para-talkie" with flexible braid antenna attached. The
change-over switch is in front, the power switch on the bottom,
and the headphone and mike connectors on top, The extra stand-off
insulator is provided for supporting a whip-type antenna for
A 'chutist is aided in his descent by instructions received
via the para-talkie, which he carries strapped to his body.
When parachute instructors of a Michigan CAP wing wanted some
means of communicating with student jumpers in the air, the
radio crew set out to solve the problem. The result was the
development of the tiny transceiver unit described in this article.
The acorn-type detector-oscillator tube and the two miniature
audio tubes operate from a pair of midget-sized 33-volt "B"
batteries and a single flashlight cell. A rig of this type also
will find many applications in WERS and ham work on terra firma.
Communications problems in the v.h.f. band shared by CAP and
WERS seem, for the most part, to parallel those experienced in OCD-WERS
operation. One CAP Group - Parachute Group 9 of Michigan Wing -
by virtue of the unique nature of its curriculum, however, confronted
its communications personnel with an unusual problem to solve.
Parachute instructors clamored for some means of communicating
with student jumpers to provide a greater measure of safety and,
at the same time, to give the student the advantage of receiving
direct instructions in manipulating his chute to control his descent.
Limited success along these lines was effected by the use of directional
loudspeakers, but the disadvantages were many. Two-way radiotelephone
contact seemed to be the only answer to the need, but to our knowledge
this had never been successfully accomplished between 'chutist and
ground. No enlightening technical information on v.h.f. equipment
for parachute being found in the books, the group communications
staff set out to do a little pioneering.
of the walkie-talkie type would not serve for obvious reasons. Any
set carried by a parachute jumper must be small and light, and yet
it must be rugged enough to withstand the shocks of landing. A whip
antenna could not be used because it would be a hazard; therefore
the antenna would have to be of flexible material and radiate efficiently
while being carried close to the body. Power and transceiver controls
would have to be placed so that they could not be shifted accidentally.
above problems in mind, a basic experimental circuit was set down
on paper and technicians of our squadron communications sections
were directed to work out their own concepts of the ideal parachutist's
transceiver. That's when the fun began! Dreams like this, we found,
were not easy to make come true. Many of the boys were doomed to
disappointment when their pet creations either were rejected by
the parachute instructors as a hazard to the jumper or could not
be persuaded to develop any acceptable degree of signal strength.
Eddie Pietrasik, deputy communications officer of Hamtramck Squadron
639-3, was the ingenious lad who developed the neat, sturdy transceiver
which is the subject of this article.
After exacting ground
tests, an aerial test was made during a routine CAP maneuver. Equipped
with the new transceiver, Lt. Ralph Berkhausen bailed out at 3500
feet and established continuous two-way contact with Lt. James Allen
on the ground from the instant his 'chute filled until he hit the
field. Lt. Allen, the group's master parachute instructor, who was
using an Abbot DK-2, was exultant. Communication had been perfect.
Here at last was the perfect instruction medium which he and his
staff had so long hoped for. To his knowledge, and to ours, this
was the first time two-way radiotelephone had been used successfully
by parachutists. It was he who christened the unit the "para-talkie."
The para-talkie is fastened under the jumper's right
arm, near the belt, by means of webb straps threaded through slots
in the rear plate of the case. The straps buckle over the left shoulder
and under the right groin. Microphone and headphone cords are attached
at the rear top of the case. They run under the 'chute harness to
the jumper's coverall opening, then inside the coveralls to emerge
from the collar near the 'chutists ear. A tiny lapel crystal mike,
clamped on the inside of the helmet chin strap, is used as a throat
mike. Lightweight headphones with headband removed are attached
inside the helmet.
The antenna is a piece of flat
metal shield braid, approximately a quarter-wavelength long. A lug
is securely soldered at one end and this is attached to the lead-in
near the bottom front of the case, the braid running down to the
toe of the 'chutist's right boot. At the boot there is a special
harness and an adjustable expansion spring which keeps the braid
taut and also compensates for long or short legs. This is insulated
from the braid by a short polystyrene bar.
is pre-tuned to 115.75 Mc. Before the jumper leaves the airplane
he turns on the set by snapping a toggle switch on the bottom of
the case. The controls normally are in the" receive" position. He
can then hear transmissions from the ground transmitter. To talk
to his instructor he presses the "push-to-talk" lever at the right
side of the case. The "receive" position is automatically returned
when he releases the lever.
Fig. 1 - Circuit diagram of the para-talkie.
|C1 - Isolantite-insulated
midget tuning condenser, 3 plates, with slotted shaft.
C2 - 50-μμfd. mica.
C4 - 0.005-μfd. mica.
C11 - 0.1μfd. paper.
C6 - 500-μμfd.
C7 - 0.002.μfd. paper.
C9 - 0.1-μfd. paper.
R1, R12 - 5 megohms, 1/2 watt.
R4 - 25,000 ohms, 1/2 watt.
- 0.5 megohm, 1/2 watt.
R5 - 10 megohms,
R6 - 1 megohm, 1/2 watt.
- 3 megohms, 1/2 watt.
R8 - 2 megohms, 1/2
R9 - 450 ohms, 1/2 watt.
1,000 ohms, 1/2 watt,
R11 - 25,000 ohms,
|L1 - 1 turn No. 14 bare
copper wire, 1/2-inch inside diameter.
- 4 turns No. 14 bare copper wire, 1/2-inch inside diameter,
spaced to fill 1/2-inch winding length.
- Modulation choke, 30 henrys, 5 ma. (Thordarson
74C30 can be used if space is available.)
RFC2- V.h.f. choke, 55 turns No. 30 d.s.c., wound
on 3/16-inch polystyrene rod (or Ohmite Z-1).
- Pole of 4-pole, double-throw rotary wafer switch with
spring return and lever.
S2 - Pole of d.p.s.t,
Headphones used are Brush Type BJ.
Any good quality light weight type may be substituted. The
microphone is an Astatic L-I. The "B" batteries are Burgess
XX22E and the "A" battery a No. 2 flashlight cell.
The circuit diagram of the transceiver
is shown in Fig. 1. The oscillator-detector circuit is built around
a 958 acorn tube which operates as an ultraudion when transmitting.
develops bias voltage for the grid. Condenser
, connected across the filament terminals of the 958,
was necessary to eliminate a tendency toward spurious audio-frequency
The receiving circuit is the usual self-quenched
superregenerative-detector type, with a positive exciting voltage
applied to the grid of the 958 through the resistor R1
is in the quench frequency-determining circuit and
the capacity necessary, dependent to some degree upon inductance
of the primary of T1
, might vary from the value indicated
with a different transformer. Two stages of audio are used, with
transformer coupling between the detector and the first stage and
resistance coupling between the first and second stages. Full gain
is not required for receiving. The grid of the first audio amplifier
is fed from a voltage divider across the secondary of T1
tapped down as indicated, so that the voltage amplification realized
from the 1S5 does not over-drive the grid of the IS4. This arrangement
also helps to guard against r.f. on the grid of the 1S5. The 1000-ohm
, shunting the 'phones is another attenuating
device to hold the volume down to a level where the headset can
be worn with comfort. In early stages of the development of this
rig, a switching arrangement was used which cut out the 1S5 and
directly to the grid of the 1S4 through C6
The revised circuit arrangement, although employing considerable
attenuation, is much more satisfactory.
Modulation is accomplished
by employing a choke in the common high-voltage line to the modulator
tube and the 958 plate. This choke has a d.c. resistance of 1000
ohms and is by-passed by C7
to avoid oscillation in the
1S4. Grid bias for the 1S4 and 1S5 is obtained by the voltage drop
between negative "B" and "ground." The antenna-coupling
turn is placed at the grid end of the tank and coupling is adjusted
for the maximum antenna loading which will allow the tube to maintain
stable superregeneration when the switch is in the "receive" position.
These views of the "para-talkie" show the arrangement of components
inside the 4 x 5 x 6-inch steel box.
The unit is contained in a
standard 4 X 5 X 6- inch steel box with removable sides. Most of
the components are mounted directly on one of these removable sides,
to make the job of assembling and wiring easier.
the shortest leads possible, the acorn detector-oscillator tube
and the tuned-circuit components are grouped close to the change-over
switch mounted on one of the fixed sides of the box. C1
is mounted on spacers, with its shaft opposite a quarter-inch hole
through which it is adjusted by a screwdriver. L2
self-supporting, its ends being soldered directly to the terminals
with the grid side going to the rotor terminal.
also is self-supporting, being mounted at the grid
end of L2
. An extension lead at one end goes directly
to the small feed-through insulator which serves as the antenna
terminal, while the other end is soldered to a lug fastened to the
is supported by the heavy leads with which
it is fitted, one being soldered to the plate terminal of the 958
socket while the other goes to the switch terminal RFC1
is mounted similarly underneath the change-over switch.
The audio transformer, T1
, is in one corner next to the
change-over switch. The two audio tubes are placed opposite S1
leaving sufficient space behind them for the two 33-volt "B" batteries
which are held in place with metal straps. Space for the miniature
modulation choke, L3
, is found next to the 1S4 output
tube. The single-cell "A" battery is clamped against the bottom
end of the case with a U-shaped metal strap.
Two cable connectors
are set in the top of the case. One is for the headphone connection
and the other for the microphone cord. It is important that a shielded
'phone cord be used. The power toggle switch, S2
mounted on the bottom end. The side of the case which was removed
for the photographs is slotted for the web straps by which the unit
is fastened to the operator.
The para-talkie has held up
well under repeated tests, our only casualty being one cracked lead-in
insulator. Careful selection of parts and a painstaking wiring job
probably are responsible for its stable operation, the frequency
shift from "receive" to "transmit" being negligible. Its general
performance as a portable rig on the ground, using a whip antenna,
is superior to commercial transceivers with considerably greater
power input. Posted