April 1945 QST
of Contents]These 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 (if any) 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.
See all available
vintage QST articles
A Parachutist's Transceiver By L.T.
Arthur H. Copland, CAP
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 ground use.
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.
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.
Existing units 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. Experimental
With the 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 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.
The set 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.
- 50-μμfd. mica.
C3, C4 - 0.005-μfd.
C5, C10, C11 - 0.1μfd. paper.
- 500-μμfd. paper.
C7 - 0.002.μfd. paper.
C8 -10-μfd. 75-volt electrolytic.
R1, R12 - 5 megohms, 1/2 watt.
R2, R4 - 25,000 ohms, 1/2 watt.
R3 - 0.5 megohm, 1/2 watt.
- 10 megohms, 1/2 watt.
R6 - 1 megohm, 1/2 watt.
R7 - 3 megohms, 1/2 watt.
2 megohms, 1/2 watt.
R9 - 450 ohms, 1/2 watt.
- 1,000 ohms, 1/2 watt,
R11 - 25,000 ohms, 1
|L1 - 1 turn No. 14 bare copper
wire, 1/2-inch inside diameter.
L2 - 4 turns
No. 14 bare copper wire, 1/2-inch inside diameter, spaced to
fill 1/2-inch winding length.
L3 - 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, toggle
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. Resistor R2
develops bias voltage for the grid.
, 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
, 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 resistor, R10
, 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 coupled
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
to avoid oscillation in the 1S4. Grid bias for the
1S4 and 1S5 is obtained by the voltage drop across R9
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.
To permit 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.
is self-supporting, its ends being soldered directly to
the terminals of C11
with the grid side going to the rotor
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 case. RFC2
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
The audio transformer, T1
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
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,
, is 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.