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Radio Control on the Citizens Band
Although designed primarily for controlling planes, this unit
may be used with ship's and other models.
By Vernon C. MacNabb*
Anyone can now control model airplanes and boats by radio. Some
readers may wonder why such an obvious statement is made. You always
could control anything by radio within certain limits. Control of
mobile or distant objects by radio is as old as radio itself, but
for the average citizen, a limitation existed in that he had to
be a radio amateur, which today means learn the code and pass a
technical examination.
In June 1949, the Federal Communications Commission, realizing
the necessity for a band of frequencies which would allow the average
citizen to operate a transmitter without technical knowledge or
ability to read code, opened a band of frequencies known as the
Citizens Band, running from 460 to 470 megacycles. This band of
frequencies falls somewhere near the indistinct dividing line between
what is known as very-high frequencies and ultra-high frequencies.
It presents a serious problem in making equipment work on these
frequencies because it is close to the limit possible with conventional
tubes and tuned circuits and at the same time it is so low in terms
of ultra-high frequencies, that short-wave plumbing or waveguides
have large and rather unwieldy dimensions. Some recent technical
developments, however, have produced some subminiature tubes which
will function properly at this frequency so the equipment described
in this article is more or less conventional in that it uses tuned
circuits instead of cavity resonators.
The photographs, Figs. 4 and 7, show the transmitter and receiver.
The transmitter dimensions are 9" x 4" x 2 3/4", and the batteries
are self-contained. The weight of the complete unit with batteries
installed is less than four (4) pounds. The receiver itself weighs
five (5) ounces and the recommended batteries to use with this receiver
will weigh an additional nine (9) ounces, making the total weight
less than one (1) pound. Dimensions are 3 3/4" x 2 1/8".
The accompanying photo (Fig. 1) of a model builder hand-launching
a plane illustrates the size airplane that will readily carry this
equipment. This plane is powered by a .019 engine. The closeup view
of the cabin (Fig. 6) shows the receiver installed in a plane.
The transmitter is a self-excited oscillator with a directly
coupled antenna and the problem in designing this unit was one of
stability. The circuit diagram, Fig. 2A, looks like any conventional
single tube unmodulated transmitter but the chassis, shown outside
the case in Fig. 3, shows that mechanically it is a rather radical
departure. The tuned circuit and antenna coupling coil is punched
out of the chassis for rigidity and grounding reasons. As a result
the chassis is connected to "B+." The comb-like piece reduces capacity
coupling between the tuned circuit and antenna pickup coil to help
pass FCC frequency stability requirements. A piece of bimetal is
used to form a small capacity which varies with temperature across
the tuned circuit to provide temperature stability. The transmitter
had to be approved by the Federal Communications Commission before
it could be manufactured, and the requirements were that it should
not drift more than 0.4 percent from 465 megacycles under a number
of conditions, such as tube warm up, decline in battery voltage,
and temperature change. It took over eighteen months of development
work to satisfy the FCC on these rigid requirements. An individual
will find it impractical to try and design a transmitter for his
own use as the FCC will not undertake approval tests unless 100
units are to be manufactured.
Fig. 1. A little shove and the completely radio-controlled plane
is ready to perform.
The folded dipole and reflector which plugs into the top of the
transmitter allows the small output of the transmitter to be concentrated
in one direction and the reflector also serves the purpose of making
the box cold as far as r.f. is concerned. The transmitter operates
with 6 volts of filament power and 135 volts of "B" battery. The
filament draws 200 milliamperes and the plate current, when the
transmitter button is pushed, is 20 milliamperes.
To operate the transmitter, the filament switch is turned to
"On" and a minute allowed for tube warm up. A conveniently placed
Micro Switch is then depressed as the unit is held in the hands
to apply "B" voltage and send out the signal.
The function of the receiver is to close a relay when the signal
is received from the transmitter. In order to make it suitable for
model planes, the weight must be kept to a minimum and one tube
is all that is used in the circuit. A superregenerative type of
circuit, Fig. 2B, is used which is so sensitive that even when a
very weak signal is received from the transmitter the plate current
change is approximately ten to one. The normal idling current is
0.1 milliampere and when the transmitted signal is received the
plate current jumps to 1.0 milliampere, or more. The sensitive relay
is set to operate at about 0.3 milliampere therefore it has a large
factor of safety and extreme reliability. This action takes place
over distances in excess of one-half mile when carried by a plane
in the air so that ground reflections and absorptions are not present.
At waist heights along the ground, the equipment will work at approximately
three-tenths of a mile. Because the plate current is so low, two
hearing aid type "B" batteries delivering 60 volts may be used to
supply the "B" voltage and their life is as long as their shelf
life. The tube in the receiver is a 6K4 subminiature and uses 6
volt filament supply at 150 milliamperes. The intermediate sized
flashlight batteries will operate the unit for about an hour.
Fig. 2. (A) Complete schematic diagram of the transmitter portion
of the radio-control unit. (B) The unusually sensitive superregenerative-type
receiver section.
Looking at the receiver, Fig. 7, it will be noticed that there
is a square or rectangular band of aluminum underneath the main
chassis base. This is the antenna which is a dipole folded into
an approximate square so it is not directional. The tuned circuit
is a copper band at the left end of the chassis and the relay is
in the right foreground. Four Fahnestock clips are provided for
connections, "plus A," "plus B," "ground," and the fourth clip for
connection to the escapement, motor, or any other device used to
control the airplane or boat. The Fahnestock clips on the receiver
also serve the purpose of mounting the receiver on rubber bands
to avoid engine vibration and shock.
Fig. 3. Transmitter removed from case and showing the unusual
way that the unit is constructed.
Fig. 4. Over-all view of transmitter with antenna. The entire
radio-control unit weighs less than four lbs. including batteries.
Radio control of model planes is not new as they have been flown
for approximately fifteen (15) years by such pioneers as Clinton
DeSoto and the Good Brothers. The Good Brothers are the most famous
for their contributions in this field. One of the simplest means
of controlling a model plane in flight is by controlling the rudder
only. Most model planes are self-stable, that is, they fly level
or return to level flight if disturbed from their normal course
without correction of elevators or ailerons. Therefore, all that
is necessary to control flight is a rudder, which will cause the
plane to turn and return to the operator at will. They are also
designed to climb slowly while the motor is running and to glide
to earth gently after the motor has run out of gas or is shut off.
It is surprising the number of maneuvers that can be performed
by rudder alone. Not only can all types of turns be executed, such
as square patterns, figure eights, etc., but by properly setting
the amount of rudder control, the planes can be made to spiral dive
and at the end of a spiral dive, by giving opposite rudder, it can
be made to loop. The spiral dive, of course, is useful in losing
altitude in case the plane is climbing too high under power.
The most common method of obtaining rudder control is by a sequential
device known as an escapement as shown in the photograph of Fig.
5. It is a small electromagnetic device weighing only one-half ounce,
which is driven by a rubber band and triggered by the closing of
the relay in the receiver. The escapement moves the rudder from
neutral to right, back to neutral, to left, always returning to
neutral when the transmitter is off. If right rudder has just been
used and the rudder is back to neutral, one pulse gives left rudder,
two pulses right rudder.
Fig. 5. The most common method of obtaining rudder control. This
sequential device, known as an "escapement," moves rudder left,
neutral, right.
The work of the Good Brothers and early experimenters was done
in the 5 meter ham band. The receiver was a superregenerative set
which normally drew about 1.5 milliamperes. This plate current dropped
to about half that value when the signal was sent to it. It was
necessary, therefore, to cause a relay to function within a two-to-one
change of plate current. There was another limitation; with this
constant "B" battery drain, as the plate voltage fell the plate
current itself declined and the relay setting might have to be changed.
Strangely enough, in spite of all the difficulties encountered
in making the system operate at 465 megacycles in the Citizens Band,
one advantage is outstanding. Because of the extremely high radio
frequency in comparison to the low audio frequency in the conventional
superregenerative circuits, the plate current increases instead
of decreases, and because of the large safety factor on plate current
change as mentioned before, decline in "B" voltage does not cause
marginal operation of the relay. In addition, because the current
is low with no transmitted signal, economy of "B" power is obtained
which obviously saves weight.
Further convenience contributed by the 465 megacycles is the
small dimensions of the half-wave dipole that makes the transmitter
completely portable. It is only one foot long. The transmitters
working on 54 megacycles require the erection of an approximate
8 foot dipole which anchors the operator to one spot.
The best example of the reliability of this equipment, which
is commercially called "Citizen-Ship Radio Control," is the fact
that the first production units to leave the factory were used in
competition at the National Model Airplane Meet in Dallas, Texas,
in July 1950, and the model builder who incorporated this equipment
in his ship won first place. It is obvious that the radio alone
was not the sole reason for winning, but without equipment that
was absolutely reliable, it would have been impossible. Never once
did the radio system fail to respond when the transmitter was actuated.
There were many cases of other contestants, some of whom had home-made
equipment and on the ham band, who would lose control of their planes
and they would fly away, resulting in not only loss of points, but
sometimes a damaged plane when recovered.
"Citizen-Ship Radio Control" equipment is virtually license-free,
as no examination or code test is required. A federal form is packed
with each transmitter and it is only necessary for the purchaser
to fill out this form and send it to his nearest FCC Field Engineering
Office and a portion of the form is stamped with a number and returned
to the purchaser. This becomes the radio transmitter's license.
With this license, anyone can use the transmitter. The only limitation
is that any individual less than 18 years old cannot obtain a license,
but it is permissible for him to use. the equipment if one of his
parents obtains the license.
Fig. 6. Close-up view of the cabin showing the receiver installed
in the plane.
This virtually license-free equipment opens up a new field to
hobbyists. The equipment is designed, engineered, and manufactured
so that no knowledge of radio is required to use it. If a person
is capable of connecting up a simple electrical circuit, which means
connecting up the batteries, the equipment is guaranteed to operate.
With this equipment in the hands of ingenious hobbyists and model
builders, it may soon be possible to control a model plane in as
complex a manner as though a pilot were in the plane itself. One
manufacturer already has a device which requires a very simple accessory
to the transmitter and in place of the escapement a selective mechanism
in the plane attached to the receiver, which will give as many as
twenty-four (24) different functions. With this number of controls,
anything is possible. The air over vacant lots may soon be filled
with model planes zooming over the heads of spellbound spectators.
Editors Note: The home construction of the transmitter described
is not advisable, as FCC approval cannot be obtained for such units.
Fig. 7. Receiver section with the dipole antenna folded into
a non-directional square.
* President. Vernon C. MacNabb Company, 915 Westfield Blvd.,
Indianapolis 20, Ind., manufacturers of "Citizen-Ship" radio control
equipment.
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