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Radio control (R/C) of a model doesn't get much simpler than the transmitter
and receiver circuits shown in the schematics of Figure 2. Of course the
cleanness of the transmitted signal and the selectivity of the receiver of that
signal leaves a lot to be desired. In 1952 when this article appeared in
Radio & Television News magazine, the airwaves weren't cluttered with wireless
communications devices, but given that these radio systems were sharing the
electromagnetic spectrum with Citizens Band (CB) radio, the chances of getting "shot
down" from nearby operators was pretty high if you lived within a few miles of
where CB'ers were communicating. More sophisticated R/C equipment was available
from commercial manufacturers, but this system targeted the do-it-yourself types
and those with limited hobby budgets. A lot of airplane models which consumed
many hours and dollars of a flyer's resources met with their demise as the
result of a stray signal blocking the R/C transmitter's signal, or the
airborne receiver mistaking the CB signal as legitimate and commanding the escapement
device
to turn the model in an unintended manner. My first R/C system, in the
mid-1970s, operated in the 27 MHz band in−between the allocated CB radio
frequencies. On more than one occasion, I experienced what was almost certainly
CB radio interference.
Radio Control on the Citizens Band
Fig. 1 - A little shove and the completely radio-controlled plane
is ready to perform.
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".
Fig. 2 - (A) Complete schematic diagram of the transmitter portion
of the radio-control unit. (B) The unusually sensitive superregenerative-type receiver
section.
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.
Fig. 5 - The most common method of obtaining rudder control.
This sequential device, known as an "escapement," moves rudder left, neutral, right.
Fig. 6 - Close-up view of the cabin showing the receiver installed
in the plane.
Fig. 7 - Receiver section with the dipole antenna folded into
a non-directional square.
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.
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.
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.
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.
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.
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.
* President. Vernon C. MacNabb Company, 915 Westfield Blvd., Indianapolis 20,
Ind., manufacturers of "Citizen-Ship" radio control equipment.
Posted May 18, 2024
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