Radio Control on the Citizens Band
March 1952 Radio & Television News

October 1958 American Modeler

October 1958 American Modeler Table of Contents

These pages from vintage modeling magazines like Flying Aces, Air Trails, American Modeler, American Aircraft Modeler, Young Men, Flying Models, Model Airplane News, R/C Modeler, captured the era. All copyrights acknowledged.


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.