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License-Free Radio Control
May 1962 Popular Electronics

February 1960 Popular Electronics

February 1960 Popular Electronics Cover - Airplanes and Rockets [Table of Contents]

People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights (if any) are hereby acknowledged. See Popular Electronics articles on aircraft modeling. See all articles from Popular Electronics.


Mag Tag: Part 15 Device

License-Free Radio Control

Check the FCC regulations; you'll find you can operate without a license

By Donald L. Stoner

Although it is not generally known, you can operate a radio-controlled model without a license. Anyone who has taken the time to read paragraph 15.205 of "Part 15 - Incidental and Restricted Radiation Devices" of the Federal Communications Commission Rules and Regulations will discover that a low-power communications device may operate between 26.97 and 27.27 mc (27.12 mc ±150 kc) provided it complies with the following requirements:

• The carrier of the device shall be maintained within the band 26.97 to 27.27 mc.

• All emissions, including modulation products below 26.97 or above 27.27 mc, shall be suppressed 20 db or more below the unmodulated carrier output.

• The power input to the final radio stage (exclusive of filament or heater power) shall not exceed 100 milliwatts.

• The antenna shall consist of a single element that does not exceed 5 feet in length.

The intended use is not specified and thus Part 15 radio-control equipment may be considered permissible. Note also that no specific channel assignments are made (subpart a) and that any frequency between the limits specified can be used. The carrier frequency could be "wedged" between two Citizens band channels where interference is at a minimum.

There is one other point which should be mentioned in connection with the rules and regulations: Part 15.208 (d) states, "The certificate may be executed by a technician skilled in making and interpreting the measurements that are required to assure compliance with the requirements of this part." Thus, if you are technically qualified to make the required measurements (second harmonic, modulation bandwidth, power input, etc.), you can build a transmitter or modify a commercial unit. No second-class radio-telephone license is required. However, if equipment is found to be operating improperly or in violation of regulations, then the certifying person may be called for a hearing before FCC examiners. If he is found to be unskilled in making and interpreting measurements required for certification, then he may be prosecuted.

Although the power input of equipment used under Part 15 is limited to 100 milliwatts (0.1 watt), this is adequate for up to 1-mile range. Radio-control transmitters are usually operated with a continuous carrier. Only the tone is pulsed to actuate a receiver escapement or relay. This system is more reliable since the carrier tends to override interfering stations. Experience has shown that battery-consuming high-power transmitters are not required since the model seldom goes more than 1/2 mile away. Modern transistor receivers are more selective and sensitive than their vacuum-tube predecessors. Completely transistorized receiver and transmitter systems are not only practical but are commercially available (Wen-Mac, etc.).

Inside the GW-30. This unit is about to be modified for license-free R/C use.

A Part 15 R/C transmitter

To prove that a low-power transmitter could be used for radio control a Heathkit GW-30 Citizens-band transceiver was modified to work in conjunction with a transistor radio-control receiver, an F & M Electronics Pioneer model. The circuit of the G-W-30 is shown in Fig. 1.

The modifications involve disabling the GW-30 receiver and inserting feedback in the modulator section to generate a tone. The transmitting frequency of the GW-30 need not be changed.

Here's how to modify the GW-30.

Take the unit out of its leather case and remove the back panel. Lift the printed-circuit board from the case after removing the three screws that secure the chassis. Familiarize yourself with the location of the pushbutton switch contacts (Fig. 2 and the photograph). This switch is modified so the transmitter carrier comes on when the volume control-switch is turned on. Depressing the button, in the modified GW-30, then produces a tone rather than turning on the carrier.

It is easier to bend the switch contacts than to rewire the switch to do this. They may be straightened later if a return to the original operation is desired. Note the dotted lines in Fig. 2. Bend the fixed contact on section B up so it touches the moving contact. This completes the emitter circuit of transmitter transistor V4. Next bend the fixed contact on section C up. This breaks the primary circuit of T1 and disables the receiver. Do not modify section D. Finally, on section A, bend the upper fixed contact up to clear it from the moving contact. Bend the lower fixed contact of section A so that it touches the moving contact at ail times. This connects the antenna to the transmitter section. Finally, the modulator is made to oscillate by introducing feedback.

This can be done by reconnecting section D of the switch to tie the modulator output to the input. Locate the wires which connect section D to the speaker and T2. Reverse these two wires by "swapping" the connections from section D of the switch on the circuit board. Thus you can see that when they are connected in this manner, depressing the pushbutton connects the speaker winding on T1 to the speaker winding on T2. When this is done, the audio stages break into oscillation and tone-modulate the transmitter. This completes the changes to the unit and it can now be reassembled.

Although the modifications sound complicated, it actually takes more time to tell how to make them than to do the job.

The GW-30 handheld CB transceiver.

To test the conversion, check the transmitter by energizing a radio-control receiver in a model on the ground. It should actuate the control surface reliably over a distance of 1,000 feet and more when the model is held aloft. In operation, the transmitter is energized, before launching the model, by turning on the volume control. As soon as the model is 50 to 100 feet from the transmitter and has enough elevation, the control button can be depressed. The exact sequence of operation will depend on the escapement used.




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