George M. Myers
Radio Technique
Reader letters: categories
- Which RC radio shall I buy?
- What's the new RC channel setup?
- Where can I get information on (who fixes) a particular old radio?
- 31M (third-order intermodulation interference) — an increasing number of letters since the January 1986 issue.
Ham-band recommendation
I rarely answer the first question directly, except to say the safest radio right now is one on the Ham band — where the channel spacing is 100 kHz and the user population is small. If you’re worried about an expensive Giant Scale model, get a Ham ticket and use Ham gear; also get a Ham receiver so you can monitor the channel you intend to fly on (hams are expected to listen before transmitting). The new Ham channels at 50.8 through 50.98 MHz exist because 52–54 MHz is being used increasingly for other services (repeaters, computer RF modems).
RF modems on 72 MHz
Several readers sent ads for RF computer modems operating on 72 MHz. The ads cite use of both odd (new RC) and even (old RC) channels. I doubt manufacturers could have obtained FCC permission to use our new channels so quickly; the AMA lawyers are looking into it.
- Current RF modems are limited to low power (one watt), so they should not generally be a problem unless someone flies right next to an operating modem.
- A battery-powered modem brought onto a flying field (e.g., in a car) could be a concern; clubs might consider rules prohibiting RF modems on the field.
- RF modems trade cost savings (no wiring) for reduced privacy and security — transmissions can be monitored on club receivers.
- The computer/industrial RF environment is very noisy. Any modem manufacturer intending to use both odd and even channels will have to solve 31M problems on a 10 kHz raster. The effort could lead to better RC receivers for us.
Channel setup
There is no new RC channel setup at present — and none expected until 1988.
Information on old radios
For information on older radios, see Fred Marks’ book: Getting the Most from Radio Control Systems, Kalmbach. ISBN 0-89024-550-9.
31M testing and results
Bob Aberle and I have done extensive field testing of receivers (see Model Aviation, January 1986, p. 42), and testing continues. If we find an outstanding performer we’ll report it; so far, none has stood out.
#### First proposition We tested AM, FM, and PCM systems, using both single- and dual-conversion receivers, in wet and dry conditions. None of them performed well against 31M. Therefore, since you can’t buy an RC system that ignores 31M, you should pay attention to it.
#### Second proposition There are too many 31M triplets to permit a general flight-line scheme that covers every number of flight stations. A three-station flight line (two for 72 MHz channels and one for Hams) will avoid practically all problems, but that arrangement may not satisfy every club. The column in February showed how satisfactory arrangements can usually be culled from existing populations.
Many readers wrote saying they understood 31M after reading our column. One reader, John Krekelberg (Minneapolis), went out with friends in −20°F weather to run tests on a heavily advertised system as well as others. While Bob and I don’t test in that weather, anyone who plans to fly in extreme conditions should test their gear.
Test procedure clarification
Our objective is to define the point of complete system failure. That’s why we test with complete systems and rock the transmitter control while watching servo response. The change from “positive control” to “no control” usually occurs within a foot of distance. Glitches often appear intermittently at different spots before the final loss of control; therefore, when replicating our tests you must continue toward the interferers until you reach complete failure. The complete-failure distance is repeatable (even across weeks), whereas glitch locations are not.
Industry feedback and possible improvements
Several electronics engineers reviewed our procedure and data and suggested improvements to receiver designs. Experts agreed better performance against 31M is possible within current technology — one significant change might be increased receiver supply voltage. Knowledgeable people are taking an interest, and that should improve RC-system design over time.
Club practices and education
In the meantime, educate your club members that 31M is real. Encourage equipment maintenance, periodic retuning of transmitters and receivers, and use of impound areas, monitors, frequency clips, and frequency flags at every flying site (not just large contests).
Modelers often show off by doing risky things (snap rolls on takeoff, knife-edge along the runway, flying through buildings). The same bravado applies to flying without a monitor or ignoring impound procedures. These risky behaviors succeed until an unexpected element (like 31M) appears; the margin for error is shrinking.
Transmitter-evaluation results
George P. Steiner helped start the AMA “Gold Star” transmitter-evaluation program with testing at the IMS show in California. Of 98 units tested:
- 31 were out of frequency tolerance,
- 40 showed a second-harmonic output greater than −30 dB,
- 17 showed a sideband at ±20 kHz greater than −30 dB.
These results are worse than findings at the ’83 and ’85 Nats. I assume Steiner tested “sport” transmitters rather than competition transmitters. If repeated elsewhere, this suggests sport transmitters may be less trustworthy.
Like pianos, RC systems need periodic retuning. The careful alignment work from the 1983 crystal-swap meet may be dissipating over time.
Car and boat radios on aircraft frequencies
Many letters ask how to keep car and boat users off aircraft-only frequencies. Anecdotes:
- A hobby shop owner had a car radio installed in a plane because his distributor said it was OK.
- A shop in the Salt Lake City area stocked only four-channel, two-stick 75 MHz radios and told customers “Go ahead, it won’t make any difference.”
- Another shop said aircraft radios don’t sell locally, while off-road car radios sell out.
These market pressures and distributor practices contribute to saturation of inappropriate gear.
Manufacturer labeling — proposed solution
Bob Aberle proposed in 1982 (reiterated in 1985 and 1986 discussions) that manufacturers permanently mark radios at the factory: label 75 MHz radios “Not for use in aircraft” and 72 MHz radios “For Aircraft Only.” If markings are not permanent and factory-applied, they will have little effect. Implementation may not occur until 1988 when shared channels go away, but we should push for it.
Request for data
Please send copies of test data or any other relevant information you have. If you’ve had a radio “go soft” in the 72–76 MHz area, tell me. Include as much detail as possible: type of transmitter, type of receiver, radio make, weather conditions, and any test notes.
That’s all for now.
Transcribed from original scans by AI. Minor OCR errors may remain.
