Radio Technique
George M. Myers
Some answers to reader letters.
Letter from Mark Scheda (AMA 125120), Rochester, NY — 20 Jan 1985
20 Jan 1985
Mr. George M. Myers AMA Dist. #2 Frequency Coordinator
Dear Mr. Myers,
This note is to bring to your attention a situation in which you might take an interest. During May and June of 1984, I experienced severe interference problems on three different occasions while using RC44 in western NY. I was flying a brand-new, seven-channel Airtronics Championship Series FM transmitter and receiver. Here's a description of the incidents:
- Flying at Grand Island, about five miles north of Buffalo. Severe interference produced a crash. (At the time, I was not convinced of the interference, as the plane was new and not yet trimmed.)
- Flying at Niagara Falls, about 15 miles north of Buffalo. Two sets of interferences—no crashes.
- Flying at Niagara Falls, about 15 miles north of Buffalo. (Second incident there.) No crashes.
- Flying at Barker, about 45 miles northeast of Buffalo. No interferences.
After the third incident, I pulled the entire system from the aircraft and returned it to Airtronics. They did a complete check and found nothing wrong, but changed the system to RC40. A note was returned with the radio, asking if there was a TV4 transmitter in Buffalo (the answer is yes!). The note explained that TV4 can interfere with an Airtronics receiver on RC44. Since the change to RC40, I have not had a single problem (summer and fall of 1984, probably 10 hours of flight time).
The folks at Airtronics were extremely helpful and courteous at all times, both when speaking with them on the phone and in written correspondence.
I hope that the least that comes of this note is that others are made aware of the problem. If any other action on my part would be helpful, please do not hesitate to let me know.
Regards, Mark Scheda
Comment on Mark Scheda's report
First, thanks to Mark for a report that exemplifies the term "academy," which my dictionary defines as "a place for teaching special knowledge." Everything associated with model aviation requires special knowledge, and AMA (and its official journal, Model Aviation) exists to give AMA members an opportunity to share what they know. The office of DFC was created to serve as a conduit for information on the new RC channels, and I am pleased to use it as intended.
Everything Mark reported agrees with the facts as I know them. TV4 can interfere with RC reception, and Airtronics has been helpful in diagnosing and fixing such problems.
The reason TV4 can interfere with many RC channels is that most RC receivers operate on the superheterodyne principle. A superheterodyne radio mixes two radio frequencies together so it can extract information from their sums and differences. TV4 sound is FM, centered on 71.4 MHz and deviating +/- 0.075 MHz in normal operation.
When an RC receiver picks up the TV4 sound carrier at a high enough level, that signal can intermodulate with the receiver's local oscillator, and with transmissions of other RC transmitters, producing detectable interference.
When you use a monitor, you often hear a pulse-modulated TV4 sound track on some RC channel, but only if an RC transmitter is operating near the monitor. I have demonstrated this for several clubs, using either the Airtronics monitor or others (like the Lange converter). What is alarming about that test is that the operating RC FM transmitter usually is on a channel different from the one being interfered with. For example, I have used my Tower Gold 500 transmitter (AM) on RC40 to demonstrate TV4 sound (FM) on OR/VH (72.400 MHz) on my Airtronics scanner. I hadn't noticed that RC44 could interfere with itself, but apparently others have.
The effect is strongest when you fly close to the TV transmitter (less than about 20 miles away) and drops off at greater distances (about 45 miles, as in Mark's experience). The interference effect is also stronger when there is an operating TV set nearby (within ~20 feet) tuned to TV4. Any operating superheterodyne receiver (like a TV) radiates some energy from its local oscillator via its antenna. You can observe this when two pocket AM radios tuned to the same station whistle at each other. From this, I deduce that an operating TV receiver in a vehicle on the flying field (in a camper, for instance) might produce interference in an otherwise glitch-free situation.
Personal computers on the field have the same potential for producing interference, although a TRS-80 was used for scoring the 1983 Soaring Nats in continuous operation for three days adjacent to the impound area with no observed interference.
Portable and commercial VCRs (video cassette recorders) similarly can contribute to interference, though I have seen many situations where they did not. Interference happens but is not absolutely predictable. When I listed possible interference combinations in my columns of May and June 1983, I got some irate responses, then decided to let nature take its course. It has.
Dual-conversion receivers should not have this particular TV4 problem. The TV4 issue is peculiar to older single-conversion superheterodyne receivers that use a 455 kHz IF amplifier. However, dual-conversion receivers might have similar problems with other TV channels—possible, not certain.
In case of doubt, turn on the receiver first and watch for a while to see what happens—which ideally should be nothing.
Letter from Peter A. Jacobitz (AMA 114113), East Boston, MA
"I am only a two-year novice at R/C. I ask questions, and find that when someone doesn't know the answer they generally say, 'Don't worry about it!' When they really mean, 'I don't know.' So, I turn to you.
"I have a Tower Gold 500 system on RC48. When I turn on the power to the RX and TX, the servos buzz, chatter, and twitch. Standing six feet away, with the TX antenna fully extended, running my hands over the case or tapping it produces a variety of twitches. Should this happen?"
Pete, no radio should act like that. Mine sure doesn't. Something is loose inside yours; it might be the replaceable crystal, or something else that plugs into something. There is also a good chance that the antenna isn't screwed down tight. Failing that, look for a wire that has broken but not separated. In any case, fix it before you fly it. From my experience with Tower Radio, they will go out of their way to make it right for you. If you can't correct it based on the above suggestions, send it back to them.
On its first outing, the FM set on RC48 was hit by a Kraft AM radio on Blue/White (72.160 MHz), which had just returned from service. I thought I had a bad FM receiver until a fellow club member flying on RC42 FM crashed as a result of interference from an AM transmitter on RC54.
All of the above radios had good ground range. Testing with my own radios showed that the AM radio produced jitters in the FM set, though not as badly as those from the Kraft radio.
When I called Futaba, an unnamed technical person disclosed my knowledge of a problem with the FM radios. He indicated that the probable cause was "splash" from the AM radios.
"If this problem is widespread, it could explain some glitches. I would like your comments, and perhaps some suggestions on how to detect the problem before the aircraft is airborne. I intend to enlist the help of other club members to carry out further testing."
Reports from other readers
David J. Watson (no AMA number given) of Taylors, SC sent a copy of his letter to Cirrus Hobbies, which reads in part:
"... I have returned my radios five times—three times for the Unlimited 8 and twice for the Century 7. You just gave me a new receiver and say you aligned it and test flew it. The Century 7 is acting up again. Both of these are FM radios, and they give me more trouble than my Cirrus Apollo 6 or my Circus 4, both of which are AM radios.
"... I am getting desperate. Five good planes have been destroyed because of radio problems. My friends, who fly Futaba and Kraft, don't seem to be having these troubles. If your radios do have problems, please tell me they do, and tell me what I can do to correct these problems."
As you can see, it doesn't matter which manufacturer you buy from—FM sets are getting hit. The problem is widespread, and reports are coming in more frequently as the number of new FM radios in service increases.
The first lot of FM radios to hit the market used the old single-conversion 455 kHz IF design (except for the Kraft KPR-8FD and KPR-8FDS receivers, which are dual-conversion types). Faithful readers know that Bob Aberle and I have been predicting for at least the past five years that a dual-conversion design would be required to meet 1991 conditions. From available evidence, single-conversion types work well by themselves (see dozens of reports by Bob Aberle in Flying Models). From reader letters like those above, they are susceptible to interference when operated in groups.
This column has offered test procedures from time to time, all based on the idea that similar tests would be performed by our readers using normally available equipment. So far, only one person has written about his testing, and that report was thorough but outside the scope of activities Bob and I defined as reasonable to expect from RC operators.
The simplest test (31M) was described last month, and a more comprehensive test program was given in the September 1984 column. Unfortunately, we haven't been testing for all the possible AM, FM, and TV combinations because Bob and I don't have all the test equipment we'd need. We will likely have to extend our test procedures, which means out-of-pocket expense for additional gear.
As for blaming all the trouble on improperly adjusted transmitters—"Prove it!" We've tested only a few transmitters (about 600) for emission characteristics and in that lot found only two with illegal spurs. Our sample is small, but enough to show that the AM transmitter is not likely the main cause of the problem. Now and then it will be, but most often it won't. I repeat my theme: the key to the future is a good receiver.
Letter from Angel Mendoza (Huntington Beach, CA)
After reading the September 1984 issue of Model Aviation, Angel bought a Kraft system with the KPR-8FD FM dual-conversion receiver (or possibly the KPR-8FDS). His experience:
- First session at Mile Square was OK (six flights).
- Next session at Sepulveda Basin. Bad glitch. Landed safely.
- Tests at home showed that his Futaba on RC48 AM would interfere with the Kraft on RC56 FM.
- TX and RX modules to Kraft for repair.
- Three weeks passed.
- Flying session at Mile Square (six flights). No problem.
- Next session. Throttle out of control.
- Back to Kraft. No fix. National repair out of business.
- Kraft told him to send it to Arizona for repairs.
- Have owned the radio 90 days and have only flown twice.
Some synthesized radios left the factory with a bad transistor that passed AM signals through the front end. Unfortunately, Kraft dropped out of the domestic RC production business around the same time, which complicated repairs. At this time I advise anyone with a synthesized radio and a problem to contact Pete Waters (AMA District VII vice-president) at 117 E. Main, Upper Level, Northville, MI 48167. Pete is one of the few who has the tools, components, and training to service synthesizers. If you missed it, read his column in the March 1985 issue.
Letter from Carl G. Godlewski (Westport, CT), AMA 143818
Received at AMA HQ 9 Jan 1985
Dear Mr. Myers,
I wrote two letters to the FCC in 1984 requesting paperwork necessary to get a Ham operator's license. I understand that the license is required to operate my JR FM radio on 53.300 MHz.
I sent the first request to the FCC office in New York City about April 1984. Three months passed without a response. Then I sent a second request to the FCC office in Washington, D.C. I have yet to receive any response from either office. What gives?
I hold a Radio-Telephone operator's license, acquired in 1969 when I took pilot training. Since the FCC has failed to answer my requests, I am tempted to do something about it.
In summary, is my Radio-Telephone operator's license all I need? Should I continue to prod the FCC, or let the comatose giant sleep? As a newcomer to this hobby, any advice or help you can give me will be appreciated.
Regards, Carl G. Godlewski
Response regarding licensing and the FCC
This letter is representative of several I've received. The FCC is undergoing administrative changes, and you will have to be patient with them. The recent restructuring of examination procedures and the introduction of Volunteer Examiner Coordinators (VECs) has caused delays in routine processing.
If you want to get a license, contact the ARRL (American Radio Relay League) or your local amateur radio club for information on scheduled VEC examination sessions. The ARRL and the VECs can supply lists of local clubs and testing sessions (which are posted in advance to allow time to attend). VECs will assist with the examination and forwarding paperwork to the FCC, which later issues the license.
Amateur radio theory courses are taught as Adult Education in many high schools and will prepare you to pass both tests. You can check with the ARRL or the Amateur Radio Testing Corp., 275 Hillside Ave., Williston Park, NY 11596, for information on examinations and study material. Also available is the Amateur Radio Theory Course from the Heath Company (Benton Harbor, MI 49022), which provides code and theory course material on cassette with accompanying lesson manuals. The Heath catalog changes from time to time, so don't hold it against me if a product has been discontinued.
Specifically, the Radio-Telephone operator's license (which every pilot has) confers a license to use the Aviation Band, 109–135 MHz. It does not permit use of the Amateur Radio bands.
George M. Myers 70 Froehlich Farm Rd. Hicksville, NY 11801
Transcribed from original scans by AI. Minor OCR errors may remain.
