NEW RCD ALL-CHANNEL TRANSMITTER RF MODULE
This is written in advance of production, but the item should be available by the time this column is published.
RCD is introducing an FM transmitter module for the Hitec Master and Challenger 720 series (which are plug-compatible with the Futaba G-series). The module selects all 80 frequencies in the 72 and 75 MHz bands. It will be narrow-band and FCC Type Accepted.
The module allows you to switch-select all 72 MHz plus 75 MHz channels. Two 0–9 switches on the inside surface of the module designate the desired channel number (11–60 and 61–90). The channel characteristics are stored in the unit’s memory, so the memory automatically instructs the transmitter to generate the selected frequency. As with similar modules, you must remove the module to choose a channel.
A matching receiver module will be available. The receiver’s channel selector switches are in the On/Off switch module, which is connected to the receiver by a cable — this avoids digging inside the airplane to change channels. The scheduled price is $169 for the pair.
The system is different from the old Kraft synthesizer and is much smaller. RCD claims 5 ppm accuracy because the RC channel characteristics are loaded into memory from a laboratory standard generator, then controlled by a very precise reference crystal. This design does not use an analog synthesizer as the Kraft system did; RCD prefers not to call it a synthesizer.
Car and two- and three-channel transmitters can’t use this module because they have no provision to accept it. For compatible transmitters, however, the channel-switching module is useful to avoid long waits at the flying field. Many RC sailors (who often also fly airplanes) should find it popular. I don’t see any safety problem with having 72 MHz and 75 MHz in the same transmitter — a band-selection mistake won’t accidentally "shoot anyone down." I hope to see the module on display at the WRAM show in White Plains, New York, or the Weak Signals show in Toledo, Ohio.
Dave Abbe reports the RCD receiver is now in continuous production. He says production quality control is tight enough that you only need to plug an RCD crystal into a single-frequency RCD receiver to use it (no tuning). A ground-range check is still recommended in case your transmitter is off frequency.
Interference will get through. Haven’t you observed a strong broadcast transmitter "break through" the station you are listening to on your car radio when you drive by? It happens all the time.
I also observed cross-responses to AM and FM signals with two particular receivers under test. Those tests demonstrate you should not try to operate both an AM and an FM system on the same channel. Although encoding schemes differ, two transmitters on one channel will produce interference. PCM systems resist such interference better than other systems, but we still must use a system that prevents two transmitters from operating on one channel, even if their encoding schemes are different.
See below for alternative plans to carry us through the interim period until the preponderance of club members have all narrow-band systems.
Thanks for writing. GMM
READER LETTERS — ERROR CORRECTIONS
Recent letters and phone calls show a big concern in many clubs in the area: what shall we do for 1991? A large number of clubs have voted to require gold-stickered transmitters and narrow-band receivers. In other words, clubs have decided to bite the bullet and switch to narrow-band radios as soon as possible. A common concern is that the change will take time. Below are interim plans and some corrections to errors reported in recent columns.
To Mr. John Lacey, Minnetonka, MN, and others: the Model Aviation office saves time if you use your home address. In the October 1990 issue the caption writer substituted a negative where a positive should have been; as I stated in the text (in three places across two articles), RCMA/AMA RF CHECK stickers are required for AMA insurance except when flying in sanctioned competition — the caption omitted the qualifying phrase.
When printed in black ink on aluminum-colored paper, the RCMA/AMA sticker is referred to as the silver sticker; it denotes performance acceptable to the FCC’s 1982 Type Acceptance criteria. The same sticker coated with transparent gold-colored disappearing ink is referred to as the gold sticker; the gold sticker denotes performance in accordance with AMA Guidelines. Put a piece of Scotch tape over the sticker to preserve its gold.
Yes, AMA Guidelines anticipated FCC actions. Since the 72–73 MHz band was mandated to a 10 kHz raster in 1981, AMA contracted a professional engineer to define the necessary properties for narrow-band radios; those definitions became the AMA Guidelines.
THE ODD/EVEN PLAN
The simplest temporary way to deal with the new odd-numbered channels (RC11, 13, …) is to fly by the clock:
- Fly odd channels on the odd hours (9:00 a.m., 11:00 a.m., 1:00 p.m., 3:00 p.m., etc.).
- Fly even channels on the even hours (10:00 a.m., 12:00 noon, 2:00 p.m., 4:00 p.m., etc.).
This assumes all odd-channel transmitters and receivers will be narrow-band. OWBAM receivers will not even know the new odd-channel systems are in use.
THE ADD-A-CLIP PLAN
The most practical scheme for mixed usage is the "ADD-A-CLIP" plan. Each flier brings his own clip to the field, marked with his name and RC channel, and followed by a "W" if he is still using obsolete wide-band equipment. If marked "W," the person must tie additional clips to the first clip using a long piece of string. These added clips designate frequencies that must not be on the air at the same time, to avoid interference.
Mark the additional clips with channels that are 23 or 45 numbers away (both above and below the owner’s channel). When it is your turn to fly, put the clips on the necessary channels to block interference.
Example: Flying wideband on RC56. Subtract 23 → RC33 (tie on a clip for RC33). Subtract 45 → RC11 (tie on a clip for RC11). This blocks possible image and other interference while you fly.
This scheme places the responsibility on the wideband user to protect himself. Practically, wideband equipment is most likely on channels RC36, 38, 40, 42, 44, 46, 48, 50, 52, 54, and 56, so we consider 11 tied groups:
- 56 + 33 + 11
- 54 + 31
- 52 + 29
- 50 + 27
- 48 + 25
- 46 + 23
- 44 + 21
- 42 + 19
- 40 + 17
- 38 + 15
- 36 + 13 + 57
People flying with narrow-band receivers don't need any extra clips.
FLIGHT STATION CARDS
There are fundamentals any flight-station card design must observe. The major problem for 1991 will be old (wideband) receivers. If you want to protect them from image and 2IM, the following groups should be placed on one flight station. Only one channel in a flight-station group is allowed to operate at a time.
A: 11 + 34 + 57 + 12 + 35 + 58 B: 13 + 36 + 59 + 14 + 37 + 60 (A and B can be combined once on one station, but A and B cannot be broken down.)
The following 23-channel pairs must be kept together to protect old receivers from 2IM. I show them as six flight stations, but you can combine them to create any number of stations. You might, for example, combine A&B, C&D, E&F, G&H to get four flight stations.
Basic ideas:
- Keep the 23-channel pairs together to avoid 2IM.
- Keep adjacent channels together to minimize 3IM and interactions between wideband and narrow-band systems.
C: 15 + 38 + 16 + 39 + 17 + 40 D: 18 + 41 + 19 + 42 + 20 + 43 E: 21 + 44 + 22 + 45 + 23 + 46 F: 24 + 47 + 25 + 48 + 26 + 49 G: 27 + 50 + 28 + 51 + 29 + 52 H: 30 + 53 + 31 + 54 + 32 + 55 + 33 + 56
IDCo. offers similar cards already made up for all 50 channels using six flight stations (S50-6 for $58.50), eight stations (S50-8 for $72.00), or 10 stations (S50-10 for $85.00), as well as for the AMA Plan 1 (S25-AP-1 for $42.50) and Plan 2 (S37-AP-2 for $51.50). These cards contain all channels in use, printed for the appropriate station. IDCo. contact: N8024 Elser Drive, Beaver Dam, WI 53916; tel. (414) 885-3675.
Bob Aberle fully describes how to use flight-station cards in his article "50 Channels, Finally!" in the December 1990 issue of Flying Models magazine.
Transcribed from original scans by AI. Minor OCR errors may remain.
