Radio Technique

Radio Technique

George M. Myers

ABSTRACT

Super transmitters, flight packs, and synthesizers.

FLIGHT PACKS

What are all of the manufacturers up to now? They have their engineers concentrating on top-of-the-line crystal-controlled transmitters (Tx) which feature computer memories that store the settings, servo directions, mixing, throws, etc. for a bunch of airplanes. But, WHAT GOOD ARE THEY, IF YOU CAN'T BUY FLIGHT PACKS FOR THE OTHER PLANES? (Editor's note: A flight pack is the part of your RC system which goes in your plane. — RMcM)

Many modelers have complained to me about the high cost and general unavailability of flight packs. They say that although you can buy servos, battery packs, and switch harnesses over the counter, it's the receivers (Rx's) which are hard to find. I gave some of my money to the phone companies to learn if that's true.

When you make up a flight pack from individual parts purchased over the counter, you pay more for it than you do for a complete new system. People complain about the irrationality of being forced to buy an extra Tx, battery charger, flag, servo trays, servo arms, screws, eyelets, neck strap, instruction manuals, and carton when all you want is an Rx, battery pack, switch harness, and a couple of servos. While I have sympathy with the complaint, the only reasonable thing to do is to buy the complete package (less money out of your pocket), then discard the parts you don't want. You do it when you buy food. Who eats corn cobs?

You can now see that the flight pack availability problem is basically a receiver availability problem. You might wish that you could convince your dealer to at least stock one Rx from each manufacturer, plus some crystals. It sounds like a good idea until you discover most manufacturers list a half-dozen Rx's and try to discourage sales of FM set crystals to consumers.

Practically everything now is FM or FM/PCM. FM systems (including PCMs) must be re-checked for direction and amount of frequency deviation following a crystal change. Since it is assumed that no consumer is set up to do that (there may be exceptions), nobody wants their product to be blamed for what happens next after you "simply" swap crystals. So they try to limit crystal availability to those servicers who have the equipment to do it right. The fact that FM SYSTEMS MEASURE FREQUENCY DEVIATION TO EXTRACT CONTROL INFORMATION is what makes field-swapping FM crystals such a bad idea.

You might be tempted to USE ANY Rx YOU CAN GET to make up a flight pack. Unfortunately, half of the FM systems on the market utilize frequency deviation in one direction, the other half in the opposite direction, so mixing brands may not help much. Even if the systems use compatible deviations, the servo plugs probably won't match. Another problem is that the new 1024 PCM codes aren't compatible with the old 512 codes.

A few words about PCM

PCM stands for pulse-code modulation. The transmitter senses the positions of all of its control sticks, knobs, and switches, encodes this information as a bunch of words made up of numbers in binary code, and transmits these to the receiver. A computer in the receiver detects these numbers and translates them into commands for servo positions (or On/Off switch commands). The word-to-position correspondence can be arbitrary and can certainly differ between manufacturers.

As a general rule, it isn't necessary to align a new FM or FM/PCM Rx to your particular Tx anymore, because Tx and Rx are now aligned to precise frequency counters and signal generators at the factory or service center. Take them off the shelf and try them. Only if you see a ground-range problem should you need to return Tx and Rx for compatibility tuning. Right now, people on the inside are expecting you to see a need for compatibility retuning of Tx and Rx after about three years of use.

Back to flight packs: The basic problem with flight packs is that SOMEBODY has to stock one Rx for each channel for each line for each manufacturer in order to make just one sale to you (assuming that interchangeable crystals are not available). With 50 channels due in 1991, that could be a lot of Rx's (about 750, if you only consider Airtronics, Futaba, JR, Polk's, and World Engines lines). Never mind that you might want your pack with different servos.

Since no dealer will stock 750 Rx's, he needs access to someone who will. Meet the "someone":

Manufacturers and distributors

• Ace R/C, Inc.
• Makes AM Rx's in finished form and kits for its dealers. It offers the single-conversion Silver Seven receiver at $53.95 (less connector) and the dual-conversion Model 91 at $99.95 (t.c.), plus a line of servos. Rx's can be consumer-tuned (kits) or factory-tuned (built-up versions). Replacement Rx crystals are available for AM and bands for Ace and ProLine equipment. No FM equipment is available from this manufacturer yet.

• Airtronics, Inc.
• Stocks every type of Rx in their catalog, on every channel, for its distributors. Airtronics used to catalog flight packs, but they didn't sell well. Distributors didn't want to stock the multitude of packages. Its in-house service center (the ONLY Airtronics service center, at this writing) will handle all necessary frequency conversions and crystal replacements. It will sell Ham crystals to Hams, 27 & 75 MHz crystals to their distributors, and will not sell 72 MHz "Aircraft Only" Tx crystals at all. Airtronics will have Ham-band equipment (50 & 53 MHz) in 1989.

• JR radios (Hobby Dynamics Distributors / Circus Hobbies)
• Hobby Dynamics Distributors distributes JR radio systems for Circus Hobbies. Kevin Burner told me that the Circus Hobbies warehouse in Las Vegas and the Hobby Dynamics warehouse in Champaign, IL are linked by a common computer system. When a dealer orders stock, it is shipped from the nearest warehouse. At the time of our conversation, there were 15 Rx's of each type on each of the 72 MHz channels in the warehouses.
• JR will sell Rx crystals, but doesn't recommend that you move more than two channels from the frequency that the system had when sold. Kevin also told me that the president of Hobby Dynamics, Mike Colli, has the JR service center turning work around in 36 hours.

• Futaba Corporation of America
• Stocks both Rx and flight packs. The flight packs are priced below the sum of the component prices, but they aren't very popular with distributors and/or dealers. Normally, at least 100 of each Rx type are available, except in the case of the newest products (where Rx is in short supply). Futaba sells only matched sets of crystals, but recommends that you move only one or two channels from the frequency that the system had when purchased.

• Polk's / Aristo-Craft
• Polk's stocks extra A.R.I.-03-2681 FM Rx on any channel for $34.95. It orders them in batches of 1,000. Polk's once offered flight packs, but they weren't attractive to wholesalers, so very few made it to dealers' shelves.
• Louis Polk told me that the new Rx's have the slim F connector, which shouldn't be a problem if you buy new servos at $12.95. A.R.I.-03-7410 Molex-to-F adapters are available at $2.95 each for use with older equipment.

• World Engines
• Stocks Rx's for its Expert series on all channels RC18 and above—but no flight packs—for its dealers. The Molex connector is still used, and World still maintains its metal-cased Tx module which is not intended to be interchanged in the field. Jim Lanterman told me that the World Engines Expert Rx will be certified to the AMA guidelines soon. Proprietary changes are anticipated which will differentiate their line from that of Polk's. The U.S.-made S-16 (large) servo is still offered.

SYNTHESIZERS

What about synthesizers? I think that it's time to develop some synthesizers. Then one flight pack Rx could satisfy any Tx, and vice versa. If you can't get your dealers to stock flight packs, then it may cost more to make up a flight pack from components—forever. Certainly the dealers are more likely to stock a couple of UNIVERSAL Rx's (i.e., with synthesized tuning) than a great many crystal ones. A dealer would only have to stock two spare Rx's: one for FM (a tiny Rx for electrics, sailplanes, etc.), and one standard-size PCM.

Most of these "super memory" Tx's have PWM/PCM capability, so when you buy one you have all those new mixing tricks available for your old FM-equipped sailplane, for instance. (PWM is the "other" kind of FM—pulse-width modulation. The transmitter doesn't send out bunches of numbers. Instead, it controls stick position by the width of a particular pulse in the train of pulses it transmits; from each "frame" of updated information.) If the Tx has a synthesizer, think how much easier it will be to use it that way. Just remember to change the old Rx at the right time (1991).

SRF radios would be a boon to the sport flier. How often do you take your Channel RC48 system (the only one you have) to the field, only to discover long lines to use RC48, and nobody on other channels?

You can buy a 40-channel CB transceiver for about $30, a 181-channel VCR or TV Rx for $200, automobile radios with cassette decks and signal-seeking synthesizers for $200, etc., so how much can it cost to build RC Tx's and Rx's with synthesizers? I'll bet that it would be cheaper than stocking 101 crystals per manufacturer. The dealers would love it, because they could concentrate on selling product instead of channel.

When Kraft offered synthesized Tx's in 1983, certain rude individuals took delight in calling them "Dial-a-crash" systems. Of course, the fact that the system had to be turned off before it would switch to a new channel didn't impress them. But the Canadians (who authorized all 50 channels as soon as possible) didn't sneer. Instead, they snapped up all the Kraft SRF systems they could find. That's why you rarely see them in the U.S. I think that U.S. modelers will snap up synthesizers if someone offers them in 1991.

I'm willing to admit the possibility that some user might forget what channel he dialed in last (but all Bob and I ever have to do is lock the synthesizer control panel). We could replace the tiny, hard-to-see Kraft rotary switches with an LCD display on the Tx and a pushbutton for stepping through the channels (like setting your LCD watch); if a 1-inch LCD was put right next to the antenna, there would be little excuse for "forgetting" which channel was dialed in.

One nuisance with the Kraft SRF Rx is that you had to dig into the airplane to switch channels. I've even got a solution for that problem. We can put the Tx antenna near the Rx antenna, and then let the Rx look for the control signal (like the serial-seeker in your car radio). Once control has been established, you can put a plug into the airplane to lock the Rx on the channel (might even be part of the charging jack). Wouldn't that be a lot simpler? It sure would.

The main problem with the Kraft SRF was that there never was a second generation, where such improvements would have been incorporated.

Another complaint about the Kraft SRF Rxs was its size. Yet mine fits nicely in my Baron 28 (one of the smallest .25-powered Heiflickers). Kevin Burner told me that the JBL PCM-10 Rx (10 channels and PCM) is smaller than the old Century 7 Rx (seven channels and FM only). See what I mean about later generations?

THREE-POSITION POWER SWITCH AND TX SCANNING

While designers have been playing the "What else can I sell them?" game with their Tx's, they have completely overlooked the relatively simple possibility of a three-position power switch. Done correctly, the three positions would be:

1. OFF — no RF transmission.
2. AUX ON — a built-in monitor has been turned on to look for other transmissions on the channel.
3. ON — RF is being transmitted to the Rx.

When a synthesizer is used for the Tx frequency generator, modifying the synthesizer to act as a scanning monitor is no problem at all. The Tx could help you find an interference-free channel to fly on. Doesn't that make a lot of sense? Wouldn't it take care of the "forgot my channel" syndrome, particularly if it were automatic to transfer the control Tx only to a quiet channel, and would only do that after you pressed an enabling button? Then you would turn on your airplane Rx, pull the plug, wait for it to find your Tx, put in the plug, and go flying. (It might be a good idea to put a retaining screw in that plug.) When system prices exceed $1,000, I think it is time to get a lot more use out of a complex Tx than we do now. Just the learning time for proficient use of the Tx has become a big investment of hobby time. It wouldn't surprise me to see the complexity systems boxed with a videotape that explains the nuances of control setups, servos, mixes. Polk's Merlin comes with an audiotape.

SERVOS AND PRICING

Servo pricing is another matter. Tom Runge told me that an American company can't make the plastic parts for a servo at the price that the foreigners charge for a complete servo. What that means is that a lot of hand labor goes into assembling a servo, so servos may never be cheaper unless the design is simplified and/or production is automated. I see a big opportunity here for a smart designer. The motor and gears are the key. Somebody should be looking into printed circuit motors, stepping motors, linear induction motors, etc.

Perhaps you have noticed that many new systems are packaged with fewer than the usual four servos, in order to make a competitive price?

CONCLUSION — GETTING FLIGHT PACKS

If you want them badly enough, it's obvious that you can get Rx's and flight packs. If you have difficulty, it's caused by the distributors and dealers, perhaps exacerbated by the lack of synthesizers.

I've been playing the "What if?" game with this essay. If you see anything that attracts you, why not Xerox the column, circle the parts you like, then send the marked-up copy to the appropriate person named below? Be polite. I don't need any new enemies!

• Ace R/C: Tom Runge, Ace R/C, Inc., P.O. Box 511812, Higginsville, MO 64037.

• Airtronics: Jack Albrecht, Airtronics, Inc., 11 Autry, Irvine, CA 92718.

• Futaba: Ron Heidt, Futaba Corp. of America, 555 West Victoria St., Compton, CA 90202. (Futaba may have relocated to an Irvine, CA address by the time this is printed.)

• JR: Kevin Burner, Hobby Dynamics, 4105 S. Highland, Champaign, IL 61821.

• Polk's: Gil Rose, Aristo-Craft/Polk's Model Craft Hobbies, 500 5th Ave., New York, NY 10010.

• World Engines: Jim Lanterman, World Engines, 8960 Rossash Rd., Cincinnati, OH 45236.

If they get a few thousand letters, it just might make some kind of an impression. It would make more of an impression if the letter could truthfully say, "I am interested in your PCM Super Whatever, but it would be useless to me without additional flight packs at reasonable prices."

Transcribed from original scans by AI. Minor OCR errors may remain.