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Radios - AM, FM, PCM - Which Should I Buy?
Author: Bob Steele


by Bob Steele

Buying a new radio? What type do you plan to buy? Do you know what criteria to use in judging radio types or brands? I get a lot of questions on buying radios and I hope to clear some of them up in this article. The first and most important thing to realize is that the quality of the radio you buy can determine how long you keep the airplane you intend to fly it in. Most likely, you have a substantial investment in both time and money in your model and you don't want to see it crash and have your investment wiped out! We're not necessarily talking about price here, but quality.

A great deal of work has gone into receiver (hereafter abbreviated RX) design since the AMA began its frequency utilization plan a few years back and it has paid big dividends! The new Dual Conversion and/or ABC&W RX designs are interference free except when someone turns on a transmitter (abbreviated TX) on your channel. In some cases, metal to metal electrical noise may cause a problem. I would urge you to check the manufacturer's RX specifications to be sure they meet industry standards. Compare one company's specs against another company, so you can be sure what you are buying! If you can't obtain specs for a manufacturer's RX, then don't buy from him!

Now, on to the transmitter! There are basically three types of TX's available, the basic four channel, the six channel with dual rates and maybe some other bells and whistles, and then there are computer programmable radios. (PCM radios have a computer in them, too, but we'll leave that until later.) Computer programmable radios are used by sailplane flyers, most all serious pattern and helicopter flyers, and a lot of sport flyers who want the versatility and 4 to 8 aircraft memory offered by these computerized marvels. Most beginngers start by buying the basic 4 channel setup, and if they stick with the hobby, switch to one of 6 or 7 channel rigs as their skills increase and they demand more from their radio. The gimbals in the TX stick assemblies are the first link in the electronic chain that conncts our hands to our aircraft control surfaces. These stick assemblies are critical to good control and should be of the best quality. Buy something that feels good to you and whose servos track the stick movements well!!

Which brings us logically to the servos! I have stopped using non-ball bearing servos except in those instances where they will be installed in a small (under .35) powered plane or a 2 meter glider. Servo resolution is much better and you do not encounter the problem of the top case output drive hole wearing egg-shaped as they do in an unbushed case. If possible, take the next step and get a coreless motor and it will cost you MORE money and more current drain during the flight, but the better resolution is worth it! Here again, it's a question of protecting your investment by buying quality.

Back to the original question -- AM, FM, or PCM? I still fly a lot of planes on AM and have not had any trouble. I use dual conversion receivers only. There is no reason to give up on this type of equipment if you already own some. If your favorite TX is an AM only TX, just make sure you use good RXs and go on flying AM. Suppose you're buying a whole new outfit? Leaving the compatability question out of it, I would opt for plain FM not PCM since you will save $40 to $60 that you could better spend on good servos! All you are buying with PCM is failsafe in case you get interference. I have seen one plane saved by failsafe (because it never left the ground) and watched two others go straight in under failsafe so my choice is no PCM. One thing we need to straighten out here! I recently read one article by a supposed expert who stated that PCM radios used 1024 bits of resolution while FM or PPM used only 512 bits of resolution and that made PCM better! FM or PPM doesn't use any bits at all! PPM stands for Pulse Positions Modulations and is infinitely variable. Incidentally, AM radios also used PPM as a modulation scheme. The radios which do use 512 bits are older PCM types which I believe are no longer available. PCM stands for Pulse Code Modulation and seems to have been developed to eliminate certain types of interference back before we had dual conversion receivers. Its reason for use has therefore disappeared. What pulse code modulation does is convert the analog signal from the TX stick into a true binary code by means of a microprocessor chip in the TX. This binary code is then sent to the RX, where by means of another microprocessor chip, it is converted back to an analog signal and forwarded to the servo for which it was intended. It is interesting to note that ALL servos are basically identical, whether they are to be used with AM, FM, or PCM radios. Although you may have to change connectors, you can use any servo with any radio.

So, it's still your money and your choice, but as far as I'm concerned, I'm not wasting my bucks on PCM when I can spend it on better servos, a new engine, or maybe even a new airplane!

[from Flying Circuits, Inc.]


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