72 MHz radios

If you’re getting started in RC in North America it may have come to your attention that there are two general kinds of radios:  2.4 GHz modern radios, and the older 72 MHz radios.  As a matter of fact, there are more kinds than that.  2.4 GHz equipment is subdivided into categories based on the specific type of coded signal used by the various manufacturers.  These radios have the capability to switch from one channel to another within their designated frequency, thus avoiding interference when more than one person is flying.  The older style single channel radios operate on 72 MHz for aircraft only, 75 MHz for cars and boats only, and 27 MHz for any kind of RC vehicle.  There are also 50 and 53 MHz, reserved for those holding an amateur radio operator’s license.  In each of these single channel frequencies, the radios can use either an AM, FM, or PCM signal.  Here’s a good chart of all of the frequencies in use for RC in North America.  http://www.modelaircraft.org/events/frequencies.aspx

What are the differences, and why would you choose one over the other?  This is the kind of question I always ask, because I’ve noticed that in a lot of cases the status quo or conventional wisdom do not necessarily reflect the best choice.  We’ve all noticed that the majority of the RC community has switched over to 2.4 GHz, and there are some good reasons for this shift, but it’s not a slam-dunk case one way or the other.

First, let’s go through the pros and cons of the old technology.  On the positive side, these frequencies are licensed by the government for use in controlling RC vehicles.  The transmitters can legally broadcast at 1 watt, which provides a very respectable range.  You can still control your plane as far as you can see it.  The long waves of the 72 MHz band can bend around objects, so if you fly a plane behind a barn, a car, a tree, or even a low hill, you are still in control.  The receiver has a long antenna, which can pick up a signal no matter the plane’s location or position.  The antenna can’t be shadowed by the engine, and it is still effective in the vicinity of carbon fiber and metal objects.  72 MHz also has some disadvantages.  That long, effective antenna has to be routed through the fuselage.  Only one radio at a time can be operated per channel.  There are 50 channels, but chances are that duplicates will arrive at the field at the same time.  Even if you practice good frequency control, some doofus can show up later, turn his radio on, and shoot your plane down.  AM and FM signals are subject to jamming interference from outside sources, sometimes even from interference patterns created by combinations of other frequencies.  And of course the most common problem experienced when using 72 MHz radios is damage to the receiver crystal caused by crashing.  Back in the old days you would just do an engine-on test, and if you got a glitch you would install a new crystal.  But it’s hard to get new crystals nowadays.

2.4 GHz radios were offered to the public as the answer to the shortcomings of 72 MHz radios.  No frequency control is required because your radio will simply switch itself to another channel to avoid interference from identical radios.  If unmanageable interference is encountered, the controls will lock in “safe mode”.  And of course, as we’ve all heard many, many times, there is no long antenna to fool with.  That pretty much sums up the advantages.  There are a few major disadvantages.  The short wavelength causes these radios to operate only in line of sight.  In other words, if you fly behind an object and you can’t see your plane, you’re not controlling it.  If your antenna gets shadowed by the engine, electric motor, battery pack or other metal object in the plane itself, it will lose signal.  This risk is mitigated by using a receiver with more than one antenna, but that seems to negate the advantage of the tiny antenna if you ask me.  When the receiver loses signal it goes into the aforementioned “safe mode” which means that it neutralizes the controls and you can’t do anything until it reboots.  If your plane was in a funny position, it will just keep going in that direction until it hits something.  Another disadvantage is that there is no government license for RC use of the 2.4 GHz band, which means that these radios can broadcast at only one tenth of the power of a transmitter on the licensed bands.  You probably don’t want to fly your plane to the extent of your control radius because you’ll lose sight of it, but the range technically is shorter than what you can get from 72 MHz.

The reason the new radios operate on 2.4 GHz is because of microwave ovens.  When the microwave was first offered for sale, it was set up to operate on this frequency to avoid interference with other bands.  Ovens don’t put out tons of radio noise, but there is always some, so these frequencies have been left open as a Wild West no-man’s land.  60 years later, everybody who wants to sell a radio control airplane, garage door opener, blue tooth, or other gadget, goes to the same unlicensed frequency.  Will it reach a saturation point and cause an intolerable level of safe mode lockups?  Time will tell.

So, what does all of this mean to you, the prospective radio user?  Should you follow the herd to 2.4 GHz, or should you use an old 72 MHz radio like some kind of hipster?  That all depends on your personal situation.  A lot of flying clubs allow only 2.4 GHz nowadays, to eliminate any possibility of interference.  Clubs used to have transmitter impounds, frequency flags, frequency boards with clothespins, or other such frequency control schemes to keep people from shooting each other down.  It sure seems easier to just tell all of the members to leave their 72 MHz gear at home.

What if you fly at an all-frequencies club?  If you fly at a small club or with a few friends at somebody’s pasture, or by yourself out in the boondocks, you can pretty much use whatever radio you have on hand.  Just ask everybody present if there is anybody else on your channel.  If you consider the fact that we had tons of fun with 72 MHz single channel FM radios for several decades before 2.4 GHz came along, it’s hard to believe that 72 MHz is as bad as a lot of people say.

For my own part, I have about a dozen old 72 MHz radios that I use regularly.  I have a few old receivers with crystals in them, but mostly I use Corona receivers from ebay.  You turn the transmitter on, push the button on the receiver, and it’s locked to that frequency.  It can be changed whenever you want.  Recently I decided to get a 2.4 GHz radio to find out what all the excitement was about.  I wasn’t impressed.  It’s just a radio.  I crashed a plane when the engine came between the transmitter and the tiny receiver antenna, but that was my own fault.  I didn’t realize until after the fact that this was an issue and I was supposed to use a receiver with more than one antenna.  I wouldn’t have guessed that, because it still seems kind of silly.  Aside from that, there really isn’t any difference.  What it all comes down to for me is that I can buy old transmitters for five bucks, get a receiver for $20, and I’m in business.  There’s only one other guy around here who uses 72 MHz, so we just make sure we’re not on the same channel.

If you decide to use a 72 MHz radio or radios, here are your rules.  First, open the back of the transmitter and check the battery leads for corrosion.  Equipment left to sit will develop black wire syndrome, and it will crash your plane.  Replace the batteries if you suspect a problem.  After your radio is definitely working correctly, take it to your flying site and do a radio check.  Check for proper operation at a distance, using a helper, to make sure it works at full range.  If you find a problem with interference, don’t use that channel.  Switch to a different one, or get a different radio.  Back in the old days different flying sites sometimes had a channel or two that were known to be glitchy, and everybody avoided those channels at those specific sites.  After you determine that your radio will work at your field, whenever you fly just make sure nobody is on your channel at the same time.  Don’t worry, we all had to follow these rules for 50 years and it wasn’t a problem.