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Written by Tony Stillman
In the Air
Column
As seen in the September 2014 issue of
Model Aviation.


If you are fairly new to the RC modeling scene, you might not have any idea what a 72 MHz RC system is. More experienced modelers will know exactly what they are and have probably owned several.

The 72 MHz digital proportional RC systems have been around since the 1960s. Evolution of this equipment provided us with first Amplitude Modulation (AM) sets, then Frequency Modulation (FM), and then the latest versions, which were Pulse Code Modulation (PCM).

This progression gave us a more reliable link between the transmitter and receiver and provided fail-safe operation in the PCM systems. The big drawback with these systems was that they operated on a specific frequency channel within the 72 MHz band, and it was vital that only one system be operating at any given time on each channel.

To ensure safe operation, a system of frequency pins was developed. You were issued a pin when you wanted to operate your transmitter. Each club had a similar system at its flying site. Before turning on your transmitter, you had to get the frequency pin from the frequency board at the flying site. You usually shared the pin with others and kept track of those at the field who were flying on the same frequency as your own, to ensure that you did not cause interference and “shoot” the other pilot’s aircraft down by mistake.

The FCC gave us 72.000-73.000 MHz as our area for operations of RC model airplane equipment. In the late 1980s, AMA worked with the FCC to obtain permission to expand the number of usable channels we enjoyed from 7 to 60. The FCC did not give us any more frequency bandwidth, but allowed manufacturers to make “narrow-band” equipment so that it could safely operate more frequencies in the same amount of bandwidth.

We now have frequencies starting at 72.010 MHz, and then spaced 20 KHz away is our next frequency, 72.030 MHz. This spacing continues throughout the band to give us Channel 11 (72.010 MHz) through Channel 60 (72.990 MHz).

Having more channels allowed more pilots to fly at the same time with less worry of interference. These additional channels fueled a growth in RC. Along with the new frequencies, improvements in encoder design led to many enhancements in transmitter design.

New features were added including programmable mixing, preprogrammed mixing, and exponential. The latest designs included microprocessor-based transmitters that were capable of memorizing complete model settings, for the first time allowing a transmitter to operate more than one aircraft. Multimodel memory exploded onto the scene and transformed sport radios into sophisticated, affordable transmitters with new features finally made available to the masses.

So, what has changed? As far as 72 MHz RC operations today, nothing has changed since the 1991 narrow banding. The 72 MHz equipment that meets these requirements is still legal to operate today and will be legal for the foreseeable future. The FCC regulates radio frequencies in the US and we work closely with them on anything that would change the frequencies we are allowed to use. At this time, nothing is in the works that would change the current rules.

With the introduction of 2.4 GHz spread spectrum equipment, the need for frequency pins has nearly disappeared. Now you can go to the flying field and not worry about interference from other RC pilots if you are flying on 2.4 GHz. As soon as this technology was proven, many modelers began trading their 72 MHz gear for spread spectrum gear and demand for 72 MHz systems plummeted.

Manufacturers of RC radio equipment have determined that sales of 72 MHz RC equipment have slowed to the point that it is no longer profitable to continue to offer it to retail consumers. They have discontinued manufacturing this equipment, and are pointing to 2.4 GHz RC equipment in its place.

Obviously, 2.4 GHz equipment cannot be interfered with by other users of RC model aircraft systems (as designated by FCC rules) so 2.4 GHz RC equipment is deemed inherently safer and more current. Some even refer to 72 MHz RC equipment as obsolete and unsafe.

Nothing could be further from the truth! It could be argued that 72 MHz RC equipment is safer today because so many fliers are using 2.4 GHz RC equipment making the 72 MHz band less crowded, so there is less possibility of interference from other RC model fliers.

With many good used RC radio systems for sale, many modelers purchase this gear to save money and to obtain a system with more features than what they currently own. This makes good economic sense! Although a company may not be making new equipment on 72 MHz, there is a large amount of it available, and good bargain hunters can save money while obtaining a quality RC system.

The AMA has not changed its stance on 72 MHz equipment, and will continue to work with the FCC to keep 72 MHz, as well as the other frequencies, available to modelers. Unless a particular event or club decides to restrict operations to certain frequencies, 72 MHz (as well as 27 MHz, 50 MHz, 53 MHz, and 75 MHz) will continue to be available to modelers in the US.

If you are looking for a good deal on RC equipment, consider purchasing used 72 MHz equipment. Plenty of it is available at swap meets, flea markets, and online. Many club members still have good gear that they no longer use.

With so many modelers operating on 2.4 GHz spread spectrum, 72 MHz is a good bet today. Most of the time you will find that you are the only one at the field on 72 MHz, so you have your own private frequency. Isn’t that just as good?

72 MHz is alive and well, but you probably won’t find it at your favorite hobby dealer.

—Tony Stillman
Flying Site Assistance Coordinator






26 comments

Tony,

Thanks for a most timely article. In fact just recently someone in the club asked if his 72 MHz equipment was still legal. I assured him it was and would be for the foreseeable future. As I read your article I was pleased to see that the answer I gave was correct.

This is a great article and very true. In the past year I've bought a lot of formerly expensive 72Mhz gear at incredibly cheap prices on ebay. It still works for me. As of now, batteries are more expensive than the transmitters that they power.

Transmitters on 72 MHz have a collapsible antenna. If you forget to extend the antenna, like I did, the range of the transmitter will be greatly reduced and you may lose control of your aircraft.

After reading this I was taken back by the idea that 72 Mhz could still be used. It can only be used in a fun fly situation where the pilot can be provided with frequency monitoring clips. The drawback of this article is that no competition planes can be adjusted and flown on 72 MHz because all contests/contest directors REQUIRE all entered planes to be on 2.4 GHz. This requirement for contest mandates that 72 MHz be eliminated as a frequency choice because 2.4 GHz does not require frequency monitoring. This article is misleading to the reader and may only apply to those pilots and planes that never advance past the Sunday afternoon flying stage.

It may come as a great suprise for a lot of you, but not all of us compete. 72 mhz is accepted at our field and as a matter of fact has proven more reliable that 2.4 There are some definite problems with some of the radio systems being sold today and I personally have put mine back on the shelf and gone back to 72mhz!

Obviously you fly in contests and have been forced to adapt to 2.4 g Radios. I would like to add that not all of us that fly R/C are competition flyers! As a sport pilot who flys mostly at a small club field that DOES still allow 72 mhz, I choose to stay with the equipment that I am familiar with and works well for me. Oh, by the way, I'd like to give a hearty THANK YOU to all that have left 72 mhz to me!

Another good way to economize if you already have a 72 megahertz transmitter with all the bells and whistles is to purchase a 2.4 mhz plug in module. The bells and whistles still work the same but now you are operating without fear of interferrence.

2.4 MHZ will not penetrate water, doesn’t work for R/C submarines.

Tony is absolutely right about 72 MHz being safer to use nowadays because so few people are using it. I still fly with 72 MHz radios and never worry anymore about getting shot down. After over four decades of both 27 MHz and 72 MHz radio systems, something just doesn't seem right with a transmitter that doesn't have an antenna sticking out of the top of it ;-)

- Kirt Blattenberger

And it makes a very handy wind sock if you put a ribbon on it! Can't do that with 2.4!

Can you still compete on 72 mk in any type of contest ?

I've been an "on-and-off" RC enthusiast for four decades now, and am back with the AMA once more!

Since I've had a Ham license for four decades now, I long ago made the move to six meter gear, and the "relic" 53 MHz gear I've still got (just two AM RF decks and compatible receivers) would still be quite usable in ground-based vehicles (land yachts-"iceboats with wheels"-are what I've thought of as ideal!). Due to the fact that I can ONLY fly with "knobby" or "single-stick" RC transmitters that I've almost always built for myself since I started in RC in 1977, I'll be using Gordon Anderson's great MicroStar 2000 (abbreviated "MS2K") computer-style Tx encoders (at http://50.62.233.209/microstar-2000 ) in all of my latest examples. My first pair of knobby MS2Ks are for six-meter VHF-band Ham flying, using the earlier Mk.III version encoder which was "current" about Y2K, with the latest Mk.V encoder going into a pair of "UHF-only", spread-spectrum knobbies — twin-band ones, for 13 cm/2.4 GHz license-free flyin' using FrSky's DHT and their Rx-es, and 70cm/433 MHz Ham band third-party SpredSpec gear (possibly DragonLink) to give me some "flexibility" to fly on UHF bands in an "either/or" manner. The pair of VHF six-meter Ham MS2Ks will each get upgraded to the MS2K Mk.V as I can afford them, and I've already got the dedicated MS2K six-meter synthesized Tx RF decks for each of those, to use with either the Mk.III or Mk.V version MS2K encoders.

Flying on the Ham bands has always been "free'n'easy" for long-time licensees...the four-meter band (72 MHz) RC flyers are nowadays almost AS rare as the Ham RCers have always been. Getting receivers - the hardest thing for the six-meter Ham fraternity - for the older four-meter band is getting just as hard as finding six-meter ones, with virtually the only "new-build" choice for my fellow VHF-band Ham RCers that remains, being Gordon's own-design six-meter receivers at http://50.62.233.209/pulsestar . The four-meter/72 MHz guys can still get used receivers, that can usually be used between brands (JR and Airtronics/Sanwa were positive-shift FM, Futaba negative-shift FM), but getting used receivers on 72 MHz can always be a "dodgy deal" in the 21st century as time goes on.

I'm looking forward to going with the "twin-band UHF-only" knobby radios I'll eventually be building, in time...for the present, getting my lone completed six-meter (of the matching pair I'll have brought-together by the end of 2019) MS2K knobby radio re-powered with fresh NiMH cells and "on the air" again is the main concern, in getting back to RC for the first time since 2003.

Great article Tony. I wholeheartedly agree with your position. I never had any issues with 72 MHZ radios, and now that most people fly on 2.4 GHZ, the chances of interference are extremely low. I flew at a club in Houston 3 years ago that allowed it, and never once had anyone asking to use my frequency flag. Though my current club in Maryland mandates use of 2.4 radios, I was pleased to see last year that the AMA site in Muncie still allows use of 72MHZ. I wish more people felt this way, but the manufacturers have established a position that's difficult to ignore, especially for those entering the hobby who don't know any better.

Glad to hear that some others still use 72Mhz as I do from time to time. I like the idea that when the batteries start to go low the ]surfaces just move slower allowing time to land. When a 2.4 gets too close to low voltage everything just stops dead. Using 5 cell packs with higher capacity has fixed most of those low voltage problems. Two of our 3 area clubs still allow and use 72. The third is just too close to another so...

I was under the impression that 72 KHz has a longer range than 2.4GHz, is this correct?

72MHz is UHF and 2.4 is Microwave. The spectrum spread of UHF is close on ground or " bounce back" like donuts shape while Microwave heads up high to the sky that cause 2.4 GHz receivers have more antennas or bidirectional for reception. We can say 72MHz is longer range than 2.4GHz the RC application specifically 72MHz has been used for long range FPV.

Very thankful for this article on 72MHz!
Proper installation of the airborne system, plenty top quality foam rubber, batteries in A1 condition and always begin the day flying on a FULL FRESH charge! 72MHz has performed well, and I hope it will continue for as long as possible!

Great article, Tony! I still have my 72 MHz radios because I prefer to fly single stick - what I learned with back in the 60s. My biggest problem is maintaining the transmitter batteries. An article about installing a modern Lithium polymer transmitter battery into these old but still legal 72MHz systems would be much appreciated.

Dave Gierke

This discussion on 72Mhz makes it look like the move to 2.4G is just a change in operational frequencies. The move to 2.4G spread spectrum is vastly more than a change in frequencies. The use of spread spectrum is what makes it possible to eliminate the need for frequency pins. The technology behind today's SS equipment brings digital coding techniques that are not done in 72Mhz systems. Also, 2.4 systems bring us packet telecommand, packet telemetry, error detection and correction (EDAC), coding gain, etc. This is a very significant technological improvement over 72Mhz.

If it ain't broke don't fix it!! I still fly 72 MHZ radios and they work great .So do my Nicad batteries!!

Great article. I fly on both 72 and 2.4 and still probably have 27 somewhere. What I find ironic is that most of the people racing FPV quads (myself included) are right back to using the control board pins, as they typically use 4 to 8 channels at 5.8 GHz for the analog video transmitter to goggles or screen. DJI and some 3rd parties are starting to provide digital video transmitters, but they are still expensive and non-standardized. Deja Vu all over again! I've been playing with some forms of remote video from my planes/gliders for almost 20 years, starting at 900 MHz. Nice that these can still all co-exist.

How can I tell if my 72Mhz transmitters and receivers are narrow band?

Great piece Tony - enjoyed hearing about those ole radios. I still have my Citizenship one channel but havent flown it for about 5 years now. Used to take it out just to feel young again. Also have a number of 72'ers and 27's along with my flock of 2.4's. Enjoy them all and its good to see that AMA will keep the 72's alive - there is so much of it out there!
Ron

As a new R/C flyer, I really enjoyed the fine explanation of those “old radios with long antennas “ that I have only seen in old club photos.

At the NCRCC club in Ellington Ct we have flyers who still fly on 72 Mhz, mostly older flyers. With age comes memory loss, and I have caught two pilots taxiing their planes with their transmitter antenna down, and warned them before they took off to raise the antenna. Others have been lucky they finished flights before realizing they never raised their antennas. And unfortunately some have not been so lucky. As older flyers leave the hobby you will find that 72 Mhz will leave with them. With all the advantages of 2.4 systems I see know reason to fly on 72 Mhz, and leave my planes "life" in the hands of fellow club members. I do not know how many times I saw people turn on a transmitter in the pit area while a flyer was flying and cause trouble. As far as buying used goes, we have a few flyers who buy used transmitters, receivers, and servos and more than once I have seen them have problems. As far as I am concerned the motto is "Buy new, not trouble."

Good article. I still have several models I fly on 72Mhz at my local fields. I'm usually the only one on 72! I like the "built-in" wind sock with a frequency flag on the end of the antenna. I still prefer the "old" mechanical trims to the new electronic ones, but I only have one transmitter these days with that feature. If you plan to fly competition, you will have to use 2.4Mhz - but that's okay.

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