Radio Controlled Hobbies - RC 2.4 GHz Radios

Ever since the first radio control systems for models were built over half a century ago, the technology has been "narrowband". Narrowband refers to the amount of space that signal takes on the spectrum of available frequencies. Today's FM/PCM radio control systems operate on a tiny sliver of space on relatively low frequencies (27, 35, 36, 40, 41 or 72Mhz).

In radio terms it means that almost any other signal on the narrowband frequency you're using will result in interference (glitches or lock-out). Clearly this isn't the best situation for controlling a potentially expensive and sometimes dangerous radio controlled model but, with careful channel management, it has served the RC Hobby well for decades.

2.4 GHz Manufacturers of spread spectrum (SS) radio systems are claiming that you need never worry about being shot down by other fliers and that all 2.4GHz systems can get along in harmony, despite apparently using the same frequencies. The first and most common type of 2.4GHz systems is what we call Direct Sequence Spread Spectrum (DSSS). This involves the transmitter and receiver staying within a fixed part of the 2.4GHz spectrum. The second type is called Frequency Hopping Spread Spectrum (FHSS) and involves having the transmitter and receiver constantly changing their operating frequency within the allowed limits of the 2.4GHz band.

Spectrum technology offers some very clever ways to reduce the effects of interference and allow many different radio sets to operate simultaneously without the need for a frequency peg. Thanks to the way these systems spread their signals thinly across the 2.4GHz band and thanks to the way some of them hop around so as to remain a moving target, it takes a very strong interfering signal to have any effect. DSSS system can be knocked out if the strength of an interfering signal on that channel is strong enough.

Reputable manufacturers realize that their systems may be in control of very large, expensive and potentially dangerous models so they try to allow for as many contingencies as possible. Futaba uses constant frequency hopping, JR/Spektrum uses a backup channel (a tactic known as redundancy). Another important aspect of 2.4GHz spread spectrum radio control systems is something called diversity. Diversity is required because the radio signals at 2.4GHz behave quite differently to those we're used to on lower frequencies such as 72MHz.

Whereas the old narrowband frequencies will pass right through most objects such as houses, trees, fences, and model airplanes, 2.4GHz behaves much more like light, being either absorbed or reflected by many parts of the environment. This absorbing and reflecting of the 2.4GHz signal results in occasions when the receiver antenna may be shielded by some part of the model, or may even be subject to the kind of ghosting that used to be seen on old TV sets when the signal was reflected by trees or buildings (called multi-pathing). The effects of shielding and/or multipathing mean that it's quite possible the receiver will be unable to hear the transmitter clearly enough to extract the data being sent. The simplest (and best) solution to this problem is to use more than one antenna and/or more than one receiver in your model. By mounting these antennas or receivers in different places (even just an inch or two apart), one can take over if the other is unable to get a clear signal.

The JR/Spectrum system allows for multiple receivers, up to four or more and some of these receivers have multiple antennas. This is surely the ultimate diversity setup. On very large models, you can be absolutely sure that there's no chance of shielding or multi-pathing by simply increasing the number and distribution of receivers within the plane.

The Futaba FASST system uses two antennas mounted on the one receiver. In theory this isn't as good as the JR/Spektrum option but in practice it seems to work perfectly adequately.

It is true that you can't be shot down by another 2.4GHz radio control system but there is still always a chance that other forms of interference can cause you to lose control of your model. The 2.4GHz band is used by a very wide range of other electronic equipment from wireless internet to microwave ovens. There's no guarantee that one of these other devices won't interfere with your RC set.

Spread spectrum radio sets work in a way very similar to PCM ones in the way they respond to strong interference. If you're unlucky enough to experience interference so strong that the link between transmitter and receiver is lost, your receiver will enter "hold/lockout" mode and then go to fail-safe mode (if set). The cause of such a lockout/fail-safe can be almost anything including, but not just limited to, interference. In fact, in the case of spread spectrum systems, experience has shown that lockouts are far more likely to be caused by inadequate batteries in the model or bad installation.