Keep in Touch Series
The Modern R/C Radio System Explained, Part 6

Spread Spectrum Radios

I suppose the biggest thing you will notice when someone brings a Spread Spectrum radio to the field is that he or she does claim the channel with a channel pin. That's strange you might think. The other thing you will notice is the short antenna. Even stranger. Let's address the short antenna first, then we'll move onto other things.

Carrier Frequency :

The "Carrier Frequency" is the frequency of the basic radio wave that is used to send information over a radio link. When you dial you car radio into your favorite program you select a specific number on the dial, let's say FM 105.7. This means that your program is being transmitted on 105.7 MegaHertz (MHz). And that means that the voltage on the antenna cycles from a positive voltage through a negative voltage back to the positive voltage 105,700,000 times per second. In the US the most common frequency for Radio Control is 72 MHz for aircraft and 75 MHz for surface vehicles although other frequencies are available (50 MHz, 27 MHz). Other countries may use different frequencies.

Since all radio signals travel at the same speed, a lower frequency causes fewer cycles to occur for a given distance. Therefore the length of each cycle varies with frequency. The higher the frequence, the shorter the "Wave Length". To efficientlt transmit a radio signal, the antennas have to be made to a particular length that is some proportion (Often 1/4) of the wave length of the radio signal. Therefore, the higher the frequency, the shorter must be the antenna.

 

Spread Spectrum:

Spread Spectrum radios use 2.4 GigaHertz (GHz) (That's 2,400,000,000 cycles per second) That's about 30 higher that our old 72 MHz standard. So that explains the tiny antenna.

Here's the problem. 2.4 GHz (Or as it is usually call - 2.4 Gig) is an unlicenced frequency. Which means there are no governmental controls on who and how it is used. This means that everyone uses it, from taxi radios to microwave ovens. In turn that means there is lots of interferrence. So why is it used for radio control? The answer lies in the technology we call "Spread spectrum" With in the 72 MHz band that we use there are 50 discrete channels. We call them channel 11 through channel 60. You transmitter and receiver will work on one of those channels for the duration. However, the term "Spread spectrum" refers to the technology that allows the channel (Frequency) to change when appropriate. That means that if I'm working on one channel (frequency) in the 2.4 GHz band and I get interferrence, the radio will simply change channels.

Encoding is another technology that is used in our Spread Spectrum radios. This is the technology that sends digital codes as well as the servo commands over the radio. If each transmitter creates different codes, then a receiver that's working with one transmitter will ignore all other transmitters. The process by which a receiver is identified with a single transmitter is called "Binding"

The benefits of spread spectrum are that when I turn on my receiver and transmitter, if it detects another radio on the same channel, mine will simply change to a different channel. That's why we don't need to specify a channel as we do with 72 MHz.

At the time of writing, there are two companies in the US that offer Spread Spectrum radios - Futaba with their FASST products and Spektrum with their DX and other products.

Futaba and Spektrum implementation of the above technologies differ quite significantly.

Spektrum will select a channel and continue to use that channel until interferrence is detected and then very quickly change to a second channel that it has previously selected as being clear. Binding is done by executing a sequence that uses a switch on the transmitter.

Futaba does it in a different way. The transmitter and receiver continually change channel every 2 msec (2 one-thousanths of a second). In this way, if one channel has interferrence, it doesn't matter because that channel only working for 2 mSecs. Binding is done with a switch on the receiver.

 

Receiver Antennas:

Why two antennas on the receiver? Antennas only work well if they are in phase. That means that if the transmitter antenna is vertical, the receiver antenna should be vertical and adjacent. If you "Point" an antenna towards another antenna, the signal will be weak. Well, at 2.4 GHz, this fact is emphasized. Of course as a plane flies around the receiver antenna change phase all the time. So there could be positions in flight that cause the signal strength to weaken to the point of being dangerous. So two antennas are used on 2.4 GHz receivers. The phase of each of these antenna MUST be different, ideally they should be set to be 90 degrees out of phase. This means that as the plane changes its position so that one antenna receives a weak signal, the other antenna will be in a phase that receives a strong signal. Spektrum has one receiver pack that actually has two separate receivers each with 2 antennas.

You will find that the transmitter antenna's position can be altered. When flying you MUST make sure that the antenna is kept vertical at all times.