FREQUENCY CONTROL

The Achilles heal with the use of RC Gear originates from Interference caused by other users, transmitting on the same frequency. In a well organised club, there should be a suitable Frequency Control system in operation at all times.

Recent advances in technology have enabled the use of the 2.4GHz frequency band, and a newly developed modulation system which should eliminate outside interference and the need for a frequency control system. However, until all users have adopted this new technology, a Frequency Control system will remain a requirement. For further information about this latest technology, please see the bottom of this page (2.4GHz Spread Spectrum Technology)

35MHz Band

Firstly we must ascertain which frequency your Radio Gear is operating on. This frequency is displayed on the crystals e.g. 35.100.

Picture of typical crystals.

To simplify matters, the available frequencies on the 35MHz band have been given channel numbers from 55 through to 90. If this number is not already displayed on the crystal, you may identify which channel number is applicable to your frequency, by using the following table:

34.950 - Channel 55           35.130 - Channel 73
34.960 - Channel 56           35.140 - Channel 74
34.970 - Channel 57           35.150 - Channel 75
34.980 - Channel 58           35.160 - Channel 76
34.990 - Channel 59           35.170 - Channel 77
35.000 - Channel 60           35.180 - Channel 78
35.010 - Channel 61           35.190 - Channel 79
35.020 - Channel 62           35.200 - Channel 80
35.030 - Channel 63           35.210 - Channel 81
35.040 - Channel 64           35.220 - Channel 82
35.050 - Channel 65           35.230 - Channel 83
35.060 - Channel 66           35.240 - Channel 84
35.070 - Channel 67           35.250 - Channel 85
35.080 - Channel 68           35.260 - Channel 86
35.090 - Channel 69           35.270 - Channel 87
35.100 - Channel 70           35.280 - Channel 88
35.110 - Channel 71           35.290 - Channel 89
35.120 - Channel 72           35.300 - Channel 90

The frequency control system that we employ is known as the “Peg-Off” system.

The pegboard displays all the channel numbers (55-90), each with an appropriately numbered peg clipped on the board. To use a channel, you must take the correct numbered peg off the board and clip it to your transmitter aerial. You may then switch your transmitter and model on.

You must not switch on, without obtaining the appropriate peg.

When you have finished your flight, you must ensure that you have switched your model and transmitter off. You should then immediately return the peg to the board. This allows others to know that the frequency is now available for use.

The B.M.F.A. advise that this system is certainly the most common and the most favoured by its members. The psychological effect of the user being in “control” of the peg (and therefore frequency) is very strong and it is generally a very good system. It is however by no means infallible. The successful operation of this system requires members to remember to return their peg to the board after they have flown, to prevent pegs from going missing. There must also never be more than one pegboard on display at the flying site, to avoid confusion as to the one in operation. Switching on without checking the availability of your frequency can have very serious consequences, as can forgetting to switch your equipment off following a flight. We therefore cannot sufficiently stress the importance of your vigilance when using this system.

As we are unable to have a permanent pegboard on site (due to possible vandalism), it is crucial that in the event that you arrive at the flying field first, you have a pegboard system to display. They are extremely simple and cheap to make.

To enhance safety, our club owns an electronic frequency scanner. This system is used as a secondary system to the pegboard as a means of ensuring that the frequency you require is not already in use. The scanner will also display any forms of possible external interference, or cross channel interference created by faulty crystals on other frequencies. Whilst it is not always possible to have the scanner available on a normal flying day, it is always used at club events.

A final note regarding crystals. You may find that the frequency that your RC system was supplied with is used by a large number of members and as such is often unavailable. In this situation you can observe which frequencies are not in regular use and simply purchase crystals on one of those frequencies. Ensure that you only use genuine crystals, recommended by your RC system’s manufacturer. If you do change your frequency, you must ensure that you know exactly which peg to remove from the pegboard. We would recommend that you choose one frequency and use this exclusively. This is because regular changing of crystals can have serious consequences should you forget which frequency you have installed, thus removing the wrong peg from the board and proceeding to switch on. Crystals are also very delicate components and should not be subjected to regular removal and refitting. Similarly, certain transmitters have the crystal located in a removable module on the reverse of the transmitter. Again the module pins are very delicate and regular removal and refitting may cause damage resulting in the failure of your equipment.

Picture of transmitter with a removable module.

2.4GHz Spread Spectrum Technology

This technology has been developed over the last few years for RC usage and in the very near future, all major RC Manufacturer’s will be selling systems which operate on the 2.4GHz frequency band. In December 2006, OFCOM issued the necessary permissions and changes to UK Government legislation which allow the sale and use of approved equipment transmitting on the 2.4 GHz band at 100mw maximum radiating power. Equipment on this band is already selling in large numbers and will soon start to appear in increasing numbers at the club field. Basically, the system operates on a much higher frequency which moves the signal up beyond the range of model generated interference from electric motors, metal to metal vibrations etc. Further to this, the system “binds” the receiver to the transmitter so that it will only accept and recognize signals from your transmitter only. This therefore eliminates the potential for interference from another member’s transmitter causing your model to crash. The system does not operate on a fixed frequency like your 35MHz set will. We will not try and offer a definitive guide to how this technology works, as this is documented fully on other websites, however the system basically works using one of two main broadcast methods. The first format is known as Frequency Hopping Spread Spectrum (FHSS). With an FHSS system, the transmitter transmits a narrow band signal and rapidly jumps from one frequency to the next, spending a few milliseconds on each frequency. This is the system used by Futaba known as FASST (Futaba Advanced Spread Spectrum Technology). The second is called Direct Sequence Spread Spectrum (DSSS). DSSS systems transmit on two selected frequencies on a very wide band. Only a small portion of that band is used for specially encoded information. This is the system used by Spektrum RC. All manufacturers have assured that both broadcast methods are compatible to work at the same time as each other. There is no need for a frequency control system to be in operation (nor would one be any use!) as both systems scan for unused frequencies before transmitting a signal. It is believed that 40 users can operate at the same time therefore this should never be an issue at our site. If all frequencies are in use when a transmitter is switched on, the system simply won’t transmit until available frequencies are found.