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Committee metings are held in May and November. If you are interested in joining, ask for details.
Five frequency bands are available for the control of models. 26.96MHz to 27.28MHz and 458.5MHz to 459.5Mhz bands have been available for a considerable time but due to interference - particularly on the 27MHz band - two additional bands were approved. 34.945MHz to 35.305MHz was allocated for aircraft use only and 40.66MHz to 41MHz for other (surface) models. The conditions of use are as published in the Wireless Telegraphy (Exemptions) Regulations 1999 (S.I. 1999 No.930) and amended by S.I. 2003 No.74 (Schedule 6). All equipment has to be type tested in accordance with ETSI Standard EN 300 220 and meet the EMC conditions of EN 300 683. The most recent addition is the 2.4GHz band. For more details see here.
From the modellers point of view all tested equipment has the "CE" mark added by the supplier to confirm it meets the Standards. It has a maximum radiated power of 100mW and is capable of working with similar R.C. Equipment at a channel spacing of 10kHz (25kHz on 458MHz band). On 27MHz band, some existing equipments are not capable of working on a 10kHz spacing. Check with other users before operating. No radio licence is required for this equipment.
A copy of the official information sheet (OfW 311) issued by the OFCOM is available at http://www.ofcom.org.uk/radiocomms/ifi/licensing/classes/rlans/technical/ofw311/
Statutory Instruments (S.I.) are available at www.legislation.hmso.gov.uk
Copies of ETSI Standards are available from:- BSI Customer Services, BSI Standards, Cheswick High Road, London W4 4AL Tel: 020 8996 7000
Frequency Modulation and Crystals
To send information, the transmitter has to be modulated. The two forms of modulation currently in use are known as Amplitude Modulation (A.M.) and Frequency Modulation (F.M.).
Amplitude Modulation is achieved by switching off the transmitter for brief periods (about 250 microseconds) and is the form of modulation most experienced modellers will be familiar with, particularly on the 27MHz band.
Frequency Modulation has some technical advantages, particularly working at the close channel spacing now being used. The modulation requires the transmitted frequency to be changed slightly (about 1.5kHz) for short periods that can be detected by the receiver. Because the way this is achieved varies between different manufacturers and types of equipment, the transmitter crystal used may vary in frequency from the nominal channel frequency. Care must be taken by the modeller to insure the pair of crystals he is using is appropriate for the particular radio in use. Alternative crystals may work but range is likely to be reduced or the transmitted frequency may deviate considerably from the nominal channel frequency causing interference with other modellers' radios. Receiver crystals are also different for standard and dual conversion receivers. It is strongly recommended that only transmitters and receivers of the same type and manufacture be used together. If a modeller decides to use an alternative receiver it is his responsibility to ensure the equipment is compatible.
Having fixed the frequency and modulation method, the information has to be sent from the control sticks to the servos in a way they can understand. Two methods are currently in use and are known as Pulse Position Modulation (P.P.M) and Pulse Code Modulation (P.C.M.).
In Pulse Position Modulation the length of time which corresponds to the position of the servo is sent as a string of pulses, the time between adjacent pulse determines the position of each servo. When the information for all the servos has been sent there is a pause for the system to reset and a startup pulse begins the process again - including any changes in the position of the sticks during the pause. The time this takes is known as the frame rate and there are usually about 50 frames transmitted per second. Each pulse is separated by a time of between 1 to 2 milliseconds as required by individual servos.
Pulse Code Modulation takes advantage of micro-controllers to send and receive the information as a digital code. The position of each servo is encoded as an 8 or 10 bit number. This is transformed back to 1 to 2 millisecond pulse by the receiver and passed to a standard servo. The encoded number includes information which enables the receiver to detect any errors in the information due to interference or loss of signal. If this happens the receiver switches to a "fail safe" code generated by the receiver. As supplied, the receiver will normally hold the servos at their current position but the modeller can change this to a "preset" mode. In particular, the engine servo should close the throttle on the loss of signal. A more detailed explanation on P.C.M. Fail-safes is available from the British Model Flying Association.
Fail Safe Systems for P.P.M.
Some models are required to have a "fail safe" fitted. A separate unit can be connected to a P.P.M. set to achieve the same setup as that provided in P.C.M. Equipment. If you change from P.P.M. to P.C.M. this add-on unit will no longer function and must be disconnected and the P.C.M.'s built in fail-safe programmed to replace it.