4.1 Introduction

(a)  Before starting to use a flying site every effort should be made to determine if there is any radio interference present on the bands it is intended to use. Particular attention should be paid to other major users of the radio spectrum in the area, such as other model clubs or hospitals, factories etc. (who might be using paging systems or other high power radio frequency transmissions).

(b)  All radio control clubs (unless they are 2.4 GHz only) should have access to some means of frequency checking or monitoring. There are several 35 MHz monitors on the market which retail from around £50 to £400. These are all good value and offer a range of facilities ranging from a basic scanner up to a combined scanner/pegboard.

(c)  BMFA has several frequency monitors which are available to clubs on loan. Contact the Leicester office for details.  An alternative is a ham radio type scanning monitor which will cover all the bands we use. These are about £400 to buy new but a second-hand unit in good condition could be a good investment.

(d)  Hand held frequency checkers are also available at reasonable cost and are a purchase that is highly recommended to any R/C club that has significant numbers of 35 MHz transmitters in use. They will enable a Club to keep a regular comparative check on its members' individual transmitters and are invaluable for spotting such things as faulty crystals, wrong frequency flags etc. One model even allows the checking of receiver crystals.

(e)   If your club feels that some monitoring information on 2.4 GHz is required, please note that a USB dongle is now available that will allow a laptop to act as quite a reasonable spectrum analyser which gives good results on the 2.4 GHz band. Contact the BMFA Leicester office for details of the supplier.

4.2      Cellphone Masts and Microwaves

(a)  It has been shown that Cellphone transmitter masts may cause short range interactions with the radio equipment we use.  To be safe, do not allow models to fly within 100 metres of such masts. If there is a mast near your field, you should arrange your flying area so that the pilots have their backs to the mast and it is in ‘dead airspace’ if possible.

(b)  The UK is crossed by many low level microwave communication beams and the number of these has increased dramatically since most cellphone masts have been converted from landline feeds to microwave interconnection.

If one crosses your field it may cause problems with interference and glitching, particularly with 35 MHz radios. If your club member’s suffer from such interference regularly (usually in the same place on the field) then it may be a microwave problem.

(c)  You can guard against it completely by simply wrapping 35 MHz receivers in a layer of aluminium cooking foil, making sure to tightly crimp the foil for about 5cm out along both the receiver aerial and the bundle of servo leads. The interference affects the components of the receiver directly and doesn’t work through the aerial.  Note that some receivers already have a conductive coating of carbon sprayed on the inside of their plastic case which has the same effect as the external foil wrap.

4.3      35 MHz Transmitter Interaction Problems

Any model using 35 MHz can suffer severe interference if it flies closer to an operating 35 MHz transmitter other than the one that is controlling it. To avoid the chance of this happening Clubs should:

(a)  Ensure that all pilots stand reasonably close together when flying. The concept of a ‘pilot’s box’ is useful, even though it will not usually be marked out.

(b) Ensure that operating transmitters are not overflown. Care should be taken by the club to ensure that flying field procedures make this very clear.

(c) Take action to prevent operating transmitters being taken out on to an active flying area when, for example, models are being retrieved. Transmitters should be handed to a helper on the flightline and should remain switched on until the model has been retrieved and switched off.

(d)  Ensure that all inactive transmitters in the pits area have their aerials retracted if possible. The extension of the aerial should be one of the last actions taken when moving out to the pilot’s box to fly and retracting the aerial should be one of the first actions when moving back into the pits area with your model and transmitter switched off.

4.4      Frequency Control at Club Sites

(a)  All clubs should operate some form of frequency control system on the flying site, such as a peg-board

(b)  At larger flying sessions the use of a transmitter pound should be considered in addition to the frequency control system.

(c)  All transmitters, except 2.4 GHz sets, should carry an easily visible channel identification pennant;

For 27 MHz, a correctly coloured ribbon and/or a white flag, approximately three inches by two inches with one inch minimum height black numerals.

For 35 MHz, an orange flag, approximately three inches by two inches with one inch minimum height black or white numerals.

For 2.4 GHz, there is no need for an identifier.

(d)  All clubs operating a mix of 35 MHz and 2.4 GHz transmitters should institute very robust pre-flight checks, especially if individual members fly a mix of frequencies.

Several incidents have occurred where a flyer has not appreciated that the 35 MHz set they have in their hand is not the 2.4 GHz set they are used to using and have neglected to extend the aerial. Constant vigilance is required.

(e)  If you are setting out a pegboard after a flying session starts, take care to identify all the models present and the frequencies they are using. Check thoroughly as a mistake can lead to a shoot down and don’t assume that a model in the air is on 2.4 GHz

4.5      Pegboard Recommendations

For 35 MHz frequency control (and for 27 MHz where still used) there are three basic pegboard systems: Peg Off; Peg on; and the Double Peg. There is also the Individual marker system which is used by some clubs.  Each of these systems is described below and it is essential you determine which system is used at your club  before you ever switch on your transmitter.

It is highly recommended that all club pegboards are clearly marked with the GPS co-ordinates or map reference of the flying field.  This will enable emergency services to find your location easily in the event of a serious incident, even in isolated areas.

  • 4 GHz – 2.4 GHz radios do not need a pegboard system to be set up to control radio frequency safety. However, there is one circumstance where a 2.4 GHz pegboard might be extremely useful to a club and that is when there is a limit to the number of models allowed to be flown at any one time.

Many clubs in these circumstances use the number of pegs ‘on the board’ to help control the number of active models and the ability to monitor the number of 2.4 GHz sets actually in use at any one time could be important.

  • The 35 MHz Peg-Off System – The pegboard displays all useable channel number/colours each with an appropriately numbered peg or marker clipped to it. To reserve a channel the flyer must take the correct peg off the board and, usually, clip it to his transmitter aerial.

Pegs must be returned to the board at the end of each flight or there can be confusion as to who has the right to fly. This system is useful for fixed base operations when the board and its pegs can be left on site without being subject to vandalism.

  • The 35 MHz Peg-on or Reversed Peg System – The pegboard is marked out with the channel numbers/colours as before but with no pegs. Each flyer carries his own named peg and to reserve a channel the peg is clipped on to the board before a transmitter is switched on. It is essential that pegs are removed from the board when a flight is finished and pegs must always carry the pilot’s name.

This system is useful for sites where vandalism might be a problem as the pegboards can be made small enough to carry easily and each member of a club can have his own, only one being used at any flying session of course.

  • The 35 MHz Double Peg System – The pegboard used is exactly the same as for the peg-off system, complete with a full set of numbered pegs. The pilot, however, also has a named peg that he carries with him as in the peg-on system. In use, to reserve a frequency, the pilot takes the numbered peg off the pegboard and replaces it with his own named peg.

This system still has to be used carefully but it is recognised as probably the most reliable system as it avoids several of the potential problems with the two other peg systems.

  • The Individual Marker System – A further popular system is where each club member has his own small individual frequency marker board with his name and channel number marked on it. These are stuck side by side in the ground by the flyers as they arrive on the site making, in effect, one large pegboard. Flyers on the same frequency place their markers one behind the other and use a peg or similar to reserve the channel between them.

This system has been known to be effective on beach and hill sites as the marker boards can carry ‘permit to fly this year’ details as well as name and frequency information.  It may also have uses with 2.4 GHz equipment, not as a frequency control system but to enable the flyers to see who is actually on the field or slope and this may have significant safety implications if anyone is injured or taken ill.

  • There are other variations and clubs should select the system which they feel is most appropriate to their flying field situation and, whichever system is chosen, should ensure that the operation of frequency control is well understood by all their members and visiting flyers.

(g)  Pegboard Problems

(i)   Switching on without ‘getting the peg’ is the cardinal sin and can have very serious consequences so Clubs should make very sure that their flyers do not slip into such bad habits. This is especially important as many flyers are now using both 35 MHz and 2.4 GHz sets and not using the 35 MHz pegboard because they have become used to using a 2.4 GHz set is becoming a common problem. It cannot be emphasised enough that all Clubs should insist on high standards of training in the use of their frequency control system. The move to the pegboard before even thinking about switching on a 35 MHz set should be second nature to all R/C flyers.

(ii)  Changing 35 MHz crystals, either on the field or at home, can also have very serious consequences if the flyer forgets that he has done it. It is very easy to then take his ‘usual’ peg and reserve a completely different frequency to that which he is actually using. The dangers are obvious.  If 35 MHz channels are changed, it is essential that channel flags are used and changed with the crystals. Pilots must discipline themselves to act correctly in these circumstances as it is very easy to make a mistake with the pegboard after a change has been made.

(iii)  With the Peg Off system, flyers sometimes take the peg home with them and the ‘missing peg’ can lead to a new peg being made. The problems then occur when both pegs then turn up at the same time. Even worse is the situation where it is assumed that a missing peg has been taken off-site and a new temporary peg is made for the rest of the day. You can very easily have two people both thinking that they have use of a frequency

(iv) With the Peg On system, under no circumstances should anyone simply remove a peg that is reserving a frequency.  If, however, you suspect that a peg has been left on the board in error (the flyer may have gone home) then you should check with senior flyers on the field and the peg may then only be removed after stringent checks that it is no longer in use. The name on the peg helps again here and is yet another reason to make sure that all pegs are named.

4.6      35 MHz Synthesised  Frequency Transmitter Control

Synthesised frequency transmitters are legal to use in the UK, as long as they have been tested and carry the familiar CE mark.

Synthesised transmitters do not have a higher risk factor than crystal controlled sets but the possible problems that may arise are slightly different because of the ease with which channels may be changed.

To help control this situation, all UK available synthesised frequency sets either have a two stage switch-on sequence where the frequency to be transmitted is clearly shown on initial switch-on or a permanent display of the set frequency that is shown even when the transmitter is switched off.

This is virtually the same procedure that the user of a crystal controlled transmitter goes through and it gives the users of both type of set the opportunity to go to the frequency control system and reserve their frequency (‘get the peg’).

Whatever type of 35 MHz transmitter control is used, the biggest risk will always be the flyer who switches on without thinking and without ‘getting the peg’ and it makes no difference if their transmitter is a synthesised one or not.

4.7      35 MHz Frequency Allocations at Club Sites

The 35 MHz frequency band is by far the most used by club flyers but, because almost every club operates in unique circumstances, it is not possible to recommend a fixed band plan for the regulation of those frequencies on every site.

There are, however, several different types of frequency allocation already in operation at club level, as laid out below, and all clubs should consider very carefully which method of frequency allocation they should use.

(a)  Use of all frequencies at 10 kHz spacing – This is the most used system and it is operated successfully by most clubs. Modern equipment gives very few problems at 10 kHz spacing, especially when common sense precautions against self generated radio interactions are taken.  As one safeguard, it is important that flyers regularly operating together on adjacent channels should perform an adjacent channel check every two or three months.  Use of all the frequencies at 10 kHz spacing, combined with the Club’s general safety precautions and the Adjacent Channel Check is probably the safest way to operate. Modern equipment is quite capable of operating to this standard and when faults do develop (usually faulty crystals) they can usually be spotted before they cause any trouble.

(b)  The 35 MHz Adjacent Channel Check – The check is quick and easy to do. Flyer A switches on transmitter (with aerial down), then switches on his receiver and stands about 4 metres from his model. Flyer B, on an adjacent channel, switches on transmitter (aerial up) and stands alongside flyer A. No interference should be noted on A’s model and it should be under the full control of A’s transmitter. The test is then repeated using B’s model and with his transmitter aerial down and A’s extended.

Note that ‘interference’ will range from ‘glitching’ with older sets to failsafe operation with DSP receivers or PCM sets.  Any interference noted indicates possible tuning or crystal problems and must be investigated further. The test may save your model as it will give early warning of problems beginning in your radio equipment, usually well before they become bad enough to cause control problems in the air.

(c)  Use of the Contest Band Plan / 20 kHz spacing – The original contest band plan, dating from the first allocation of 35 MHz frequencies, is as follows:

...... R/C power...... all odd frequencies

...... R/C silent flight    all even frequencies

The original allocation of competition channels was the responsibility of the BMFA Technical Committees but it was also used by most clubs as their standard 35 MHz frequency band plan.  It gives an automatic 20 kHz split between frequencies in use on a site and this was important in the early days of 35 MHz, when the equipment available was not as reliable as it is today.

However, increased demand for frequencies and better standards of radio equipment has led to this system becoming used much less, both at club level and in competitions. This system may still be useful for some ‘silent flight’ clubs and for some power clubs with sites near to known slope or thermal soaring sites. Its use has, however, been overtaken by frequency requirements and availability and the ‘block frequency allocation’ is now more appropriate in many cases.

(d) Use of Block Frequency Allocation – Where a club has a large site and is able to set up two or more flight lines, or where two clubs operate closely together then each is allocated an agreed block of frequencies.  Each flightline then has its own pegboard, allowing only the agreed allocated frequencies to be used.

Many combinations can be worked out to suit individual needs and the increased number of channels made available in recent years has made this type of frequency sharing much easier. The ability to have targeted pegboards on each flightline or site is very important in avoiding frequency clashes.

4.8      Lone Flyers

Lone flyers, or people who fly in small groups of two or three, must take the greatest care that they are not operating in situations where they can cause interference to a local club or flying group. As a general rule, you should not fly within two miles of a recognised club flying site unless you have some arrangement with the club who fly there.

This is for the benefit of both parties as interference works both ways and could result in the loss of aircraft on either site.

Lone Flyers, in fact, are far safer when operating on 2.4 GHz as there are no interference implications and the radios can be used anywhere that the flyer wishes.

If you wish to fly alone and are still using 35 MHz radio but are not sure if there are clubs sites local to you, contact your local model shop or the BMFA’s Leicester office for information. You will usually be able to get into contact with clubs quite easily through these sources and it is essential that you do so to ensure safe flying for all concerned. You should also consider the purchase of a hand held scanner, although remember that at ground level it might not pick up a signal that is apparent at your model’s altitude.

Next Section 5. INTERFERENCE

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