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Radio Control Technicalities   Back to Club Members' Pages

Quick links down this page : The legal bit — faqs — Aerial lengths — 35MHz channels — Other topics


Introduction for Beginners to Radio Control

The radio control equipment now available to modellers is extremely reliable and, like most electronics, has never been cheaper. You can buy any of the well known brands with confidence. Members of the club use Sanwa, Futaba, Spektrum, JR and Hitec radios, but some other brands you see advertised in the aeromodelling magazines may be equally good.

The radio control equipment we use is made up of :

You will probably buy your first radio as a complete set (sometimes referred to as a "combo") for around £140 upwards. It will arrive in a large colourful box and, very importantly, should come with a good instruction manual. It is cheaper to buy a set than individual components and in the set you will also receive re-chargeable batteries for the Tx and for the Rx plus a suitable charger.

There will also be an on/off switch to mount in the aircraft, mounting accessories for the servos, an instruction manual and perhaps a few other goodies such as servo extension leads and manufacturers stickers to go on your model.

There are a few essential things you should know before buying your radio gear :

  1. In the UK, a small band of radio frequencies around 35 Mega Hertz (MHz) has been allocated for the exclusive use of aeromodellers. This means that it is illegal for anyone to use this 35MHz band for any other purpose. The result is that we as aeromodellers should not experience any radio interference which could cause our models to go out of control.
  2. In the UK, a band of frequencies around 40MHz has been allocated for surface-use radio control, meaning RC cars and RC boats. You should NOT attempt to fly an aeromodel using 40MHz radio as you could crash due to interference from somebody (legitimately) using a RC buggy or RC boat nearby.
  3. A licence is no longer required to operate RC equipment in the UK in the authorised bands.
  4. Note that on continental Europe, some countries use the 41MHz band for aeromodel RC, some 40MHz, but there is a move to standardise on 35MHz throughout Europe. In the USA, the 72MHz band is in use for aeromodel RC. Be aware of these frequency band differences when buying on the internet, only the 35MHz band is to be used in the UK.
  5. To enable several models to be flown at a site at the same time, the UK 35MHz aeromodel band is divided into channels numbered from 55 to 90. Only one model can fly on each channel at any one time. The most common method to change channel is by inserting the correct crystal into your transmitter and the corresponding crystal into your receiver.
Since these notes were written in 2006, there has been a revolution in radio control for models. Because of interference problems on 35MHz, most modellers have now transferred to the 2.4GHz band. However, all of the notes above still apply. And there are still plenty of models flying perfectly satisfactorily on 35MHz. 35MHz transmitters have a long telescopic aerial, 2.4GHz a stubby 12cm aerial.

NOTE : You use 2.4GHz entirely at your own risk, it is an "open" frequency widely used by many other users (such as Wi-Fi).
2.4GHz transmitters are limited to 10mW transmitting power, 35MHz transmitters have a much higher radiated power limit (1 Watt ?).

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A few Frequently Asked Questions (with answers) for beginners to radio control.


What is the range of the transmitter? Simple answer for 35MHz and modern 2.4GHz, as far away as you will ever want to fly your model. That is, whilst the aircraft is in line-of-sight with the Tx and still in the air. The exception is early 2.4GHz radio sets meant for indoor use, they had limited range. Newer 2.4GHz sets are all "full range".


Can I use a Futaba crystal in my Hitec transmitter? Simple answer, NO. Some crystals are interchangeable between different manufacturers, but to be on the safe side, stick to the crystal from the Tx's manufacturer. On a similar vein, crystals designed for single conversion receivers should not be used in dual conversion receivers and vice versa.


How long should the Rx aerial in my model be? It hangs out of the back, can I wind it up to keep it tidy? The piece of wire emerging from the Rx which acts as a Rx aerial should NEVER be shortened or coiled up. As supplied by the manufacturer, the aerial wire fitted to a "standard" Rx is generally about 90-100 cm long. It is not a problem to leave the wire trailing from the rear of your model. If you shorten the Rx aerial wire or coil it up, the range of your radio will be severely reduced. More below.


My Tx aerial is a bit tight to pull out. Can I put some gease on it? No. The oil or grease could cause bad electrical contact between the segments of the aerial and reduce the efficiency of the aerial, leading to a reduction in your control range. It is an advantage to clean the aerial from time to time by wiping with a small amount of solvent (such as meths) to remove any dirt or grease that has accummulated.


All the RC sets I see advertised seem to have computers in the transmitters. This seems a bit complicated for me, can I buy a non-computer Tx? Simple answer, no, unless you can find a shop selling off old stock or buy second-hand. Having said that, my advice would be to buy a computer transmitter. You will quickly find that the advantages offered outway the initial complication.


What is this "failsafe" thing? All aeromodels weighing more than 7kg must be fitted with a device that (at least) closes the engine throttle if the on-board Rx fails to pick up a controlling signal from the Tx, that is, if there is a failure of the radio control link. Most everyday models weigh 1kg to 4 kg and so are not required to be fitted with a failsafe device though it would do no harm at all to do so and might avoid a "fly away".


Will I have to do any maintenance on my RC equipment? Unless you are an electronics whizz, your maintenance will be confined to keeping everything clean and dry and looking after the re-chargeable batteries. You should also inspect all items for physical damage or wear and tear from time to time. It is most important that you check the operation of your RC equipment BEFORE EVERY FLIGHT. Any anomalous behaviour, such as a twitchy servo, a servo that seems to stick occasionally or anything at all out of the ordinary, requires investigation and YOU SHOULD NOT FLY.


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NOTE 21st November 2013. : The comments below here refer to 35MHz and are still relevant. But an update for 2.4GHz is required.

Digging Deeper - more details

Radio frequency, wavelength and aerial lengths

35MHz is in the "short wave" radio band. The wavelength is about 8.57m. Optimum transmission and reception would be obtained with a half-wave dipole aerial of overall length of 8.57m, which is hardly practical for portable equipment!

The next best thing for good reception is to use an aerial whose length is an exact fraction of the wavelength, a half, a quarter or an eighth, etc.. But as the aerial length gets shorter, its efficiency diminishes rapidly.

In practice, our radio control systems work satisfactorily with a transmitter aerial length of about 1.25m and a receiver aerial of about 1m length. These are the lengths of the aerials supplied with standard Hitec RC equipment, and are typical of all makes.

Important note : the Rx aerial length is the straight line distance from where the aerial leaves the receiver to the end of the aerial. If you coil up excess length because it will not all fit inside the length of your model, you are reducing the aerial length and therefore reducing the strength of the signal which it is capable of picking up from your transmitter.

Additionally, and potentially even worse, a coil in the Rx aerial wire is particularly good at picking up electrical noise from the varying currents flowing in the servo and battery leads. This noise will generate "glitches" in your servos, which will get much worse as the range between the Rx and Tx increases, that is, when you are flying!

It is normal practice to leave part of the Rx aerial trailing behind a model aircraft or or to mount the Rx aerial in the wing. Either way, the maximum length will give you the best possible reception.

You may see some receivers labelled as "full-range". This is actually the normal standard, in contrast to the very small, light receivers with shorter aerials which are meant to be used with indoor models, for which a limited range is perfectly acceptable.

Flexible "whip" aerials are available for both transmitters and receivers. These are shorter and look cool, but don't work any better than the telescopic Tx aerial or the wire Rx aerial which most modellers use.

Back to Frequently Asked Questions

35 MHz Radio Control Channels

The 35MHz aerial radio control band is divided into 36 channels each channel separated from its neighbours by only 10kHz.

All transmitters need to comply with European Standard CE0681 to meet the required bandwidth and operating frequency accuracy criteria. There is usually a sticker on the box and on the transmitter of complying equipment. You will not be at all popular if you use a very old or non-compliant transmitter which causes interference with other models, maybe causing somebody else's pride and joy to crash.

Receivers need to be sufficiently selective to avoid interference from transmitters operating on adjacent frequencies (adjacent channels). Older receivers and very lightweight modern receivers tend to be of the single conversion type instead of using the modern dual conversion system. Single conversion receivers are more susceptible to interference from any source.

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

You select the channel you are going to use by plugging a crystal into the Tx and a matching crystal into the Rx. The crystals will be clearly marked "Tx" or "Rx" and with the channel number. Note that the frequency marked on the crystal bears no obvious relation to the channel frequency in the above table.

A recent development has been the introduction of "synthesised" transmitter and receiver tuning, eliminating the need for plug-in crystals. This system is well proven, having been in use for more than 20 years in other areas of radio communication and once the price falls (high at the moment due to its novelty) it will become the norm. If you are buying a new set of radio equipment, consider spending a little more for the convenience of being able to dial-up any one of the 36 channels. This could be very useful at crowded fly-ins, when you may be frustrated by having 4 or 5 others models sharing the same channel as yourself when many other channels are not being used.

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WORK STILL TO BE DONE ON THE SECTION BELOW.

Computer transmitters

EPA, model memory, Tx - computer, trim memory, later use, more in advanced
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Re-chargeable batteries

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Becoming An Expert

How a failsafe should operate, failsafe >7kg throttle only, but PCM, in Rx, separate device
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Other topics

Frames
modulation
AM
FM
PPM
PCM
digital servos
servo resolution
Rx selectivity
Rx sensitivity
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