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Radio Transmitters The Full Story!


In any radio control system the transmitter will be the part of an integral system that you feel and it goes without saying that it should be comfortable to hold. But of course it goes much further than that.

Should it be 27mhz AM or 40mhz FM, AM, PCM, multi model, 2 channels or more, have ATL, ATV, Exponential, Dual Rates, Electronic trims, etc. etc. What do all these initials mean? And what are the pro’s versus the con’s? As with most things, you only get what you pay for, this applies equally to radio control equipment . Reliability, accurate and precise control without adjacent channel interference should be the minimum requirements.

27 MHz AM

This is the original radio control frequency and generally, used by models of all types surface, air, scientific, citizens band and hospital equipment. The latter two not normally on our band but at times with enough power to cause interference.

The information about the servo and winch positions required by the receiver are superimposed on the 27 MHz as a signal amplitude change, hence the term AM which stands for Amplitude Modulation. Originally the transmitters and receivers could only operate at a frequency separation of 50KHz. (Solid colours) Brown Red etc. 6 in total. Now modern 27MHz radio’s can work at around 10KHZ spacing hence the ‘Split’ colours. These are arranged in a 20/30KHz division. This gives us 12 ‘spots’ on the 27 MHz band to sail our boats.

40 MHz FM/AM

When the 27 MHz band became more and more crowded, User Groups lobbied the Home Office this eventually led to two new bands for model use. 35MHz for Aircraft only and 40 MHz for surface models only. One of the conditions imposed at that time was that all transmitters and receivers here in the UK would use FM as standard. But now, due to current technology, 40MHz AM can operate at 10 k/c spacing with FM. 40MHz FM Xtals cost more because the specification is more stringent. Whereas AM xtals 27 or 40MHz are cheaper to make.

Frequency Modulation (FM) uses a small variation either side of the transmitter crystal (xtal) frequency to encode the signal with the winch and servo positions. The more sophisticated circuitry to enable this, also allows the adjacent frequency to be much closer, 10KHz. With the Band allocated to 40 MHz, means up to 34 yachts (plus 12 on 27mhz) could be operated simultaneously!!

Of course we sensibly restrict this to a much lower figure with the Heat Management System. But the higher technical performance and greater choice of frequency means less xtal changing between races at larger meetings. Also because the band is exclusively for surface models less outside interference.


With the advent of digital technology and IC’s (integrated circuits) A very sophisticated communication method was developed here in the UK for missile guidance control. Eventually the technology became available for model use. Futaba have been at the leading edge of this type of control with their 1024 bit processors.

This system offers the ultimate in model control with many safety and fail-safe features. Called PCM this stands for Pulse Code Modulation. To enable this to work the model transmitter and receiver now uses computer technology which in turn has many benefits to offer us.

For instance, multi model capability. This is where one transmitter can be set up for several boats. At first glance with only one Yacht, something for the future, but model 1 can be set for the ‘A’ rig, model 2 the ‘B’ rig and model 3 for the ‘C’ rig. This means the boat can be optimised for each rig. The winch throw and end points can be set for each rig.. for that extra fine tuning. Also when sailing on ‘C’ rig in very windy conditions, the exponential function on the 3VC can be used on the rudder function to give a ‘soft’ response around the centre very good for controlling the course towards the ‘reaching’ marks whilst retaining maximum throw for tacking in difficult conditions. This should be seen as a refinement over Dual Rates where a switch has to be operated to change rudder throws.


This is done with ATV and ATL. Lets take these two in turn.


ATV stands for Adjustable Travel Volume and is also referred to as ‘end point adjustment’. This is very useful when setting up total winch throws…because it allows the close hauled position to be set independently from the running position. You can see it is not imperative that the winch has a throw adjustment with ATV’s, although it does give an added dimension to the trimming science.

The same applies to the rudder, although the ATV would normally be the same for port and starboard. But if your horn geometry caused unequal rudder movement when checked with a degree gauge or ruler, the ATV function can be used to balance the throws and ‘feel’ of the yacht.


What is ATL? and how is it a benefit for the Yachtsman. Adjustable Throttle Limiter is another very useful feature of the computer style transmitters. For us, change throttle to Sail and it becomes Adjustable Sail Limiter (ASL). This function relates to the sail trim lever next to the stick and when activated means the trim lever only works when the stick is in the ‘close hauled’ position. So once the throws have been adjusted , by moving the trim it is impossible to ‘stall’ or damage the winch at the running position because the trim lever position does NOT move the opposite end point. So now we can ‘tune’ the close hauled sail position for different conditions.


The higher specification Futaba radio’s feature Electronic trims which enable fine adjustment with the added bonus that they only work when the Tx is switched on. Whereas the lever type can be inadvertently moved when handling the transmitter between races.

On the Futaba 3VC the ATL function is displayed as a percentage with two buttons (electronic trims) rather than a lever, adjusting the sail position by 1%-5% per press depending on the setting. This fine setting capability obviates the need for rows of holes in the boom and gives a more precise control anyway, plus it can be done while sailing! This very precise control when close hauled, enables a fine balance between boat speed and ‘pointing’ ability to be constantly adjusted to suit racing situations.


With some of the antics on the bank while racing resembling ‘Fencing’ rather than sailing its little wonder we see so many works of art! On a serious note we should always use some form of aerial tip protection.

Not always appreciated is that a damaged or shortened transmitter aerial can create the following problems:
a) Reduce the range of the set.
b) Increase the risk of interference from adjacent Tx’s
c) Generally increases the current taken from the nicads. (Shortening sailing time).
d) With a dirty aerial (sliding joints) the transmitted frequency can be affected and cause a) and b) above.

To maintain the aerial it should be treated with a water dispersing spray (WD40) doing each section in turn and carefully cleaned with meths after. This should be done regularly especially after sailing in the rain, with a Tx cover of course. The added benefit is that the aerial is unlikely to ‘fold up’ on you and cut your hand or need early replacement. So, the often ignored but vital aerial deserves a thought and some maintenance. If you have a repaired aerial then give serious consideration to replacing it ASAP.

While on aerials, the receiver or boat aerial should always be fully extended, never partly coiled in the ‘radio pot’ and not run near the winch or rudder servo if at all possible.

All these points are important to get and maintain a ‘solid’ radio link with the boat. How many times have you seen a yacht with the servos ‘glitching’ and being ignored by the skipper, yet an oscillating rudder and shaking sails will reduce the boat speed enormously. Without this attention to the radio link, performance will suffer particularly when adjacent frequencies are in use and can be happening on the water without you realising it.

PCM (Pulse Code Modulation)

With PCM, the aerial comments still apply, but the ‘glitching’ problems mentioned do not occur because the PCM receiver samples every new piece of information before telling the servos to move. If it finds the information is corrupted (interference) then it ‘Holds’ the last command until it receives valid data, this all happens very quickly.

This means the yacht will sail unimpeded, switching the transmitter off briefly will not cause any problems, whilst not recommended its even

possible to change the Tx battery while sailing! Whereas with a conventional AM or FM radio set you must always turn the TX on first and off last to avoid winches de-spooling etc.