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Radio Control Myths and Legends

There is available within the RC Laser community some very good documentation about care for servo’s, batteries etc. From experience I believe that the importance of both the Transmitter and Receiver antennae are not fully understood by skippers, in any class of RC boat. This is not surprising as they are quite specialised subjects. There exists much good advice about the care of antennae, however this is generally prescriptive and does not explain why one should do certain things. 


The Link - Transmitter to Boat Receiver

Modern radio control equipment ranges from simple two-channel Amplitude Modulated (AM) equipment to very complex data transmission methods. Whether you use simple or complex radio equipment, the common element between them all is how efficiently the radio frequency element is transmitted and how well it is received in our boats. Poor performance from either the Transmitter (TX) or Receiver (RX) antennae, or both can mean intermittent loss of signal hence temporary or complete loss of control.


Transmitter and Boat Receiving Antenna

 There are two fundamental points to remember.

1.      To obtain maximum design radiated signal the TX and RX antenna must be fully extended. Failure to do this results in serious loss of radiated power, hence potential loss of boat control.

2.      For maximum signal strength reception in the RX antenna, the TX antenna should be aligned in the same plane. The reason for this is that antennae have polarisation, either vertical, horizontal or in some special cases circular.  In the case of the RC Laser, the majority of RX antennae are laid horizontal along the deck. For maximum signal reception therefore, the TX should be held with the antenna in the horizontal position.


In reality with good TX and RX antenna and the distances involved the orientation of the antennae may not be critical. However, on the margins or, with a poor antenna the correct orientation may help you to keep control of the boat.


Skippers with poor antenna installations, who race on small lakes, may not suffer from loss of control. Problems may arise when visiting other venues where greater distances between the skipper and boat can reduce the received signal strength. This loss of control may be temporary or permanent until the distance between the transmitter and receiver is reduced.


The end result is that the skipper loses race places or worst case declares that the boat is out of control. Once this is declared the skipper must pull out of that particular race. Frustration sets in when the boat is retrieved and the radio system works fine.


The Transmitter Antenna


The designer of the equipment knowing the speed of light and the radio frequencies of operation can work out the length of the whip antenna at which it will resonate. The matching circuitry from the TX power amplifier (PA) to the antenna can be designed for maximum transfer of power to the antenna. This in turn generates the maximum Effective Radiated Power (ERP) for this class of equipment. 


Throughout this process the designer has to make one major assumption? That is that the electrical continuity of the whip remains throughout the equipments working life.


Poor electrical contact along the whip reduces the ERP, as the antenna is no longer at resonance, which in turn can cause a mismatch to the PA, which may reduce the power even further.


The Solution

 From the above the only thing that the skipper has any control over is the preventative maintenance of the TX antenna.

 The majority of transmitting Antennae are telescopic whips, which when collapsed makes the transmitter easier to transport. Two common problems can occur over time.

 Dirt / Corrosion

 Dirt / corrosion can accumulate between the sliding joints resulting in poor electrical contact and reduction in radiated power. The solution is to regularly clean the antenna with either switch cleaner or WD40. WD40 does tend to leave a residue film behind which can hinder good electrical contact; this residue needs to be polished off to ensure good contact between the joints. Switch Cleaner is better as it does not leave a residue.

 Marching to a Rhythmic Antenna

 Due to ware and collisions with other skippers, antenna can become bent and the joints damaged and loose. Early signs of this are: -

 o       As you walk down the bank a rhythmic jangle comes from the antenna; this is a sure sign that the antenna is on its way out.

 o       As the TX is held vertically, one or more of the telescopic elements slides into the lower one.

In either of the above two cases a replacement antenna must be purchased.


The Receiver Antenna


The piece of wire dangling from the receiver is the antenna. This too is cut to be resonant to the same frequencies as the TX. One problem with the receiver is that the majority are built for the radio control car market. As an RC car is small compared to a sailing boat the antenna length is made shorter and with the use of discrete components is made to resonate at the correct frequency. This means that the length of wire is sometimes not long enough to bring out of the boat. This can be readied quite simply as follows.


1.      Measure the length of the existing antenna and note it.


2.      Cut the antenna leaving sufficient wire to attach an in-line plug. Solder one end of the plug to the wire attached to the RX.


3.      Take a length of wire and solder the mating half of the plug to it.


4.      Connect the plug half’s together and measure out twice the length of the original antenna. Cut the wire to this length.


5.      Route the antenna as required.


Not only have you a longer antenna, but also a rapid means of changing the receiver should you require to replace it at some point in time. The ability to rapidly swap a receiver is very useful when fault finding.


A Measure of Quality


It is difficult without specialist measuring equipment to assess how good the radio link between the transmitter and installed receiver is. The following is a very good wet finger method of assessing this or comparing different RX antenna installations.


Ready the boat for sailing but do not put on the water, place it in its boat stand.


Obtain the help of a fellow skipper, who minds the boat and observes it.


The skipper walks away and at approximately 10m intervals operates the TX controls in a predetermined sequence e.g. Full Right Rudder; then Full Left Rudder; Winch Fully Out; Winch Fully In. After each action the observer signals to the Skipper what has happened.  To do this an agreed set of hand signals is required.


When two to three hundred metres separation from the boat has been achieved the tests can stop as this more than sufficient for yacht racing.


Next try to estimate the distance to furthest mark you sail on your lake.


If we assume that the distance to furthest mark you sail to is 50m and that you still had full control of the boat tests at 300m; then you have a signal strength reserve of 6:1.  


When should these Tests be Performed?    

         o       If it is suspected that performance has deteriorated.

         o       The boat has been out of commission for some time.


o       If you want to experiment with a different boat antenna, comparisons can be made with the old and new and a figure of merit for each determined.


  One last thought; complex radio control equipment does not guarantee racing success. Poor TX and RX antennae do contribute to race failure.