Radio Control Systems - Antennas
The biggest issue with antenna or ariels is the quality of reception of signals. The arena used for recording Robot Wars in for television is a-buzz with electrical noise: cameras, lights, other robots, talk-back radio system to name but a few. To have complete, safe control over your robot it is essential that you have the correct antenna and transmission form.

As discussed in the AM/FM and PCM sub-sections here, the type of transmission will affect your reception with PPM-AM being most prone to noise and FM-PCM being most resistant. But how much does the ariel affect things?

Common knowledge in the world of radio controlled models is that the ariel does not normally need to be extended or unravelled. This is true - to a point. If your robot is generally well insulated from noise (especially the plastic bodied machines) then there is no problem with having the antenna wire tied up and well inside the robot. Indeed, this is what we did for Panda Monium in the 1998 Wars.

Sometimes the body of the robot can cause reflections and interference from the motors, speed controllers and other equipment can cause problems. Now is the time to get a grasp of the basics.

Antenna length

The antenna should be a fraction of the actual wavelength of the transmitted signal (ie 1/4, 1/2, 1/8 and so on) to get best results. So, if the box is transmitting on 40Mhz, we can find the length of one wave thus:
  
lambda = v/f

where v = velocity of the wave (assume for practical purposes that this
is speed of light in a vacuum = 3x108ms-1)

      f = frequency of the wave = 40x106Hz

 lambda = length of wave in metres

 lambda = 3x108 / 40x106

        = 7.5m
So, a good size for your antenna would be 0.9375m (1/8th lambda) which is about the size of a typical 40Mhz Tx ariel.

Reflections of signals

Of course, in the arena or battlebox there are lots of places where signals can be reflected - lots of nice flat metal surfaces, for example. Reflections of the same signal are a problem as they are the same amplitude and frequency and can easily cause interference to the orginating signal. Remember also that your own robot body may cause reflections whether the ariel is internal or external. If the trough of one reflected wave appears at the ariel at the same time as the peak of the originating signal, the two will cancel out giving little or no signal.

Reflections cannot be eliminated altogether, but you can be aware of them - it's certainly worth considering if in testing you find the signals are erratic or the range is significantly reduced. 

Types of ariel

There are three main types of ariel that can be used for ariels, although they are really all variations on the same theme. These are wire, stubs (and rubber-ducks) and metal whips.

Wire is the most common ariel. This is what is already connected to a commercial receiver when you buy it and is normally just gathered up and tied with a cable tie to keep it from getting damaged. It can be extended but this is not normally much of an improvement - it may cause partial correction of a more serious underlying problem. Some competitors push the wire up into plastic tubing, such as pnuematic pipe, and use this as a make-shift whip ariel. This can be used to hold the ariel upright out of the top of a robot and if problems are being caused by reflections off the robot body, this may make a difference. The obvious downside is that you are leaving yourself wide open to cutting weapons and once you lose the ariel in part or in whole, the Rx will struggle to make sense of the signals it is receiving.

Stubs and rubber ducks come in two flavours; passive and active. The antenna is basically a coil of wire inside a sprung rubber sheath. If it is knocked or bent, it will normally deform until the obstruction is removed and then return to it's upright standing. Active stubs use signal amplification circuits to boost the incoming signals and correct for any background noise generated by the inductive effect of the coils. The good thing about these ariels is that they are pretty compact and are fairly robust when used outside the robot. The downside can be that the gain is not as good as the wire ariel. How much difference the active systems make, given the cost of them, is not really known at the moment.

Whips are the type of ariels used on the transmitters. They are tall metal 'sticks' which are sometimes retractable. They can also be made of flexible metal inside a plastic sheath. These antenna are brittle and tend to be bent or broken easily. They are best used for the Tx only.

Positioning of the antenna

We have discussed the problems of interference and reflections and these will affect the choice of ariel site. The Tx will also suffer in the arena as you are encased behind a polycarbonate shield with scaffolding supporting the structure. To get a good signal you need to get the ariel outside the box and clear of the metal scaffolding. One way is to poke the ariel out through the hole provided. Another, I believe used in Battlebots, is to retract the normal ariel and connect a 'peg' to the antenna. This 'peg' is connected to an external ariel either provided by the organisers or hung out of the safety box by yourself.

Also, be aware of the issues in the arena such as high boundaries and walls. If you intend to go behind these and they are higher than your robot, perhaps you should consider making the ariel as high up on or in the robot as possible to maximise the signal. Perhaps you should even consider a different route!

Finally, time to state the obvious. If you have a 'bot that can run bothways up, don't stick an ariel out perpendicular to the top - you may find it's not there or badly deformed when the robot runs the other way up. Remember also that putting the ariel right next to high current wires or the motor and/or speed controllers is really not helping your cause as these are havens for interference. Generally try to site the antenna near the top, middle and as far from electronics as possible.