[h=3]Your tail rotor servo wire, on a boom mounted servo, tends to end up with its cable running a long distance from the rear of the heli up to the gyro (or from the gyro to the receiver). It's also usually routed along side the heli as a straight shot to where ever the other end plugs into the gryro or receiver. Quite often, a servo extension is added to the circuit. As such, it makes a pretty good antenna for not only picking up external electrical noise that could end up in your power and signal lines, but it also makes a good antenna for radiated noise -- the electrical noise generated in the servo (especially if its a digital servo with a microcontroller inside), and the electrical noise generated in the gyro by its microcontroller. This noise is random frequency loaded with many harmonics, and could possibly interfere with your receiver's ability to process the transmitter's signal correctly (noise radiated back into the power and signal lines can also mess up your radio's ability to function properly, too).
The purpose of the ferrite "donut" is to change the inductance of the wire and substantially reduce the ability of the wire to act as a noise emitter, and as a noise receptor - within a small band of frequencies. Ferrite beads and other special ferrite shapes are commonly used in most consumer, industrial, laboratory, and aviation/space electronics to attenuate and control Radio Frequency Interference (RFI) generated by the equipment , and reduce its susceptibility to Electromagnetic Interference (EMI).
And you're correct. You really need to know which band of frequencies you are trying to suppress, as the ferrites are designed to attenuate noise in different ranges of frequencies. One size does NOT fit all.
I don't believe their use is that common in the RC helicopter world. They may show up a bit more in the large "giant scale" aircraft world, as these folk not only have lots of servos scattered all over the plane, but they use a lot of servo extensions. The longer the wire, and the more more of them you have, the better chance you have that they will act as antennas that receive and radiate RF noise.
One place where they seem to be used quite frequently is in the electric powered airplane market. The use of brushed electric motors is like putting a HUGE RF noise generator in your plane. The brushless motors and their controllers also present a good opportunity to act as noise generators. I've seen quite a few references in RC Modeler, Model Aviation, and other model magazines recommending the use of ferrite donuts in electric powered planes. I suspect that the electric heli crowd will discover the advantages of ferrites soon, if they already haven't.
The ferrite donut with the wire passed through it forms an "RF Choke". A radio frequency choke (RF choke) is a circuit element that presents a high impedance ("high resistance") to radio frequency energy while offering low resistance to direct current (like that from your battery).
RF chokes are frequently used in series with power supply leads both to keep RF from coupling to power supply paths and to prevent power supply components from paralleling undesired impedances across the RF circuit.
In the earlier days of RC, servo cables, battery cables, and switch harnesses were made out of wires that were twisted -- say twisted pairs, triples, or quadruple wires in a bundle. As systems have evolved, we now see that the cables used in today's systems are made up of flat cables, with parallel wires (look at a Futaba or JR cable).
The older, twisted wire configuration was actually advantageous for controlling RF interference and EMI susceptibility. The twisted wires, especially since the ground wire was included in that bundle, prevents the cable's ability to act as a noise radiator or receptor. The newer, parallel sets of wires actually enhance the cable's ability to act as an antenna. Simply putting a few twists in your cabling as you route those wires goes a long way towards reducing your chances of the long, straight cable runs acting as an antenna. It may not look as pretty, but electrically, it is beneficial to your system's operation. Dave[/h]
