Date: Thu, 06 Jul 00 15:43:09 GMT From: amesnopsamrak.demon.co.uk (Andrew Stephenson) Subject: Re: Radio Interference
In article <39642EBC.50DB8B00nopsamnet> tignopsamnet writes: > 2) Install a filter in the power lead to the radio. This > should consist of a> series (in line) choke (coil) and a > shunting capacitor of approximately .25 microfarad. Choke > values will have to be determined experimentally - just be sure > the coil wire is heavy enough to carry the current you will be > drawing. I am in general agreement with these suggestions but would add a small modification, which may sound strange but will be explained in a moment. Instead of having just one capacitor, have two in parallel, one of them suitable for frequencies of up to a few MHz (eg, polyester?), the other suitable for higher frequencies (eg, ceramic). The VHF one need only be small (say, 0.01 microfarad) while the other can be the main one (say, 0.22 muF). The reason for this split is that different kinds of capacitor construction give different behaviours at higher frequencies, ceramics being good at VHF. Fortunately, as frequency increases the capacitor's necessary size decreases. The voltage ratings should not be a problem, for the technologies suggested; be sure they are rated at, say, 25volts or higher -- trivially easy. If you cannot find a suitable choke coil, talk to a radio supply place and see if they can provide a ferrite bead/tube which slips over the power leads. Tell them what you want it for. It should have a frequency rating suited to the affected waveband(s) (there are many ferrite formulations) and be put near to the radio. No need to insulate, as ferrite is a ceramic material; but fix it in place with, say, some kind of tape. (There are fancier ways of doing this but I suspect they would be too fancy.) Also consider fitting other ferrite beads/tubes over other wires connecting to the radio (loudspeaker feeds, eg). Don't do this to the ground lead, though. Other posters may wish to modify these suggestions, in turn. -- Andrew Stephenson
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