Thursday, July 1, 2010

I have seen advertisements for in-line coaxial cable filters ...

Q Rick, K8SV, asks, “I have seen advertisements for in-line coaxial cable filters. How effective are these devices at reducing or eliminating RFI?”

A If an RFI problem is caused by RF getting onto the outside of a transmitter’s feed line, a filter of this type will help suppress it. This RFI scenario arises only when susceptible equipment is much closer to the feed line than it is to the transmitting antenna. This could happen in an apartment location, or in a ham’s own home (such as when the radiating coax passes near your computer).

If your neighbors are located a distance away from the ham station, radiation from the feed line is not generally a problem. In that case, a low-pass or band-pass filter would be more appropriate, if the interference is to an over-the-air service, such as antenna-connected TV or FM broadcast reception. Even so, in most of those cases, fundamental overload also plays a role.

See on the Web for some good information packages on various aspects of RFI. Of course, The ARRL RFI Book ( is another good source of help for all types of RFI problems.

From QST August 1999