[RFI] Re: RFI to intercom

[Jim Brown]

On Mon, 23 Aug 2004 11:18:54 -0500, Skip Cameron wrote:

>On late 80's vintage Model N350 Nortek intercom/AM/FM system that also 
identifies M&S Music on the Model sticker at my house (which has no  RFI 
problem) and an identical unit at my neighbors, who can hear me on all 6 speakers, 
when I operate on 12 and 10 meters, I have tried chokes, bypass caps and toroids 
at the main console and at each speaker end and the RFI has not been attenuated.
>Any other ideas?

Some major thoughts.

1) If the wiring is not twisted pair, replace it with twisted pair. Shielded CAT5 would 
be first choice, braid-shielded audio cable second choice, unshielded CAT5 third 
choice.  Connect the shield to the CHASSIS of the equipment.

2) Chokes and other filters should go right at the electronics, not at the speakers. It 
is the electronics that detects the RF, not the speakers. 

3) Most "clamp-on" ferrites are made from materials optimized for VHF 
performance, and are "just starting to kick in" at 25-30 MHz. You can lower the 
range of frequencies at which they are most effective by using two turns around the 
ferrite material. The best materials for this frequency range are what Fair Rite calls 
#31 or #43. A single turn through #31 may be good for 25-30 MHz, but you'll 
probably need two turns with #43. 

4) The #77 and #78 materials are poor choices, because they are optimized for 
much lower frequencies. #61 is optimized for VHF/UHF, and is also a poor choice. 

Jim Brown  K9YC