[RFI] Recent Posting on RFI with intercom.

[Jim Brown]

On Mon, 23 Aug 2004 14:46:27 -0400, Van Fair wrote:

>Solution seems to have been found. My RF is getting into the intercom via 
a
>connected FM antenna. With it disconnected 95% of the RFI goes away. I 
>plan to use a high pass filter and hope this will allow FM with no RFI. The 
FM
>radio is not on when the RFI occurs in the intercom. STRANGE.

That's because it isn't the FM radio that is causing the problem, but rather 
that the RF current flowing on the shield of the FM antenna is getting into the 
intercom's electronics. In other words, the coax is acting as an antenna for 
your ham transmitter. 

I would do three things. 

First, forget the high pass filter -- it won't help at all with RF that isn't 
detected  by the input stage of the FM radio. And, as Tom Rauch noted, the 
high pass filter is only configured for differential mode RF. You've got 
common mode RF.

Second, pay careful attention to the RF path to ground for the FM antenna 
at the intercom's electronics. Make sure that the shield is going to the 
CHASSIS of the intercom, NOT to the circuit board.  

Third, get a real good common mode choke (ferrite) on the coax from that 
antenna, and make sure that the choke looks highly resistive at the 
frequency of your transmitter. What you need to do is reduce the shield 
current. 

Finally, remember that virtually ALL detection in audio gear either follows 
square law or is rectification that only occurs below a threshold of 
rectification. That means that any dB change in RF strength will result in 
double the number of dB of loudness of the interference. In other words, a 6 
dB reduction in RF current on the coax will reduce the detected RF by at 
least 12 dB.  In other words, your filter doesn't have to completely eliminate 
the RF to get it "good enough" to make your neighbors happy. And if the 
detection is by rectification, all you have to do is get below the threshold 
and it goes away completely. 

Jim Brown  K9YC