[RFI] Common mode noise

Pete Smith N4ZR n4zr at contesting.com
Wed Feb 29 08:58:05 PST 2012

I just built a bias-tee to inject 12 volts into my receive antenna 
feedline for a relay box and preamp at the far end.  I used what I think 
is a standard design, with a series 1 mH RF choke and an 0.1 uF bypass 
capacitor to prevent RF ingress from the DC line coming into the tee.

To my surprise, with no antenna connected, the 160M background noise 
level in a receiver connected to the tee was still several S units 
higher when the DC supply (a Radio Shack regulated wall wart) was 
connected than when it was removed.

I had a #31 core handy, so I wound 10, then 15, then 20 turns of the 
parallel wire from the wall wart to the plug on the end.  This 
progressively reduced the noise being contributed by the wall wart to a 
level such that I now cannot hear the DC supply on any band above 160 

I just thought this might be useful info for someone else out there, and 
I'd also appreciate any critique of the methods I used.

73, Pete N4ZR
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