[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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