[RFI] Common mode noise

[Dale Svetanoff]

Pete,

Very interesting, but one big question:  You say that the Radio Shack wall
wart is regulated.  OK, is it linear or switcher?  One way to tell without
cutting or sawing it open is to look at the power input requirement.  If it
says "120 V" only, then it is likely a linear regulator (with an input
power transformer, rectifiers, a few filter caps, and a 12V regulator chip
or circuit).  If it says something like "90 to 240 VAC", then it is
guaranteed to be a switcher and everything you wrote makes sense.

If it is a linear, I do know of a few instances in which linear regulator
chips (such as LM7812, or equivalent) "took off" as oscillators because
they were not properly by-passed at their input and/or output pins.  A
properly functioning linear regulator should not contribute to the noise on
your system.

73, Dale
WA9ENA


> [Original Message]
> From: Pete Smith N4ZR <n4zr at contesting.com>
> To: RFI List <rfi at contesting.com>
> Date: 2/29/2012 10:58:07
> Subject: [RFI] Common mode noise
>
> 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 
> meters.
>
> 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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