[RFI] How much RF does your power company deliver along with the AC?

Christopher E. Brown cbrown at woods.net
Sat Aug 22 16:22:49 PDT 2009

On Sat, 22 Aug 2009, Jim Brown wrote:

> On Fri, 21 Aug 2009 21:44:29 -0500 (CDT), Christopher E. Brown wrote:
>> Perhaps I did not state things as clearly as they could have been, but you
>> may want to re-read my original message.
> Something else that comes to mind based on the symptoms is the grounding of
> the power line and your shack. The ideal is a very low impedance (very
> short) bond between the power system ground electrodes and your ham station
> ground electrodes. If there is noise on the power system and it is flowing
> to ground, you want it going to ground by the shortest path, one having a
> very low impedance, and one that radiates the least. You don't want it to
> have to go through your building wiring to your shack ground.

Since the shack is battery run, disconnecting the charger and the shack 
gnding bus connection to the shack ground and the external antenna removes 
_all_ connections to anything.

Running the radio straight from battery with only connection leaving the 
desk being the coax running to the attic dipole the only change is more 
noise when totally ungrounded.

A small loop near any AC wiring shows strong wideband noise, move a couple 
feet away and nothing.

I am assuming that I have a large air core transformer effect happening 
here.  A fair amount of common mode RF on the AC side with nowhere to go, 
with the 20M dipole acting as a single turn secondary.

I only posted after I had exausted everything I could think of, including 
current to ground via the shack, etc.

> One thing I did in Chicago to attack a similar high noise level was to put a
> string of #31 "big clamp-ons" on the 120V-0-120V twisted triplet overhead
> service to the building that ran more or less under my dipoles and more or
> less parallel to them. What I was trying to kill was the radiation from the
> power line into the dipole. One limitation of this approach is that at some
> frequency, the power line, with a high inductance at one end (my choke)
> becomes a resonant end fed antenna. :)

I remember the pictures in your papers, I want to do this myself.  The 
service leaves the ground and goe to the disconnect/meter.  I can get to 
the feeder to the sub-panel with 10 feet of the back of the meter, but the 
cable is too large to snap a largest clamp around by about 1/2 inch.

> It seemed to help a bit on 30 and 40M, but there was a lot of noise that
> came straight from my neighbors' houses (battery chargers, power supplies,
> etc.). I had about seven of them in the string, and I think more might have
> been better, but I used what I had. About the time I would have added more,
> I found this new QTH in CA and started moving. :)
> The "big clamp-ons" are the 1-inch i.d. parts documented in my RFI tutorial.
> A string of these is far from being an ideal RF choke for HF, because it
> will look inductive, not resistive.

I used about 5 of them around here for coax choking.  I know it would not 
be optimal, but if buying 10 a month for the next year would let me use 
attic antennas it would still be a win.

I only started looking at brick filters after I tried to place one and 
found it would not fit over the 4 wire feeder to the breaker panel.


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