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Re: [RFI] 120Hz period noise ID

To: "Leigh L Klotz, Jr." <Leigh@WA5ZNU.org>,"rfi@contesting.com" <rfi@contesting.com>
Subject: Re: [RFI] 120Hz period noise ID
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Tue, 30 Oct 2007 12:09:27 -0700
List-post: <mailto:rfi@contesting.com>
On Tue, 30 Oct 2007 10:58:50 -0700 (PDT), Leigh L Klotz, Jr. wrote:

>Since the gas pipeline is radiating like crazy and runs next to the
>electric conduit which is presumably carrying the noise, and since the
>water heater doesn't look bonded nor the gas line, I'm thinking there may
>be some NEC problems here and she should get an electrician out pronto to
>make sure they don't get 120VAC onto the hot water line or the gas

I suspect you're right that this may be a "two-problem problem." Look for 
one (or more) of the following:

1) An extra bond between neutral and ground at some point.  NEC requires 
ONE, and ONLY ONE, bond at the service entrance. There must also be a bond 
between neutral and ground on the SECONDARY of any power distribution or 
isolation TRANSFORMER downstream of the service entrance. There must NOT be 
a bond between Neutral and Ground at any other point. 

2) A LOAD connected between one side of 240 and ground. NEC does not permit 
this. If there's a 120V load on a 240V circuit, there must be FOUR wires 
going to the load -- +120, -120, Neutral, and Ground. The 240V load is 
connected between +120 and -120, the 120V load is connected between either 
120V+ and Neutral, or between 120V- and Neutral. 

3) A variable speed drive motor with the transformer that feeds it and the 
controller for the motor widely separated from each other. There will be 
lots of capacitance (both stray capacitance of the motor and bypass 
capacitors on the controller) from line to ground, and those capacitors will 
create a large loop for RF current. That current is square waves produced by 
chopping DC, typically at 10-20 kHz, with the speed varied by the width of 
the pulse. LOTS of strong harmonics. 

See http://audiosystemsgroup.com/RFI-Ham.pdf  

Bottom line -- #1 and #2 are violations of NEC. #3 is not, but you've got to 
somehow get the noise off of those ground conductors, because the radiation 
of that current WILL be picked up by your antennas, just like any other 
radio signal. 


Jim Brown K9YC 

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