[RFI] Finally located my RFI

[Dave (NK7Z)]

Hi Jim,
Thanks for the reply, and suggestions...  I have six mix43 FT240s in 
stock now...  I also have a pile of mix31s, but in this case, I think 
the 43s will be better...

Can either you, or Dave (W0LEV),  suggest a commercial unit that can be 
purchased, that I can have an electrician install?

If it were my unit, I would install however, being a neighbors unit, I 
want an electrician to install due to liability issues.  I am fairly 
sure I can get the HVAC folks to wrap a 43 core on the control lines at 
both ends...

73, and thanks,
Dave (NK7Z)
https://www.nk7z.net
ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

On 6/20/24 00:01, Jim Brown wrote:
> On 6/19/2024 6:08 PM, David Eckhardt wrote:
>> Dave, install ferrites on BOTH the input AND output.
> 
> I think you mean on both the mains power side and the control lines. 
> Strongly agree. BUT -- it's going to take more than single-turn clamps 
> that peak around 140 MHz to make much of dent between 17 and 29 MHz.
> 
> Couple of thoughts. First, dedicated twisted pair for the current 
> carrying conductors.
> 
> Second, look for anything that would divide some of the return current 
> with a random ground path, like building structure. The logic -- when 
> forward and return current can form a transmission line, their fields 
> cancel as long as they're equal, (and are confined to the narrow region 
> between the conductors that make up the line), AND the transmission line 
> is a much lower impedance than the wider random path, so all the current 
> goes to it. But if their is no transmission line, the return current in 
> that random path creates a large magnetic field, and also radiates as an 
> antenna.
> 
> Henry Ott got me to see this light years ago, and I got the same 
> analysis with an EMC engineer I encountered when he showed up as a 
> friend of the client at a church in Dayton for which I was designing a 
> sound system. The first thing he said after I'd outlined the issue was 
> "twisted pair."
> 
> 73, Jim K9YC
> 
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