[RFI] Thermostat/HVAC Susceptibility

[Dave (NK7Z)]

Dan,

I should have mentioned Jim's site, I consider Jim's site the Gold 
Standard for ferrite use information, and testing...


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

Random Quote of the day:
Did you hear about the physicist who got chilled to absolute zero? They 
are 0k now

On 1/6/26 22:15, Jim Brown wrote:
> On 1/6/2026 6:39 PM, Dave (NK7Z) wrote:
>> If it were me, and I had that problem, I'd get some FT-240-31 ferrite 
>> cores from Arrow, or one of the parts houses.  I have some of core 
>> part numbers, and where to find them at web site:
>>
>> https://www.nk7z.net/rfi-now-house-cleaning/
>>
>> The ferrite part numbers are listed there, about half way down the 
>> page, with links to the manufactures site.
> 
> Hi Dan,
> 
> You didn't say what bands were a problem, but yes, some of the wiring 
> connected to the electronics controlling the furnace is functioning as a 
> long wire receiving antenna, and the solution is multiple turns through 
> a #31 Fair-Rite core. I try to solve problems like this with clamp-on 
> cores just large enough to wind 4-5 turns through them, ideally without 
> breaking existing wiring. I don't see the need go inside any of the 
> hardware, but you do want the choke (the windings on the ferrite) as 
> close as possible to the victim.
> 
> I keep three sizes of clamp-ons in my stash for this use. 1/2-in, 
> 3/4-in, and 1-in i.d. The same cores are useful for killing noise 
> generated by the dozens of switch-mode power supplies that are in all of 
> our homes, so I suggest that you buy a bunch of the two smaller sizes, 
> at least enough to get the first price break and to get you free shipping.
> 
> There's tutorial material on my website about how to look for noise in 
> our homes and in the homes of our neighbors (if they will let us). And I 
> also like Arrow as a source.
> 
> 73, Jim K9YC
> 
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