Dan ~
Often an RFI problem is a 2-way street:
- *Inbound to the device*: the device is susceptible to receiving and
being affected by RF energy, and also
- *Outbound from the device*: the device may also radiate RFI out onto
the HF spectrum.
I had a similar HVAC situation as you describe.
My chief complaint was the outbound RFI from the device - and a raised
noise floor on HF. But, I noticed transmitted RF (especially at HP)
getting into the furnace controller.
While, you are on a path to solving your inbound problem . .* it would be
interesting to see what that device is radiating out*. Sniffing around the
device with a portable receiver (or SDR) may be enlightening.
Measuring the signal strength of the radiation before and after
installation of the ferrite (at both ends of the control line) would help
confirm your fix.
In my case, such measurements after the showed substantial (but not
complete) reduction in outbound RFI. Then, after the first round of
ferrite, I added more - and confirmed by measurements that it was done.
*Link to report of my case*:
http://lists.contesting.com/archives//html/RFI/2021-11/msg00024.html
73,
~ Alan K0AV
On Wed, Jan 7, 2026 at 7:32 AM Dave (NK7Z) <dave@nk7z.net> wrote:
> 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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> > RFI@contesting.com
> > http://lists.contesting.com/mailman/listinfo/rfi
>
>
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