Hi Ford,
Noise is best stopped at its source. (A filter at the furnace) I had this
problem at another QTH years ago. Not only can the wiring conduct the noise
like a poorly matched transmission line, but can act as an antenna. Noise like
this is broadband and once radiated into the air can not be filtered easily.
Phased signal filtering devices are not the way to go, if you can stop it cold
at the source..
You said the noise is there with the fan off. (The ignition transformer may be
on, bringing the temperature of the furnace up, before the blower kicks in.)
Above all, the furnace must be either the problem or close to the problem as
the start winding of the blower can stop your noise.
Furnace transformers can have constant ignition in some furnaces. The electrodes
can have carbon tracks to ground. Having the furnace serviced is an option, but
not necessarily a cure.
Try to find a good AC rated Brute Force Filter. . You could check the reflector
DXers for
known sources. ( A tune able filter would work also but good for one band and
tuning can also drift)
Years ago I used a coaxial capacitor filter at the burner transformer primary
and blower motor connection.
A home built dual pi section filter was used at the furnace power connection.
(Right at the furnace).
The furnace chassis was the filter ground, and the furnace chassis was grounded
to earth. (Short ground wire needed) I rented a jack hammer to get a hole
through the concrete for my ground rod. Reddy Mix brand concrete was used to
re-seal the hole.
Good luck,
73
Bruce-K1FZ
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