The AM Forum

Need some input from the audience. The ground fault feature of my radiant heat thermostat trips whenever I get on 75m. I’m sure it’s RF getting in either via the AC line or the wire grid in the tile floor. I’m pretty sure that a few chokes on the AC line and the temperature sensor should fix the problem.

Scavenging through some PCB’s I found three toroid’s that had two separate windings, each at 6.7mh. Had a QSO last night and while discussing this, two stations said that I’d be better off using two discrete chokes, rather than feeding the AC line through the two separate windings of one choke. Obviously, I’ll need one on the ground as well.

Opinions and suggestions?

 Not knowing anything about the toroid, and the impedance of your AC line, I'm not sure how anyone could give you explicit instructions.

That said, if you haven't already, check out W9YC's work.

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

He has a single core "brute force" AC line filter design:

A Home-Brew Filter The "brute force" line filter of Fig
30 provides effective line filtering for the lower HF
bands (10 MHz and below). The choke is wound with
a twisted pair of #12 THHN stranded wire on a #31
2.4" o.d. core, so it can be safely used in a 20A circuit.
As in the commercial filters, the capacitor forms a differential
mode filter with the imbalance in the inductance
of the choke, and should always be on the end of
the filter opposite the noise source. Since only the
"phase" (hot) and neutral conductors pass through the
choke, the equipment ground conductor (green wire)
must be carried around the filter (that is, from power
source to load). The capacitor should be about 0.47 μF,
Only Type X1, X2, Y1 and Y2 capacitors, which are
specifically tested to withstand the high voltage spikes
that can occur on power wiring should be used. Choose
this capacitor carefully – if it fails, it couldcatch on fire!

Steve...It's K9YC.  Here's a page that has lots of interesting info.  Scroll down about a third of the way to the Amateur Radio selections, and there is audio stuff as well as more good info on chokes.
Any scrounged toroid will usually make some degree of improvement, but for best results, it's best to do things right.  Besides the brute force line filter that you cited, he has some other solutions. 
http://k9yc.com/publish.htm

de Norm W1ITT

Great Info - Thanks!

I must have posted reference to Jim Browns stuff ten times.

His is one of those sites that should be included in what some call a 'sticky' thread.

Lots of good info, besides that pdf.

--Shane
KD6VXI

For sure. His "book" is awesome.

AC line filters almost always contain common mode chokes wound on a common core. Differential noise is very seldom the problem.