Topband: Elimination of Treadmill RFI on 160 meters

[Bill Wichers]

If you're trying to find a suitable box to mount *in the wall* that will fit two duplex receptacles and some filters, try a 4-11 box (4-11/16" square) that is 2-1/8" deep (this is a standard size). You can use a "plaster ring" on this box to allow two duplex receptacles to mount properly and then mount the box in the wall (this last part will be a bit tricky since the 4-11 boxes don't usually have mounting wings). You could also get a "double duplex raised cover" for the 4-11 box if you want to mount the box on the surface. Any of the electrical supply houses should have all this stuff, just ask for a "four eleven" box. You might need to ask for a "deep" box (which is the 2-1/8" version) since there is also a shallow box around 1-1/4" deep. The 4-11 boxes can take any standard clamps that "normal" electrical boxes take.

With the raised cover option you could make a kind of "filter cord". Use the raised cover on the 4-11 box. The raised cover adds about 1/2" so that most of the internal space in the 4-11 box would be available for filter components without interference from the receptacles. Ideally you'd use a "cable gland" (sometimes also called a "rubber cord grip") to bring flexible power cable into the box (like type SJ cable). The cable gland is a round fitting with a big tapered rubber grommet inside. When you screw the metal ring over the grommet, the grommet inside compresses in the tapered fitting to grip the cord all the way around. It's much better than any of the metal clamps that are intended for NM cable (Romex). If you want a REALLY strong grip to the cable you can use a kellums grip (a wire basket that works like the "Chinese handcuffs" that grab fingers), but that's really not necessary.

Any of this stuff is available from commercial electrical supply houses but probably not the big box stores. The wholesalers will usually sell to anyone as long as you're friendly and don't ask lots of crazy questions since they don't want to take time to explain things to non-professionals. Just ask for the following items:
Four-eleven deep box
Double duplex raised cover for the four eleven box
Cable gland for 14/3 SJ (or whatever cable you're using)
Lock nut for the cable gland (they usually don't include one)
Get a few spec-grade receptacles -- they're good and only a few bucks at the supply houses. NEMA number 5-15R is the "regular" one, 5-20R is the 20 amp version with a "T" shaped slot.

If you need even more room, I'd use a 6x6x4 PVC junction box and mount the receptacles on the blank cover using a "normal" flush wall plate. This would give you a LOT of room inside, but you'll need to cut your own holes for a cable gland as well as for the receptacles themselves. Some of my electricians have found the cheapest cutter for this is the "conduit size" step drill that is available from harbor freight for around $10. You can also use a 7/8" hole saw.

   -Bill KB8WYP

> -----Original Message-----
> From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of
> Carl Clawson
> Sent: Thursday, January 29, 2015 1:24 PM
> To: Topband Reflector
> Subject: Re: Topband: Elimination of Treadmill RFI on 160 meters
> 
> Nice to hear all the comments, recommendations, and indeed some success
> stories here. I too have a treadmill problem. Although I'm blessed with a
> radio-friendly spouse who always asks before she starts it up, I'd like to add a
> line filter. Ferrites on the cord don't by themselves squelch it.
> Murphy dictates that the new one will show up when she's in the middle of
> her workout.
> 
> How about some outlet box recommendations?
> 
> The electrics are not the hard part for me, it's the mechanics. I've been
> looking for a suitable box to mount it all in and coming up kind of dry.
> Maybe I'm not looking for the right keywords. Seems simple, a box with a
> cutout for two 120VAC outlets and enough room inside for a line filter and
> some toroids. A strain relief clamp would be a big plus but something else can
> always be worked out for that. In the past I've bolted a surface mount outlet
> box to another project box but it's kinda big and ugly, and more expensive
> than it ought to be.
> 
> 73 and thanks in advance,
> Carl WS7L
> _________________
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