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Author Topic: RFI Toroid winding question  (Read 1257 times)
AC2Q
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Posts: 348




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« on: November 06, 2012, 02:43:03 PM »

My station is "reasonably" grounded, as in an 8'solid copper ground rod, clamped and soldered to #6 solid copper wire, 6 feet from radio, etc.

Having some RFI issues, affecting usb devices when running legal limit.

So, QUESTION: I have some appropriate mix toroids.

Suppose I fold a cable in half, and wind it through the toroid, effectively then having 2 inductors in series wound in opposite directions.
(doing this because cable ends will not fit through center of toroid.)

While I believe this results in a net inductance close to 0, will it still serve as an effective RFI choke?

Constructive Comments Welcomed.

AC2Q
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G4IJE
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« Reply #1 on: November 06, 2012, 03:06:35 PM »

It may be better than nothing but the best solution would be clamp-on ferrites, which get around the problem of bulky connectors that wont go through a torroid.
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N3OX
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« Reply #2 on: November 06, 2012, 03:17:25 PM »

While I believe this results in a net inductance close to 0, will it still serve as an effective RFI choke?

No it will not.  The flux cancellation from the opposing windings will reduce the choking impedance to practically nothing.  

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
M0HCN
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Posts: 473




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« Reply #3 on: November 06, 2012, 03:24:19 PM »

Use clamp ons or remove one plug, wind the choke and reassemble.

Fitting RF connectors is a skill well worth learning, and is not that hard (For all that I rate the PL259 as one of the nastier things to fit, a monster of a soldering iron helps).

If you have quite a lot of excess length you can get some choking effect by winding a coil of a dozen or so turns and using cable ties to hold the turns together, it is more effective on the higher bands, but does offer some effect across most of the HF band.

Regards, Dan.
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KD0REQ
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« Reply #4 on: November 06, 2012, 03:56:26 PM »

use a large toroid and wrap around it as many turns as you can spare in the cable length, or do the same with a clamp-on.  coils wind the same direction all the way through to be effective.  you get all sorts of capacitive coupling and field cancellations if you double the wire over.  plus, those insulation coverings in USB cables are thinner than a tax man's smile, and you're begging for a short.  nice gentle wraps, all the same way, and use a cable tie lightly where the cable enters and exits so it doesn't unwind on you.
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AC5UP
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« Reply #5 on: November 06, 2012, 04:04:00 PM »

Suppose I fold a cable in half, and wind it through the toroid

Wouldn't that mean the input side of the coil will be flush up against the output side? With enough capacitive coupling (depending on frequency) that could allow a fair amount of RF to bypass the choke.

It should also be noted that most cables have a bend radius spec. Folding it could be a bad thing........   Tongue
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WX7G
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« Reply #6 on: November 06, 2012, 05:18:15 PM »

I suggest buying a good number of clamp-on RF chokes from DX Engineering. By a good number I mean a couple dozen or more.
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