By going from the conventional to the crossover method I was able to get a
G5RV to cooperate that I was using at the summer cottage decades ago. No
amount of twidling of the conventional turns worked.
At that time and a few hundred miles from the test equipment at work I was
more interested in getting on the air than the whys. Obviously I had found a
"sweet" spot.
Carl
KM1H
----- Original Message -----
From: "Ian White GM3SEK" <gm3sek@ifwtech.co.uk>
To: <towertalk@contesting.com>
Sent: Thursday, July 26, 2007 3:20 AM
Subject: Re: [TowerTalk] Ferrite beads for common mode current
> jeremy-ca wrote:
>>On higher HF and 6M simply winding all the turns in one direction adds
>>a lot of capacitance which defeats the effectiveness.
>>Using RG-142 or RG-303 has been popular for decades on these toroids.
>>Wind half the turns in one direction, then cross the toroid and reverse
>>for the next half.
>>
> Capacitance between turns always exists, which is what creates the large
> peak in common-mode impedance at resonance. This is particularly
> desirable for a monoband antenna because the peak can be placed right on
> the operating band. Or at least, it can in principle....
>
> Winding all the turns in the same direction brings the opposite ends of
> the winding closer together, so it moves the resonance lower in
> frequency; but this does not defeat the effectiveness of the choke at
> its new resonant frequency.
>
> The 'crossover' winding method keeps the ends as far apart as possible,
> so it moves the resonance higher. However, when the toroid begins to
> fill up with turns, there are now *two* places where the half-way point
> comes close to one of the ends, so there is still plenty of
> inter-winding capacitance. This means that the difference in resonant
> frequency between the two winding methods may not be so pronounced (on
> an 11-turn choke for 14-30MHz, I've measured as little as 1MHz
> difference 20MHz).
>
> The greatest advantage of the crossover layout is purely practical: the
> input and output are at opposite ends of the enclosure, so the main
> feedline doesn't have to double back.
>
> With both layouts, you need to tweak the number of turns and the
> positioning of the turns to bring the resonance onto the band - and with
> RG174/303 that ain't easy. You then need to keep the choke well away
> from the metallic boom to avoid detuning it. Fortunately the impedance
> at resonance is so high that you don't have to hit the band exactly.
> Anywhere close will be OK.
>
> If it isn't OK, or if a well designed choke shows signs of overheating,
> that indicates a pathological case of common-mode current. Don't blame
> the choke for that! Something about the antenna or the matching system
> is 'broken' and needs to be fixed.
>
> A large part of the secret is to use a well balanced feed system so that
> very little common-mode current would ever 'want' to flow on the outside
> of the coax. That leaves the choke with very little work to do, and
> almost any choke will be fine.
>
>
> Coming right back to the original question, G0JHC is using the standard
> Palomar bead balun kit on a 6m beam with no sign of problems at high
> power. Naturally, he is using a well balanced matching system.
>
>
> --
>
> 73 from Ian GM3SEK
> _______________________________________________
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