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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|