[Amps] My NCL-2000 warc band experiments

[Manfred Mornhinweg]

Hi Frank,

> To operate on 30m, I shunt out approx 3 turns of the coil.
> To operate on 17m, I shunt out approx 1 turn of the coil.
> I shunt out the coil turns at the tuning capacitor end of the coil
> rather than at the bandswitch end of the coil.

Bad idea!

> The shunt wires got VERY hot during operation (enough to discolor the
> alligator clips due to the heat).

No surprise there. And the power that's heating those wires is RF power 
that doesn't make it to the antenna!

> Since the bandswitch tap wires don't heat up nearly as much as my shunts
> at the other end of the coil - I wonder if it's because there is much
> higher current where my shunts are?  i.e., like the way current and
> voltage change at different points along a  feedline?

A perfect coil has exactly the same current all along it. If there is 
lots of parasitic capacitance, then the current indeed changes along it, 
much as in a transmission line. But I'm pretty sure that this is NOT 
your problem!

The real problem is another: Let's picture what you have created there. 
While I don't know the exact number of turns, a typical configuration 
for your tank coil might be like this:

1 shorted turn at the start, with your jumper and clips;
6 active turns;
30 shorted turns at the end, with the bandswitch.

What now happens is this: You have, say 15 amperes of RF flowing in the 
active turns. This part of the coil imperfectly couples to the other two 
parts. The coupling of the six turns to the 30 shorted by the bandswitch 
is rather poor, because they are spread out quite far, so that the last 
end up physically far away from the active turns. Also, these far-away 
turns provide a significant inductance of their own. As a result, the 
current induced in the shorted 30 turns is rather small, and is not a 
problem, neither in heating nor in power lost.

Instead your single shorted turn on the other side is very well coupled 
to the active six turns! And it has little inductance of its own, to 
limit the current! So, that single turn is trying to conduct six times 
as much current as the active turns! And trying to put 90 amperes of RF 
through that wire will obviously heat it A LOT!

So, I would suggest that you drop that idea of shorting turns on the 
high side of the tank coil. It's bad enough to have shorted turns on the 
other side! As someone else said, the amp might be able to tune to the 
WARC bands with the band switch set to one of the neighboring bands. And 
if not, the correct way to modify it is to short out additional turns
on the low side, not on the high side.

Manfred.

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