Mike Coslo wrote in
:

On 10/21/10 10:55 AM, Cecil Moore wrote:
On Oct 21, 8:32 am, Mike wrote:
After taking a good look at the loading coil, its apparent that
there isn't much choice. The bottom of the coil is attached to the
lower mast, and a four pronged plate that the tap wire is attached
to at the same junction. So unless no tap is used, some portion will
be shorted/ bypassed or the like.

So which would be better (less lossy) for a 75m Texas Bugcatcher coil
used on 40m? Short out each turn individually or use one jumper to
short out all of the turns that need to be bypassed?


Kinda my original question.

Intuition tells me that ideally - in order of preference:

1. the entire unused portion of the coil should just disappear.

Yes, but even better, a new coil optimised for the band. Not an original
thought though!


2. A shorting sleeve that renders the unused portion of that loading
coil as a fatter part of the mast.

Mike, some thoughts.

A word explanation of what my simple model draws out quantitatively for
an ideal coil (meaning insignificant distributed capacitance)...

Just one turn shorted / open...

If you short just one turn at the end of the coil, a current is induced
in the shorted turn. The current depends on the flux coupling factor k
and the number of turns in the rest of the coil. The loss depends on the
current squared and resistance... so whilst the current squared might be
high, resistance would be relatively low. Nevertheless, if k is high,
then the current induced in that shorted turn will increase total loss
significantly. The effect of a large current in the shorted turn is to
reduce the inductance of the combination, the higher the current (high
k), the greater the reduction in inductance.

If on the other hand, that one turn was left open a voltage is induced in
the open turn. The voltage depends on k and the number of turns in the
rest of the coil.

More turns shorted / open...

If more turns are shorted, the induced current is lower, and R is higher,
the net effect is that additional loss is reduced. Inductance is reduced,
but at a slower rate than intially.

If more turns are left open, the voltage induced in the unused turns
increases and could be many times the voltage developed across the used
turns. A first approximation is k times the turns ratio times voltage
developed across the used turns.

Shorting each of several adjacent turns...

This approach assures the highest circulating current in the shorted
turns, assuring the worst loss at all tappings.

Screwdrivers...

The approach taken in some screwdrivers uses a very low resistance
shorted turn (the tube) to make contact at the tapping point, and it
protects the unused coil turns (now inside the tube) from flux. Done
properly, that might well be a fairly good solution... I haven't tried to
measure it.

k...

Avoiding high k provides finer granularity of inductance adjustment and
lower loss when few turns are shorted... but of course lower k means a
physically larger coil for the same inductance, and potentially higher R
which increases loss.


Owen