Jim Lux wrote in
:

Owen Duffy wrote:
Mike Coslo wrote in news:i9n3jq$ds62$1
....
The model suggests that shorting the unused turns is a poorer
solution when the flux coupling factor is relatively high, and a very
small number of turns are shorted. Poorer both because of loss and
the granularity of L adjustment.

In that situation, the voltage induced in open unused turns is not
very high, whereas it can be extreme in cases where most of the turns
are unused.


But in that situation, the voltage across the unshorted turns is
likely to be low, because you've got low reactance, right? So the net
effect is small.

Hi Jim,

It depends what you take as a reference. A rough estimate of the voltage
across the unused turns is the turns ratio (unused /used) time the flux
coupling factor, which in practical applications is likely to be in the
range 0.2 to 0.5.

If the application was a valve amp pi coupler, the voltage impressed
across the used turns could be a couple of thousand volts, and on a 10m
tap, the voltage across the unused turns could easily be 5 to 10 times
that.

In the case of a loading coil for a multiband whip, the current in the
used turns is probably similar from band to band, but the used turns are
lower for higher bands and so the flux cutting the unused turns is lower,
but there are more unused turns. In this application, insulation problems
are probably less severe than the PA with a bandswitch.

To my mind, the interesting thing is why shorting some turns is 'ok', and
then to understand that doesn't make it 'ok' in all scenarios. For
instance, someone taking a valve amp pi coupler and implementing it on a
single powdered iron core changes the situation significantly.

Re the Tee circuit, yes, but it takes a while to draw it up... not nearly
as quick as grinding a few numbers and creating a graph! I have a backlog
of hand drawing scanned into articles to redraw already.

Owen