On 26 May 2005 00:17:44 -0700, "MarkAren"
wrote:

Hi All,

I keep noticing something which looks odd to me - I am sure it is
correct because this appears to be standard practice, but I would
appreciate it if someone could shed some light on the reasons...

Take a peek at http://web.telia.com/~u85920178/tx/500w-txt.htm (first
example I found). Specifically look at the 5 position switch on the
output matching, in position 5, the bulk of the inductor is shorted
out. This just looks plain wrong to me.

I assume that having shorted turns on an inductor like this drops the Q
drastically, or is this the whole point ?

Is the same true for roller coaster type variable inductors ?


Mark,

Nominally those coils are loaded to a Q of less than 10. So the
shorted turns do not significantly affect the Q of the coil. Also the
turns are off the "end" of the inductor where the least effect is
felt.

In some cases the shorted turn can be used to tune the coil there
is a 6M amp that used a shorted turn inside the primary
inductor the make the inductance variable with out mechanicaly
altering the primary coil.

Part two of that is if t he unused sections were left unshorted
they may resonate and develope high voltages or other
undesired parasitic effects.

Roller inductors are indeed used that way as well.

Allison