Tim Wescott wrote:
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 ?

Comments appreciated.

Thanks,

Mark.

I've pondered this question myself. I've only seen this done on
air-wound coils -- on a coil wound or a core this would be disaster. I
think the reason it's done is because if you _don't_ short the turns
you'd have all sorts of wierd resonance and/or high RF voltage effects
in the unshorted coils.

Most commerical receiver designs that used band switched coils
had a special rotary switch that shorted out all the unused coils
while they unshorted and selected the coil for the current band.
The reason was to avoid 'suckout' at the resonate frequency of the
unused coils.

Be aware that even if you do short out the unused section of the
coil in a tank circuit, if the lead lengths and the switch form
a resonate ground loop you will have arcing while you tune up
on some bands. Been there done that! (2kw amp with a pair of 4-400A's
in grounded grid. Selecting a good bandswitch is VERY IMPORTANT!!!!)