>I wonder how bad the varicouplers really are. The they
>often don't have a
> rolling or rubbing contact like roller inductors because a
> piece of very
> flexible wire can be used.
The area that primarily causes Q reduction in a roller
inductor at lower frequencies is the shunt capacitance from
the inductor to the frame and the countering flux generated
by end plates if metallic At high frequencies it is the form
factor error as the number of active turns is reduced, as
well as all that coil hanging there unused.
The least worry in the world is the roller wheel. As long as
it is clean and silver plated, a few inches of braid would
have more loss.
>Also, you can go from max. inductance to minimum
> 80 degrees. At minimum their Q's would be low
Q's are terrible if you try to get a variometer to go much
less than 4:1 inductance ratio.
1.) It has all the stray capacitance of the full coil.
2.) It has all the copper path length of the full coil.
3.) Flux is forced out of the coil by opposing fields into
the surrounding area.
Variometers tune fast, wear well, and are capable of very
large amounts of inductance but they have the poorest Q of
any system, and they are a disaster when operating over wide
frequency and inductance ranges. They are generally pretty
poor except in very specific applications where the speed of
inductance change or operating life is a major concern.
That's why they have very few and very limited applications,
not because people aren't aware they exist.
>but L-networks have lower
> Q's than T-networks. The Q goes as the square root of
> impedance ratio. So
> if you matching 50 ohms to 5000 the Q would still only be
> 10.
As likely would the Q of a variometer coil that would cover
160 meters when used on ten meters, if it didn't melt from
internal resonances.
73 Tom
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