Sorry I meant 180 degrees. The point is if it is used in an application
where a high Q is not needed there would be no problem because the
circulating currents would not be high. Often they were used to fill in the
gaps between taps of larger coils. In a L-tuner and reasonable impedance
ratios the Q would be on the order of 2 to 5. And the Inductor would be in
series with either the input or the output impedance thus would not see any
higher currents than either of them. If the antenna feedline or the coax
from the amplifier were #10 wire and the varicoupler was in series with
one of them and was made of same size wire, why would it melt down first?
My point was that in certain applications it would be better because you
would not have to worry about contact corrosion or roller arcing or any of
that stuff.
73 Bill
wa4lav
At 01:10 PM 11/17/2006 -0500, Tom W8JI wrote:
> >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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