Yeah, why use a ball of wire for an inductor when you can bend up some
serious water pipe to do the same thing?
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Tom W8JI
Sent: Friday, November 17, 2006 3:11 PM
To: amps@contesting.com; Bill Fuqua
Subject: Re: [Amps] Russian power L-coupler
> 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.
I don't think everyone has a grasp of how poor that design
is for wide range inductances.
Let's assume it takes 20 feet of #12 wire to have enough
inductance to cover 160 meters. On ten meters the same 20
feet of wire is there, along with all the distributed
capacitance (that is what really kills Q) of the coil large
enough for 160.
>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?
.....because current in the coil is not the same as the
terminal current entering or leaving the coil when the coil
has significant distributed capacitance. That is what kills
the Q more than anything. Like trying to do a plate choke
that has all those turns hanging there and covers 160-10
without burning up on some bands....or like trying to do a
tank circuit and not shorting the unused turns in the
inductor.
> 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.
Instead we trade a simple problem like an occasional roller
arcing for an inductor that is crummy all the time. It has
severe series resonance problems over wide ranges of
frequency, and serious Q problems over wide ranges of
inductance.
Maybe if we had a good fixed inductor of 20uH with switch
taps and wanted a trimmer of +-4uH it would be a good idea.
A variometer in series with a tapped inductor is a workable
idea, if the frequency range or inductance range is limited.
I don't think people realize how poor a system that really
is. It isn't a good system for large inductance changes. It
isn't good for large frequency changes. The reason we don't
see them much isn't because people haven't heard of them.
The reason we don't see variometer inductors is they are
limited to narrow frequency or inductance range
applications.
73 Tom
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