Hi Ian,
This is for a more broadband application.
I saw a thing a while back (don't recall quite where...) that had a row of
discs which could be rotated into a coil.
I looking for a range of perhaps: .1 to 2 uH
If it wasn't for the current, I'd try making a variometer.
73 & Good morning,
Marv WC6W
P.S. -- The link doesn't come up here... maybe I have the wrong version of
Acrobat or something else wrong...
-- <ian@ifwtech.co.uk> wrote:
Marv wrote:
>
>Anyone happen to know anything about the numbers involved with tuning coils by
interposing copper (or silver!) vanes between the turns?
>
>Specifically, what order of max-min ratio is obtainable?
>
>This is for a low impedance application therefore, the currents run high,
indicating against a roller inductor or bandswitch.
A rotatable shorted turn is sometimes used as the tuning element in single-band
pi-
tanks for 6m, but even when the shorted turn is buried completely inside the
coil,
it will only change the inductance by 10-15%. There's an example in:
http://www.newsvhf.com/6mstripline.pdf
(5MB file. Also the filename is misleading - it's not a stripline, but a
pi-tank.)
A vane would operate by induced eddy currents, but it would have poorer
magnetic
coupling to the main coil than a 'buried' shorted turn does. I'd guess that the
available decrease in inductance would be significantly less than 10%.
The surprising thing is how little heat is lost in the shorted turn, even at
the
1kW level. The secret seems to be to avoid making a joint in the loop. Methods
that
work include making the loop from a very short length of seamless copper pipe,
or
as a flat copper disc with a large hole in the middle.
73 from
Ian GM3SEK
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