Marv Gonsior wrote:
>
>For those interested in variable inductors, there was an oldie but I
>think worth reviewing, that was a rotating shorted turn inside the
>winding. Old handbooks showed the construction and it worked but just
>how well, by today's standards and measurement capabilities, is unknown.
It works fine in 50MHz amplifiers, with no noticeable heating in the
shorted turn if it is made from continuous copper (eg a slice from a
seamless tube, or a disk with a hole punched out). The shorted turn
floats inside the winding, typically supported on a rotating Teflon rod.
The application at 50MHz is in a Pi(L) network with no input tuning
capacitor. "C1" is only the tube output capacitance plus strays, and the
variable L does the tuning instead. This gets around the problem of
embarrassingly high loaded Q values (often 20 or more on 50MHz) that
result from having too much C1, so tank circuit losses are dramatically
reduced.
I haven't made any detailed measurements, but there is no detectable
temperature rise in the shorted turn after prolonged key-down at 1kW
output.
The only small problem (same as with the "stretch/squeeze" variable
inductor) is that the tuning range is rather narrow, but once the coil
is dimensioned correctly it works fine.
I believe there's a scanned article about W2GN's 50MHz amp using
shorted-turn tuning, on Skipp's website - does anyone have the URL?
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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