Jim
I have a couple of nice hi power litz wire wound variometers, maybe I should
try one. Except the variometer is remotely driven for adjustments the TAP
method is better, one tap up or down does the trick in my case where the
matching coil is a hundred yards away.
Another method is the rotated shorted turn loop within the coil for
inductance variations. I have used this and it works well.
de mal/g3kev
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Monday, December 19, 2011 11:49 PM
Subject: LF: Re: Re: Re: TX system at DK7FC, schematic
> Dear Mala, LF Group,
>
> > I avoid using a variometer which would degrade the main coil Q
> > mala/g3kev
>
> This is a bit of a myth - A variometer by itself often has relatively low
Q
> at the lower end of its inductance range, since the RF resistance stays
> roughly constant as the inductance varies, and Q = L/R. But typically, you
> would aim for the variometer making up only a small fraction of the total
> inductance, perhaps 5 - 10 percent, in order to allow tuning over the band
> of interest and accomodate changes in antenna capacitance due to weather,
> etc. Correspondingly, the variometer resistance will only be a minor
> fraction of the total coil losses, and total coil Q will only vary by a
> similar percentage as the tuning range. In a well-engineered system, the
> coil losses will in turn only be a small fraction of the total antenna
loss
> resistance, so the overall increase in loss caused by including the
> variometer will be miniscule.
>
> With a high loaded-Q antenna, small changes in reactance result in a large
> mismatch, and the continuously-variable variometer inductance allows
precise
> adjustment of matching. If you used some arrangement of tapped coils
> instead, the inevitable slight errors in inductance caused by only being
> able to adjust in discrete steps would probably result in a greater
> reduction in antenna current than that caused by variometer losses. Adding
> the taps , wire links etc. would introduce other losses anyway.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>
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