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LF: Re: Re: Re: TX system at DK7FC, schematic

To: <[email protected]>
Subject: LF: Re: Re: Re: TX system at DK7FC, schematic
From: "James Moritz" <[email protected]>
Date: Mon, 19 Dec 2011 23:49:59 -0000
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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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