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

["James Moritz" <james.moritz@btopenworld.com>]

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