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

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

Dear Mal, LF Group,

> I can see you resonate your 3mH coild with a motor driven variometer but 
> how
> do you match this to exactly 50 ohmz for a SWR of 1:1 to the TX
> I can see your coil and transformer secondary are  in series to earth but 
> no
> adjustment for matching.
Essentially the same arrangement is in use here. The transformer ratio is 
adjusted to match the antenna resistance to 50ohms.
A Q of 1000 is typical for a coil of this size wound using Litz wire. You 
might increase that somewhat by optimising length, diameter, winding pitch 
etc. For something big like the Balboa loading coil in Alex's mail, Q can be 
considerably higher - Watt's "VLF Engineering" has data on this particular 
antenna system - the coil resistance at 25kHz is about 0.06ohms, and the 
reactance 225ohms, making the Q about 3800 - it might be higher at 136k, 
since reactance often increases faster than loss resistance as the frequency 
goes up. Incidentally, I estimate L of the main loading coil about 1.3mH, so 
GW0EZY would need the variometer in series as well ;-)
Cheers, Jim Moritz
73 de M0BMU