Re: LF: Variometer es Loading coils

["mal hamilton" <g3kevmal@talktalk.net>]

Stefan

Your proposed solution is a Compromise so there will be losses or problems,  If a switch is necessary use a large motor driven ceramic switch. I have seen some very large ceramic switches about that will handle your power. or a Decca type relay with a very large GAP

Problems can be over come, there is hardware about for the job, but keep it simple is the best solution !!

 

73 gl de mal/g3kev

 

----- Original Message -----

From: Stefan Schäfer

To: rsgb_lf_group@blacksheep.org

Sent: Sunday, January 13, 2013 4:29 PM

Subject: Re: LF: Variometer es Loading coils

Am 13.01.2013 17:09, schrieb mal hamilton:

If I needed two loading coils for one antenna to cover LF es MF I think I would use one Loading coil and short out turns not required for MF and one Variometer would do both.

But then the switch has to withstand the HV when TXing on MF. Also the switch is not at ground potential. And if a matching transformer is used near the coil (to transform from or to 50 Ohm), it has to be dimensioned for both LF and MF. However if the LF losses strongly differ from the MF losses, the transformation is not ideal. Also the range that is covered from the small variometer coil may be much to large for MF, so it is tricky to resonate on MF :-/

I do not have a loading coil on MF because the antenna is a full size 1/4 wave inv L for MF and a separate antenna system for LF with around 400 uH loading/matching coil, no variometer, turns tapped up/down for a 50 ohm match.

This is one method to go. But a (isolating) transformer near the coil has some advantages like decoupling the earth (clean LF earth from noisy mains earth) and a high inductance of the transformer then the 50 Ohm side is not connected (that detunes the antenna and may help to reduce local QRM).

73, Stefan

 

de g3kev