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