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
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