I'm building a 160m/80m duoband amplifier with a Pi_L network and would
appreciate comments on a issue pertaining to switching the L2 inductor (a
toroid). On 160m I'll be using the full inductance (7.8 uH) and on 80m I'll
tap the inductor at 3.9uH (half the inductance). I plan to use a vacuum
relay to switch in the 80m tap. So my question is whether it's preferable to
switch the inductor so that on 80m the unused segment is short-circuited, or
whether it's preferable to leave the unused segment floating? If the
former, there will be appreciable circulating currents in the shorted
segment (i.e., heating and some loss), and if left open, then there will be
RF voltage induced on the unshorted segment.
My instinct is to leave the unused segment open-circuited and place it at
the downstream (antenna) side of the inductor, so that it's in a 50 ohm part
of the circuit. That would eliminate any circulating current and also keep
the voltage maneagable. The only downside seems to me that I'd have to use
an SPDT relay (with the common on the antenna), whereas with the
short-circuited configuration I can use an SPST relay, and on the 160m band
the relay is out of the circuit. Now that I think about it, I probably could
use an ordinary open frame relay instead of a vacuum relay, since the RF
voltage won't be more than a few hundred volts.
So whaddya think?
Jim W8ZR
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