On Aug 31, 10:13 am, Tim Shoppa wrote:
I am postulating a full-height quarter-wave 40M vertical that is
switchable (relays halfway up?) to a 20M vertical dipole.


Myself, I would base feed for both bands. Say if you used a
"gamma loop" matching scheme for 20m.. Use a relay/relays
to cut the connection from the inductor to the radiator. With
multi-pole relays, a set could be used to connect the feed
directly to the radiator when the inductor is disconnected.
Of course, it would need to be weatherproofed.
I used a similar setup to switch my 40m elevated GP to
a 5/8 GP for 17m. If I left the inductor used for 17m inline,
my resonant 40m frequency was about 6.900 or so.. So I used
a 24v relay to disconnect and bypass that coil for use on 40m.
You could also use a regular coil for tuning the 1/2 wave,
but I prefer a single turn which is tapped at the best point.
If a cap is needed, and it probably would be on 20m, I use
a length of coax. "open at the far end, not shorted".
One thing about splitting for a 1/2 wave.. You gotta have
insulators in the middle of the mast, and then rig a way
to bypass for 1/4 wave use. I guess it's do-able, but I'd
just as soon use a one piece radiator and feed at the base
for all bands. BTW, with even more rigging, it could be used
for 3 bands, if you added 17m running as a 5/8 GP.
All you need for that band is a series coil at the feedpoint.
I used 24v fan relays usually used for switching fan motors,
and ran wire down to a 24v transformer here in the shack.
I'd unplug it for 17, and plug it in for 40m.
BTW, yes I did also unplug and throw that wiring outside
in bad T-storms.. Same as my feedlines.. :/
That wiring ran from the feedpoint at 36 ft, down the mast
to the shack. Could carry a hefty pulse if I took a strike
same as the feedlines.
The radials should be no problem. IE: 1/4 WL for 40m.
I think most BC users that run 1/2 waves, use a field
of 1/2 wave radials.. So, should work out ok I would
think. Besides, any radials on/in the ground are going to
be fairly detuned anyway. A 1/2 wave really needs no
radials, except that ground losses will occur when
ground mounted, same as any other antenna. But
they will be less than a 1/4 wave used on the same
ground mount, and same band. The set of 1/2 wave
radials should keep that loss fairly low. Will be
better than the 1/2 wave ground mounted with no radials.
MK