Topband: Inverted-L Spacing Question

[Tom Rauch]

> This question came up recently in a conversation with
> one of the locals. He is considering installing a 160M
> inverted-L (wire) along side his 80M vertical (4 inch
> irrigation pipe). I'm thinking a spacing of 24 inches
> or so for the vertical section of wire would be OK. I
> haven't ever played with this configuration so I may
> be incorrect. I also remember reading that the
> inverted-L should be 3/8 wavelength instead of the
> usual 1/4 so when it's open-circuited (when on 80M),
> there is minimal interaction. The 80M vertical would
> then be open-circuited when on 160M.

I'd never make the L 3/8th wave.

I'd make the inverted L a 1/4 wl. All he would have to do is
ground it when on 160. That could be done automatically with
a shorted stub or a parallel resonant circuit. As a matter
of fact the parallel resonant circuit could be used to
increase bandwidth (because the residual reactance
compensates the feedpoint as you move off resonance on 160)
and could also double as impedance matching.

It's actually a pretty bad idea to open an antenna and float
it.

Ask W0FLS what happens when the impedance gets real high and
you excite a nearby conductor! The relay would have to use a
heck of an air gap to be safe, perhaps a very large vacuum
relay would work.

If he made the antenna 1/4 wl long, he could use a normal
run of the mill relay to short the antenna to ground when on
80, or the stub or parallel network to do automatic
switching.

73 Tom