I said that "A vertical above your beam will not work. An elevated vertical
must have resonant radials". At the end of your post you say the same thing.
With a quarter wave vertical mounted on the mast above a tri band beam You
would be very lucky if the beam provided a resonant radial system.
While it is true that you can use a combination of matching networks to tune
the combination and get it to work, it is not an easy task.
I agree that neither the ground plane nor the vertical element need to be
resonant in order to radiate. However getting power into a combination that
does not present resonance requires matching networks in order to bring the
feed impedance close enough to the feed line impedance in order to avoid
large losses.
Simply sticking a vertical on top of a mast above a beam is not going to do
the job unless luck is with you.
I tried doing this very thing several years ago. Played with it for a couple
of weeks and never could get anything close to a match. Then one day I hung
a quarter wave wire from the ground side of the feed at the vertical and got
a decent input impedance.
73
Gary K4FMX
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of David Gilbert
> Sent: Wednesday, October 21, 2009 11:39 PM
> Cc: towertalk@contesting.com
> Subject: Re: [TowerTalk] Ground mounting vs roof mounting verticals
>
>
> That's not really true. Elevated radials don't have to be resonant for
> an antenna to work well, or even be resonant.
>
> It is possible, for example, to have the COMBINATION of the vertical
> element and the radials be resonant ... the feedpoint impedance will
> just be higher than the classical 33 ohms. Think off-center fed
> vertical dipole, except with a portion of the lower part of the "dipole"
> splayed out. I've built antennas that way and I've modeled them with
> EZNEC as well ... it works. Several hams have compensated for a
> not-quite-quarter-wave vertical by making the elevated radials longer
> than a quarter wave, and it works the other way as well (longer
> vertical, shorter radials). Either way, tuning is a bit trickier than
> if the vertical element and the radials were each a quarter wavelength,
> though.
>
> And, of course, if the combination of the vertical element and the
> radials is not resonant, it will still work fine if you use the proper
> matching network to feed it.
>
> As far as feeding a vertical element against a yagi for a ground plane
> .... well, that's pretty much a crap shoot and it would take a lot of
> trimming of the vertical element if you want to make it resonant.
>
> 73,
> Dave AB7E
>
>
>
>
> Gary Schafer wrote:
> > An elevated vertical must have resonant radials.
> >
> > 73
> > Gary K4FMX
> >
> >
> >
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