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
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.
> -----Original Message-----
> From: firstname.lastname@example.org [mailto:towertalk-
> email@example.com] On Behalf Of David Gilbert
> Sent: Wednesday, October 21, 2009 11:39 PM
> Cc: firstname.lastname@example.org
> 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,
> 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.
> Dave AB7E
> Gary Schafer wrote:
> > An elevated vertical must have resonant radials.
> > 73
> > Gary K4FMX
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