Hi Dave,
Do you mean that with the tower tuned to 160
it seems to become a reflector??
Many Thanks for your info on this subject...
73,
Ted K2QMF
On Wed, 16 Jan 2019 12:04:56 -0700 David Gilbert
<xdavid@cis-broadband.com> writes:
>
> Pretty much this. I modeled K9OM's setup for him, and although I
> couldn't accurately copy his ground conditions and radial field, the
>
> model said there was virtually no interaction at all. He doesn't
> have a
> lot of top loading on his 57' tower so that makes qualitative
> sense. He
> is now planning to put his effort into upgrading his radial field
> and I
> referred him the N6LF's work on that.
>
> I was, however, able to significantly distort the model to show a
> major
> effect on his pattern, but only by adding enough inductance into the
>
> tower to essentially tune it to 160m. With the right value of
> inductance the pattern turns cardioid with about a 20 db notch in
> the
> direction away from the tower. That also makes qualitative sense,
> but
> I'm not sure that's a typical condition. A tower that isn't
> approximately close to resonance shouldn't have any more effect on
> an
> adjacent Inverted-L than a 15m element would have for 20m on a
> tribander. The fact that the tower is grounded is irrelevant.
>
> I think hams often like to generalize far too much without bothering
> to
> investigate further, especially in the case of antennas. We have
> tools
> available to us now that can turn rules of thumb (at least some of
> them)
> into science, but somehow the myths still perpetuate.
>
> 73,
> Dave AB7E
>
>
> On 1/16/2019 10:43 AM, Rob Atkinson wrote:
> > Where does the idea come from that a grounded tower holding up an
> > inverted L is a problem. Is this just some notion that took hold?
> > The tower is some how an RF sponge? Hams have been doing this for
> > decades. Of course the tower and ground system are bonded to each
> > other so the tower provides a return current path as with a ground
> > system. How is this different from an AM broadcast skirt fed
> grounded
> > tower. I have a 50 foot mast holding my inverted L--the L wire is
> out
> > about 3 feet from it. The mast has a 15 foot stinger on top to
> make
> > it a 1/4 wave 80 m. vertical. I can ground the mast or float it
> above
> > ground when using the inverted L on 160 but I see no difference in
> > performance. Anyone really concerned about this could always drop
> a
> > second wire on the opposite side from the inverted L, and tie the
> two
> > together at top and bottom with the single inverted L horizontal
> wire
> > continuing on, but I doubt if it would make much difference.
> >
> > 73
> >
> > Rob
> > K5UJ
> > _______________________________________________
> >
> >
> >
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> >
>
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