[TowerTalk] Delta Loop Info and Opinions?

[Michael Tope]

If you have a NEC program like EZNEC, Joe, you can actually
model a detuning shunt arm and then look at the current in the
tower and in the shunt arm. When the properly adjusted the
current inside the shunt arm/tower loop actually peaks at the
same time the series current below the shunt arm/tower loop
connection goes to a minimum. It seems a little counterintitutive
at first, but makes perfect sense after you think about how a
parallel resonant circuit works (the section of tower and the
parallel shunt arm form a big parallel resonant tank circuit)
that effectively inserts a high RF impedance in series with
the tower).

73 de Mike, W4EF.............

----- Original Message -----
From: "Joe Giacobello" <k2xx at swva.net>
To: "Tom Rauch" <w8ji at contesting.com>
Cc: <Towertalk at contesting.com>; <antennas at mailman.qth.net>
Sent: Thursday, September 16, 2004 9:26 AM
Subject: Re: [TowerTalk] Delta Loop Info and Opinions?


> Tom, thanks for your reply.  In all honesty when I read the article, it
> made sense to me because if one were trying to minimize the interaction
> between the delta and the tower, minimizing the RF current flow on the
> tower seemed logical.  I went back and rechecked the article to make
> sure I hadn't gotten it backwards, and I hadn't.  I am going to have to
> reconsider the whole issue in light of your explanation.
>
> BTW, I did mention that the deltas were for 30 and 40M and I had planned
> on vertically polarizing them and feeding them with balanced line.  The
> reasons for using the loops are that I am looking for some gain and
> directionality on 30 and 40M with a low takeoff angle, simplicity and I
> have a lot of experience with quad loops.
>
> I appreciate your input.
>
> 73, Joe
>
> Tom Rauch wrote:
>
> >>    * ON4UN in his book recommends detuning the tower by
> >>
> >>
> >running a wire
> >
> >
> >>      down the side in series with a variable capacitor
> >>
> >>
> >and tuning for
> >
> >
> >>      minimum current with an RF current probe.  However,
> >>
> >>
> >it seems that
> >
> >
> >>      this would only work for one of the bands, and you'd
> >>
> >>
> >have to
> >
> >
> >>      accept some interaction with the tower on at least
> >>
> >>
> >one of the bands.
> >
> >I always wonder where some of this stuff comes from.
> >
> >That's generally the opposite of how you should tune if you
> >are measuring current. You would generally tune for MAXIMUM
> >current in the sectionalizing drop wire, NOT minimum. That
> >insures the sectionalizing area is resonant. Since it is a
> >"loop", current flowing up equals current flowing down, and
> >that canels radiation. The end-to-end impedance is highest
> >at resonance, so the section you tune acts like a
> >high-impedance parallel tuned network isolating the tower
> >above and below that area.
> >
> >The exception to tuning this way is if the area below or
> >above the tuned area is self-resonant. It also is not the
> >preferred way to detune a structure, but the preferred way
> >requires a FS meter.
> >
> >
> >
> >>    * Does anyone have any experience with this detuning
> >>
> >>
> >trick?  Did you
> >
> >
> >>      find that it was really necessary?
> >>
> >>
> >
> >Well, I've done hundereds of them commercially. I'm not sure
> >how necessary it is in a Ham installation, especially since
> >you never said if the antenna was vertically polarized or
> >what band it was on.
> >
> >
> >
> >>    * Do you have any suggestions for how to accomplish
> >>
> >>
> >detuning for two
> >
> >
> >>      bands?  Two wires and two capacitors?
> >>
> >>
> >
> >That works, but it interacts if the wires overlap the same
> >area of the structure.
> >
> >
> >
> >>    * Does a crankup tower alter this detuning requirement
> >>
> >>
> >in any way?
> >
> >Only if the connections are intermittent.
> >
> >I have a question, why are you using a delta loop or a loop?
> >There are good mechanical reasons for using a loop, but
> >generally electrical reasons are not good. Will this be
> >vertically polarized?
> >
> >73 Tom
> >
> >
> >_______________________________________________
> >
> >See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
> >
> >_______________________________________________
> >TowerTalk mailing list
> >TowerTalk at contesting.com
> >http://lists.contesting.com/mailman/listinfo/towertalk
> >
> >
> >
> >
> _______________________________________________
>
> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
> TowerTalk mailing list
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> http://lists.contesting.com/mailman/listinfo/towertalk
>