Mike, thanks to you and Tom, I now see what's happening here. Defining
the arrangement as a parallel tuned circuit makes sense.
BTW, I do plan on modeling the antennas before erecting them. I'll let
you know how I made out.
73 and thanks for your input.
Joe
Michael Tope wrote:
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@swva.net>
To: "Tom Rauch" <w8ji@contesting.com>
Cc: <Towertalk@contesting.com>; <antennas@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
_______________________________________________
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_______________________________________________
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
_______________________________________________
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.
_______________________________________________
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_______________________________________________
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.
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