----- Original Message -----
From: "Gene Bigham" <jbigham2@kc.rr.com>
To: <TowerTalk@contesting.com>
Sent: Wednesday, December 24, 2003 7:13 AM
Subject: [TowerTalk] Sloper & Tower
Merry Christmas to all, and to all, antennas at new heights!
I have been playing with EZNEC, which is dangerous since I really do not
know what I am doing. But here is what I have gotten so far. I have a two
section aluma tower 50' crank up that is nested at about the 30 foot level,
a rotator on top, a 10 foot mast on which 2 feet above the top of the
rotator is mounted a Cushcraft A3S, then at the top of the mast is a
Cushcraft vhf/uhf 4 element beam. At the 23 foot level up the side of the
tower I ran a wire down to X= -30 and 5 foot off the ground, there it turns
and runs an additional 50 feet along fence toward X= 20. I actually have a
dip in the swr on 160 of 2:1 with this arrangement. The tower has 10
radials on its base fanned out in a semicircle toward the back fence line,
laying just on top the ground, which I am not able to model due to the 20
segment limitation on the demo EZNEC.
> You might try downloading a copy of 4nec2 from one of the various
sources.. versions that support thousands of segments, and I think the
latest version might read EZNEC input files. In any event, it's got a
geometry editor that's fairly useful.
When I model this on EZNEC with the source right where the wire attaches to
the tower (coax center to the wire, braid bonded to tower), I get the 3D
pattern EZNEC makes, BUT what is interesting is raising the attachment point
on the tower lowers the gain of the antenna structure, and lowering this
point raises the gain on the patterns.
>> So what does the model look like? Are you modeling the tower? The yagi
on top? all of the other conducting stuff, even just as a single wire to see
if it's significant?
Lowering it makes the pattern more a donut with gain around 13-19dbi,
raising the feed point on the tower turns the donut into a bottom flattened
sphere and the top of this sphere begins to grow upwards as the feed point
is raised lowering the gain drastically.
>> 13 dBi? For a dipole? 13 dBi would be impressive for a multielement
beam. There's something weird going on here if you're getting that kind of
gain.
>> but, as for the qualititative changes.. as you move the feedpoint down,
the wire gets more "flat" and parallel to the ground, no? I assume you're
not running it as a free space model, so what you're seeing is the usual
cloud burner effect from a low antenna, where the antenna and its image in
the ground form a directive beam pointing up.
Looking at the currents on the model shows the entire tower as a vertical
with current highest at the base and tapering up the entire tower structure
just like you would expect on a vertical, there is also some current
distributed on the wire.
>> This is quite weird, as you say.
I would have expected the opposite results.
Am I doing something wrong?
>> almost certainly, but that's no sin.. and you're doing the right kinds of
things to figure out what might be the problem. There's no question that
modeling, particularly with limited models, can give very nonsensical
results that are arithmetically correct. This is why it's nice to have some
practical experience actually building and using antennas!
>> did the geometry checking in EZNEC flag anything?
>> Is your vertical mast touching the ground (in the model?)
>> Can you post the NEC input file? (or email it to me)
Thanks
Gene Bigham
_______________________________________________
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.
_______________________________________________
TowerTalk mailing list
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http://lists.contesting.com/mailman/listinfo/towertalk
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