I was going to say the same thing - well stated Guy!
I'd guess your results Don aren't an indication of anything to do with
deficiencies in the translation from 'model' to reality (which I was/am
looking to better understand) but rather simply that the antenna isn't well
modeled yet. At those frequencies EVERYTHING is 'nearby' and potentially
having an effect. Add in as much detail as you can and see if that brings
the resonant frequency down (even if incrementally just confirming indeed
this supposition is correct). Let us know please.
Gary
K9RX
-----Original Message-----
From: Guy Olinger K2AV
Sent: Thursday, June 11, 2015 4:33 AM
To: AntennaWare
Subject: [Antennaware] EZNEC v6, sloper supported by tower
On Wed, Jun 10, 2015 at 1:31 PM, <donroden@hiwaay.net> wrote:
I hope to confirm how a sloped wire works from a tall tower ..
The 130 foot slope is almost straight down . 140 feet up 50 feet out to
the property line. My antenna analyzer says it's down in the Broadcast
Band instead of the target 1.825 . Climb and cut seems the next step.
Don W4DNR
With the sloper you mention, what you are describing is the tower and
the rest of the conducting environment controlling the resonance of
the antenna system. Simply told the particulars of all the conductors
there, my guess would be that the self resonance of the *system* would
be down somewhere in the broadcast band, and would not respond to
attempts to retune it by messing with the length of the sloper.
The antenna system actually consists of the sloper you describe, plus
any radials either to the feedpoint and/or to the base of the tower,
plus the tower, possibly plus its guy wires if metallic, plus a mast
and any high band antennas on top of the tower if that exists, plus
all wire and coax running down the tower and along its length length,
and conceivably into the shack.
One can create models of an inverted L with the bend supported by the
tower, that have more current in the tower than in the L even though
power is fed to the L. What's actually going on is rather subtle and a
little demonic :>)
To unravel what is going on, you need a complete model, that in
addition to the sloper, includes the tower, mast, top antennas, all
wires and coax on the tower, any 80 or 40 meter dipoles or inverted
vees within a half wavelength radius.
Now fire up your sloper in the model and see how other things are lit
up by looking at the current traces in the View Antenna window.
The solutions to all of these is not so much modeling, as by dealing
with all the miscellaneous coupling in a manner to restrict unwanted
radiation from miscellaneous conductors. This is, as one might expect,
a fairly wide ranging subject.
Sometimes these unintended couplings can be made to serve a purpose,
but mostly just need to be defeated in some manner. Many times, using
the tower as a close support simply must be abandoned.
73, Guy.
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