Thanks Grant,
I worked with Greg, '8WWV, for quite a bit of the modeling - he would run my
models using NEC4 ... and although he'd get a slight shift in frequency his
results were similar to mine that clamps seem to take things further away
from 'reality' ... or at least at that time the only reality I knew. The
assumption I made, I believe somewhat legitimate, was if I used antennas
that had been around for a long time AND had patterns shown I could compare
to the model. I used the M2 20M4DX. It seemed to do better without the
clamps in the model. Better being as to their published results. Now that
stated that was before a single electron had gone through coax to a real
antenna here ... the results on 15 might suggest that indeed that antenna
isn't optimized where most would want it to be.
I find the peak of the FB to be the best variable to range test to since it
is easily measured. It is used as a sanity check - a strong indicator. And
although I too have seen myriad different patterns using AutoEZ/NEC2 I've
seen mostly correlation between FB/SWR and then gain. Gain is after all the
'easiest' variable to meet - one almost has to work to reduce it! That is
for a wide(r) spaced antenna... so I feel confident of the range test
results.
Gary
K9RX
-----Original Message-----
From: Grant Saviers
Sent: Monday, May 30, 2016 10:53 PM
To: StellarCAT ; towertalk@contesting.com
Subject: Re: [TowerTalk] [Bulk] Change in Frequency As Antenna Height Rises
Re the element to boom plates/clamps. I've found they make a noticeable
difference with NEC4. However, they are hard to model since the Lawson
substitute element diameters are so short to cause NEC 2 or 4 to not
like the segment length. They also have a big diameter step which NEC2
doesn't like. Lawson states his taper correction formula is based on
small diameter steps. I conclude that those who swear by insulated
elements do so to not have to swear at plates in the modeling.
Lawson (page 7-12) shows a 0.5% increase in resonant F for a 6x4"
element to boom plate on a 1" diameter 46Mhz dipole. Your measured
21375KHz measured vs 21140KHz modeled with no plates which is a 1.1%
change in freq with perhaps a larger plate for a 3" boom, but at half
the frequency. Direction ok, magnitude seems high. Does the plate
effect add up with 5 elements? Perhaps.
So if "relative results" (does that mean range vs model?) are better by
leaving them out using NEC2, it may be some combination of these
modeling problems.
I'm not as sure as you are that 2 out of the 3 variables (swr, gain,
f/b) fully define the performance of antennas over a band. Sure, f/b
and gain correlate, but the shape of all three curves vs frequency is
another matter, with many possibilities. Especially after seeing dozens
of end point "triple curve plots" that the AutoEZ optimizer has
generated after many thousands of trial designs.
Congrats on having set up a range - a significant accomplishment.
73,
Grant KZ1W
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