agreed - but I have modeled my 6 element 15 and 5 element 20's. They are
conversions using a 15M6-125 to create a high strength 15M6DX (as it models
much better, especially in a stack) which I call 15M6DX-125 ... and using a
20M5 to make a 20M5LGS-100 ... again better performance version ... and I
have range tested these. I don't have a way to measure gain but using both
SWR AND measureable peak FB points I can see how and if it agrees with the
model (and assume, I believe correctly, that having these 2 variables means
the gain will follow as well).
So does it agree with the model? Generally it does not - or at least not
absolutely - it does relatively ... so the model is off in frequency ... or
stated another way using the dimensions from the manufacturer produced
results that were frequency-wise different from the model - but the patterns
are the same. So the only change is a shift in where it all is. I'm sure the
gain is also appropriately placed when I've done this shift.
Also I'll note, and leave it up to others to debate, I built the 15 exactly
(no worse than 1/8") to the M2 manual - the only differences are the boom
size going from the lighter weight 2" - 2 1/2" used on the standard 15M6DX
to the -125 boom (3") ... and the clamps are different.
Now I've found that clamps seriously mess up the models - I find it is more
accurate in results (again relative results) to leave them out - as well as
the swaging (my first models included it) and I couldn't find a clean way to
model the boom with NEC2 but using Lawsons boom compensation values it
should be trivial.
The results? The 15M6DX showed a FB peak at 21375Khz! I had to lengthen all
elements by 3" each to get it to I believe 21140 where I wanted it. The peak
FB was ~48db b4 the change and about 44 db after. So - are their dimensions
actually producing high results? Or are the clamps and/or boom causing THAT
much of a change (I don't believe that at all).
Finally the 20M5LGS made form a 20M5. Here the boom is the same, 3", the
clamps are different but because its 20 meters (lower frequency, less
percentage difference) I suspected they would have little effect overall. I
built exactly to the manufacturers dimensions ... the range testing showed
very close results re FB peak frequency and distribution as well as an SWR
curve that was nearly identical to the model ---- with again the caveat that
the model was at a different frequency! [I was able to make one single
change to the manufacturers dimensions - I changed the hairpin moving it by
1" and that improved the already very nice SWR curve making it no more than
1.30 across the band).
So my take is that these tools are not useable as an absolute tool - they
are accurate in their results but the relationship of the results to the
actual frequency is relative and needs to be compensated for.
oh - and I'm told - if the elements were isolated and I was using NEC4 that
the results would be accurate both in a curve by curve comparison (model to
actual) as well as absolute frequency placement... of course starting with
all M2 antenna I wasn't about to isolate them - and NEC4 is a pretty penny
additional.
Gary
K9RX
-----Original Message-----
From: Grant Saviers
Sent: Monday, May 30, 2016 1:45 PM
To: Jim Thomson ; towertalk@contesting.com
Subject: Re: [TowerTalk] [Bulk] Change in Frequency As Antenna Height Rises
Ok, another cut at this with a much more complex model - 6 element 17m
beam on a 48' boom. (I happen to have that model in EZNEC Pro4 and
AutoEZ). Finding "resonance" isn't of much interest. What is of
interest to me is the change in swr, gain, and f/b over changes in
height for a design I've been optimizing.
Here is a verbal description of results:
1. Gain changes smoothly upward from 0dbi at 5' to 16.7dbi at the
optimized height of 100'.
2. SWR decreases from 2.6:1 at 5' to 1.3:1 at 15' and then decreases to
design height value 1.16:1 by 30' up.
3. Minimum f/b is 28db vs the optimized value 30db also at 15 ft.
However, maximum f/b increases significantly at selected frequencies as
the antenna is elevated.
A quarter wavelength on 17m is about 14ft, so this modeling of this beam
would indicate that a bit more than 1/4wl height is sufficient to
predict results at much higher elevations.
My modeling (as G3TXQ comments) shows that max gain, min swr, and max
f/b all happen at different frequencies. So, "tuning" a beam with swr
seems like a trip to Vegas re what the performance will be. I think it
is at best a quick check that there are no gross assembly errors. I've
had the AutoEZ optimizer generate beam models with 1:1 swr, but with
reverse patterns, straight up patterns, no gain, etc.
So for beams, I think good models/modeling provide the correct
dimensions. The above results were from EZNEC Pro/4, dbl precision,
driven with AutoEZ to generate 250 test cases, frequency x elevation. I
wouldn't try analyzing this without AutoEZ.
Grant KZ1W
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