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
On 5/30/2016 9:20 AM, Jim Thomson wrote:
Date: Mon, 30 May 2016 03:42:47 +0000 (UTC)
From: Dan Maguire <djm2150@yahoo.com>
To: <towertalk@contesting.com>
Subject: Re: [TowerTalk] Change in Frequency As Antenna Height Rises
Above Ground
K9RX (StellarCAT) wrote:
There's no "probably" to it! It is lowest (in frequency) closet to the ground.
[snip] Finally on 40 meters I saw as much as a 300Khz change from 32" (on saw horses) to
37' on just one element of the yagi... I believe it was about 150 Khz from 8' to 37'. And
think there might be an additional 20 Khz or so shift from here on up to probably 50' or so.
For a 40m dipole model (length 66 ft, diameter 1 inch, aluminum, 29 segments, over
"Average" ground) the chart below shows (red trace) the frequency of minimum SWR(50)
and (blue trace) the frequency of resonance (jX=0 at feedpoint) at heights from 2.5 ft
(30") to 100 ft.
http://s33.postimg.org/vjtgi392n/resonate3.png
While it is true that the frequency (either one, take your pick) is roughly 300 KHz
higher at 37 ft vs 30", and is higher still up to about 60 ft, from there to
about 100 ft the frequency *decreases* as the height is increased. It also
*decreases* from about 10 ft to about 25 ft. So it seems that no blanket statement
can be made. Whether the frequency increases as the height increases depends on
where you start looking at things, at least for this simple model, which may or may
not be an accurate representation of Gary's one element from a Yagi.
Dan, AC6LA
## OK, so resonant freq increases from 7025 to 7240.... when height
changed from 30 ft...to 60 ft. Great, so how the hell am I to tweak each
ele on my 3 el 40m yagi ??
F12 sez to tweak the REF at 6910 DE at 7070 DIR at 7500. ( 340N)
I find it really hard to believe that the resonant freq will increase by
215 khz.... simply by changing height from
30 ft to 60 ft. Thats a huge amount. Something is amiss in the model.
Jim VE7RF
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