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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