[TowerTalk] Change in Frequency As Antenna Height Rises

[Jim Thomson]

Date: Mon, 30 May 2016 03:42:47 +0000 (UTC)
From: Dan Maguire <djm2150 at yahoo.com>
To: <towertalk at 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