[TowerTalk] Change in Frequency As Antenna Height Rises

[TexasRF at aol.com]

Jim, that was 30 inches to 60 feet.
 
Gerald K5GW
 
 
 
In a message dated 5/30/2016 11:54:48 A.M. Central Daylight Time,  
jim.thom at telus.net writes:

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  

_______________________________________________



_______________________________________________
TowerTalk  mailing  list
TowerTalk at contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk