[TowerTalk] Change in Frequency with Height above Ground

[Jim Thomson]

Date: Thu, 26 May 2016 20:37:09 -0700
From: Jim Brown <jim at audiosystemsgroup.com>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Change in Frequency with Height above Ground

On Thu,5/26/2016 8:24 PM, Don W7WLL wrote:
> Lowering the height of an antenna above ground, such as a beam, will cause the resonant freq to decrease, increase?

Decrease. But not always -- I have a 10M beam at about 15 ft that goes 
up! The variable here is mutual coupling between elements and between 
elements and ground.

Another error in using SWR as the indicator of resonance is that 
feedpoint Z changes with height, and in a very complicated way (and 
because of mutual coupling to ground). In general, low dipoles (as a 
fraction of a wavelength) are lower Z, high dipoles are higher. AND -- 
feedpoint Z is also affected by the quality of the ground.

SO -- we may THINK the resonance is affected by the height as indicated 
by the low value for SWR, but it's actually the feedpoint Z changing!

73, Jim K9YC

##  so why does  eznec  depict RESONANCE,  moving all over the  20m band, as height is varied 
from 0-100 feet ??    ( this also depicts  3 x different ground condx).    http://s33.postimg.org/69rm0qscf/resonate2.png

##  f12  sez  to NOT  use a MFJ-259B   to measure resonance, but to instead use the freq of lowest  swr. 
When tweaking 40m  REF  +  DIR..and also DE, no matching coil is used.   Ele tips  ( or loading)  is tweaked, till
the REF / DE  /DIR  is  resonated at whatever freq the manual /software sez it should resonate.   And that’s  done
by measuring each ele, one at a time, by measuring the freq of lowest swr.  Meanwhile the other els that are not being tweaked
will temp have their  centers  opened off.   IE:  shorting strap removed if a REF  or DIR,  matching coil + coax + balun removed if a DE. 

##  Even  doing it that way, resonance will  shift up in freq on all 3 x els...as  yagi is raised above the ground,  like  20 khz, when 
moved  from 15 ft to 90 ft.    Another method is to tweak em, one ele  at a time..at a greater height, like 50 ft.  The entire yagi does not
have to be elevated to 50 ft.... just the  individual eles..... one at  a time.   Each tweaked up  ele is then mounted to the boom.  Then the
completed, tuned up 40m yagi is raised to its normal height, like  90 ft  etc.    

Jim VE7RF