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Re: [TowerTalk] Change in Frequency with Height above Ground

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Change in Frequency with Height above Ground
From: jimlux <jimlux@earthlink.net>
Date: Fri, 27 May 2016 08:40:28 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 5/27/16 8:05 AM, Jim Thomson wrote:
Date: Thu, 26 May 2016 20:37:09 -0700
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk@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.



Resonance is where the reactive component of the feedpoint Z is zero.
Lowest SWR could be at some point where the the impedance happens to work out closest.



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