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.
## 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
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