Date: Fri, 27 May 2016 08:40:28 -0700
From: jimlux <jimlux@earthlink.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Change in Frequency with Height above Ground
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.
@@ That’s what I thought. Resonance = zero reactance. Perhaps it had
something to
do with the length of coax from choke balun at feedpoint ..... to MFJ-259B ??
Unless
the coax jumper to MFJ-259B had zero length, Z and R could easily shift
about, depending
on band, etc. What might /should work is a double male connector between
balun and MFJ ?
@@ I went through this with F12 years ago, and they were adamant about using
the lowest swr freq
as the resonance point. Some of the junk baluns out at the time, would easily
shift the resonance point
by 100 khz on 40m...... IE: van gorden ‘baluns’. The wire leads from balun
to feedpoint will also lower the
resonance point.... but that is moot, since most folks know the leads are a
part of the ant.
@@ so what is the correct method then ?
Jim VE7RF
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