[TowerTalk] Change in Frequency with Height above Ground

[Jim Thomson]

Date: Fri, 27 May 2016 08:40:28 -0700
From: jimlux <jimlux at earthlink.net>
To: towertalk at 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 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.
>


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