[TowerTalk] Change in Frequency with Height above Ground

[jimlux]

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.