[TowerTalk] Change in Frequency As Antenna Height Rises

[StellarCAT]

The purpose of the saw horses was to assemble the antennas - not to be used 
for any real testing beyond that other than a sanity check that things 
aren't way off (i.e. 10:1 SWR). I do find that testing at 8' on all three 
bands, 15, 20, and 40 produced quite good results that are predictable and 
it most definitely can be used for testing with the caveat of frequency 
shift in mind.

Gary
K9RX

-----Original Message----- 
From: Bob K6UJ
Sent: Monday, May 30, 2016 11:39 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Change in Frequency As Antenna Height Rises

Jim,

With my 2EL Force 12 40M yagi, I followed Tom Schiller's guidelines and
set it up initially at 15 feet
above the ground.  When I raised it up to 70 feet the resonance
frequency increased by 20khz.  From 70
to 90 about 2khz.  This is what he said to expect in his manual.  No
modeling or study done.  Just
my results with the install.
I have tried the start out on the saw horse routine in the past with
yagis. The resonant freq ramps up on a
very steep curve and I had erratic results.  On sawhorses, even the dog
walking by would change the tuning :-)
If the model shows that increasing the height from 30 INCHES  to 60ft
results in a 215khz increase in resonant freq this could be realistic,
but I think we are spinning our wheels to do the initial tuning this
close to the ground.

73,
Bob
K6UJ



On 5/30/16 9:20 AM, Jim Thomson wrote:
> Date: Mon, 30 May 2016 03:42:47 +0000 (UTC)
> From: Dan Maguire<djm2150 at yahoo.com>
> To:<towertalk at contesting.com>
> Subject: Re: [TowerTalk] Change in Frequency As Antenna Height Rises
> Above Ground
>
> K9RX (StellarCAT) wrote:
>>>> There's no "probably" to it! It is lowest (in frequency) closet to the 
>>>> ground. [snip] Finally on 40 meters I saw as much as a 300Khz change 
>>>> from 32" (on saw horses) to 37' on just one element of the yagi... I 
>>>> believe it was about 150 Khz from 8' to 37'. And think there might be 
>>>> an additional 20 Khz or so shift from here on up to probably 50' or so.
> For a 40m dipole model (length 66 ft, diameter 1 inch, aluminum, 29 
> segments, over "Average" ground) the chart below shows (red trace) the 
> frequency of minimum SWR(50) and (blue trace) the frequency of resonance 
> (jX=0 at feedpoint) at heights from 2.5 ft (30") to 100 ft.
>
> http://s33.postimg.org/vjtgi392n/resonate3.png
>
> While it is true that the frequency (either one, take your pick) is 
> roughly 300 KHz higher at 37 ft vs 30", and is higher still up to about 60 
> ft, from there to about 100 ft the frequency *decreases* as the height is 
> increased.  It also *decreases* from about 10 ft to about 25 ft.  So it 
> seems that no blanket statement can be made.  Whether the frequency 
> increases as the height increases depends on where you start looking at 
> things, at least for this simple model, which may or may not be an 
> accurate representation of Gary's one element from a Yagi.
>
> Dan, AC6LA
>
> ##  OK, so resonant freq  increases  from  7025  to  7240.... when height 
> changed  from  30 ft...to  60 ft.    Great, so how the hell am I to  tweak 
> each ele on my 3 el 40m yagi ??
> F12 sez to tweak the REF  at 6910    DE  at 7070    DIR  at  7500.  ( 
> 340N)     I find it really hard to believe  that the resonant freq  will 
> increase by  215 khz.... simply by changing height from
> 30 ft to  60 ft.   Thats  a huge amount.   Something is amiss in the 
> model.
>
> Jim   VE7RF
>
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