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[TowerTalk] Fwd: Change in Frequency As Antenna Height Rises

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Subject: [TowerTalk] Fwd: Change in Frequency As Antenna Height Rises
From: Hans Hammarquist via TowerTalk <towertalk@contesting.com>
Reply-to: Hans Hammarquist <hanslg@aol.com>
Date: Wed, 1 Jun 2016 16:45:31 -0400
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
It seems to me that Steve's, G3TXQ and Dan's, AC6LA diagrams coincide pretty 
well. You find the lowest frequency at about 1/4 wave and max frequency at 
about 1/2 above ground. What I don't understand is how the frequency start to 
decrease at the high increases before it reaches 1/4 wave above ground. In my 
simple mind the fx should start to increase as soon as the antenna "leaves 
round". 


To me, and obviously I'm wrong, the capacitance to ground should affect the 
frequency with the result that the fx should increase when the ground 
capacitance decreases up to the point where speed of light start to have an 
effect.


Anybody with a "simple" explanation?


Hans - N2JFS




Just found one a plot I did a while ago:
http://www.karinya.net/g3txq/temp/dipole_vs_cobweb_tuning.png

Steve G3TXQ

On 01/06/2016 09:42, Steve Hunt wrote:




-----Original Message-----
From: Dan Maguire via TowerTalk <towertalk@contesting.com>
To: towertalk <towertalk@contesting.com>
Sent: Wed, Jun 1, 2016 4:31 am
Subject: Re: [TowerTalk] Change in Frequency As Antenna Height Rises

Jim VE7RF wrote:
>>> The F12 N style 40m eles are not full size.  They range from 59 ft for the 
>>> REF...down to aprx 52 ft for the DIR.

Well, now we're making some progress at reaching common ground.  I created a 
dipole model with 1.5" diameter aluminum wires and adjusted the length for 
precise resonance at 7.15 MHz in free space.  Then I changed the length to 85% 
of the self-resonant length, added loading coils mid-way out each leg, and 
adjusted the coil inductance to again get precise resonance at 7.15 MHz.  And 
finally I changed the length to 70% of the self-resonant length and again 
adjusted the coil inductance to get precise resonance at 7.15 MHz.  Coil Q was 
kept constant at 200.

Then I switched from free space to average ground and varied the height of each 
dipole from 10 ft to 100 ft, this time adjusting the *frequency* for precise 
resonance at each height while keeping the length and coil inductance (if any) 
constant.

http://s33.postimg.org/v51yi2n5b/resonate6.png

100% length (69.79 ft), no loading, +218 KHz change from 30 ft to 60 ft.
85% length (55.93 ft), 3.297 uH coils, +137 KHz change from 30 ft to 60 ft.
70% length (46.06 ft), 6.721 uH coils, +78 KHz change from 30 ft to 60 ft.

So it seems that more loading results in less change in resonant frequency as 
the height above ground is varied.

Dan, AC6LA
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