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

[Hans Hammarquist]

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 at contesting.com>
To: towertalk <towertalk at 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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