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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|
