Jim VE7RF wrote:
>>> There is NO way in hell that the 66 ft long dipole, made from 1 inch OD
>>> tubing, is gonna shift 210 khz upwards in freq, when the tubing dipole is
>>> elevated from 30 ft to 60 ft.
I sure don't see anything in the model that would tax the capabilities of NEC
so I guess we'll have to agree to disagree on that. However, in playing around
a bit more I did stumble on something that may be of interest to those folks
who still put at least a little faith in antenna modeling.
I started with model 340-48H.YW from the ARRL Antenna Book CD, a 3L 40m Yagi
with a 48 ft boom, and used the Scale.exe DOS program to convert the elements
to monotaper. I also changed the units from inches to feet. Here's the result:
http://s33.postimg.org/hhm5hxebj/resonate4.png
Then I calculated the frequency of resonance (jX=0 at feedpoint, not
necessarily minimum SWR), first for the 3L Yagi and then just for the
stand-alone driven element (a dipole of length ~64.8 ft and diameter ~1.5
inches), at heights above Average ground from 2.5 to 100 ft.
http://s33.postimg.org/4yn6idnsv/resonate5.png
>From a height of 30 ft to 60 ft the Fres of the *Yagi* changed from 7.278 to
>7.258 MHz (-20 KHz). Over the same range the Fres of the *Dipole* changed
>from 7.132 to 7.349 MHz (+217 KHz). Ground reflection works in mysterious
>ways when there is also mutual coupling between multiple elements.
Bob K6UJ wrote:
>>> 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.
This is a 3L Yagi, not a 2L, but the model shows similar results. From a
height of 15 ft to 70 ft Fres changed from 7.244 to 7.255 MHz (+11 KHz). Since
a *Yagi* model seems to be in the same ballpark compared to actual measurements
I don't see why a *Dipole* would be any different.
Dan, AC6LA
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