VK3QI wrote:
>>> Are you really measuring what DXers are really after when comparing
>>> vertical antennas at different heights above ground and ground mounted.?
Putting aside for the moment the distinction between measuring and modeling, I
wanted to investigate what VK3QI said about "DX angles" (in this case 10°) vs
"zero" angle. First thing I did was create an adjustable vertical dipole model
that also has ground conditions that can be set via a variable. Here's the
AutoEZ "Variables" sheet showing (to start) the base (variable B) at 20 ft, the
length (variable D) at 35 ft, and the ground type (variable G) as "Average".
http://ac6la.com/adhoc/VertDip1.png
With those starting conditions I used the Resonate button to reset the length
(variable D). After that the length was not changed.
http://ac6la.com/adhoc/VertDip2.png
Then I set up a series of test cases with the base at 0.5 ft and with the
ground characteristics varying through all the non-water choices, from
Extremely Poor to Very Good, per the EZNEC definitions of such.
http://ac6la.com/adhoc/VertDip3.png
Here's an animated gif showing the results. The value for "G" (Ground type
index) may be seen in the lower right corner.
http://ac6la.com/adhoc/VertDipB05.gif
As the ground gets better the gain at 10° gets better (pretty much). Then I
ran a similar series of test cases with the base of the dipole at 40 ft.
Again, look in the lower right corner to see the "G" value for any given frame
of the animation.
http://ac6la.com/adhoc/VertDipB40.gif
That was interesting. With the base at 40 ft the gain at 10° gets _worse_ as
the ground gets better. More of the energy is going into the second (higher)
lobe. However, note that the outer ring is the same for both these animations
so you can compare magnitudes as well as pattern shapes. For any given ground
type the "DX angle" (10°) gain is always higher for the higher dipole. For
example, here's a comparison for Average ground.
http://ac6la.com/adhoc/VertDip4.png
So that seems to address the first point that VK3QI made. Gain at "DX angles"
(in this case 10°) gets better as the dipole is raised and it doesn't matter
what the ground conditions are. Now for his point about actual measurements:
>>> Measuring a vertical at another location 5 miles away, but at the same
>>> relative height is really measuring the ability of the vertical antenna to
>>> couple to the ground to produce a vertically polarised ground wave.
And that seems to be in regards to page 76 from the K9YC presentation:
http://ac6la.com/adhoc/VertDip5.png
K9YC measured an increase in gain of 9.5 dB as the dipole base was raised from
0.5 ft to 40 ft. If both he and the 3 watt transmitter at W6GJB had been
located on the wheat fields of Kansas that would have been one thing, but they
are both in the Santa Cruz mountains and I'm betting it's not exactly 5 miles
of flat ground (as NEC assumes) between them. Hence I'm not sure how to use
NEC to verify the K9YC measurements since there is no Far Field at an elevation
of 0°. So I fudged a bit.
Here's an animation with the green dot marker at 1° elevation, not exactly the
same as equal heights but close. The ground type is fixed at Very Poor per
K9YC's comments. The dipole base ranges through 0.5, 10, 20, 30, and 40 ft.
http://ac6la.com/adhoc/VertDipFF1.gif
As the base height is raised the gain at 1° increases by about 6.6 dB (from
-19.34 dBi to -12.72 dBi). That's not identical to the measured increase of
9.5 dB but it seems to verify that the gain at "ground level" (almost) should
increase as the dipole height is increased, which seems to corroborate what
K9YC measured.
Not sure what any of this proved or didn't prove, just thought I'd share it.
Here's the model file I used if anyone else would like to play with it. (Note
that you can't just click on the link to open the model. Save it to your
computer then open it from within AutoEZ, as explained in Step 3 of the AutoEZ
Quick Start guide.)
http://ac6la.com/adhoc/VertDipole.weq
This model will work just fine with the free demo version of AutoEZ which you
can download from here.
http://ac6la.com/autoez.html
Dan, AC6LA
http://ac6la.com
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