>I don't know what quantity "startling" is. Perhaps the problem is my fault,
>because I call a change "insignificant" while others correctly define the
>very same change as "startling" or "substantial".
>Here's what EZnec sez for two wave angles:
>
>Flat dipole at 30 feet, 8.4 dBi at 90 degrees and 2.6 dBi at 25 degrees.
>
>Dipole with 30 ft apex and ends 3 feet off the ground is 7.5 dBi straight
>up and 1.3 dBi at 25 degrees.
>
>Pattern shapes are almost exactly the same! Bringing the ends to within
>three feet of earth results in about 1 dB change, the smallest amount
>perceptible to the average human in a controlled test. One dB is a few
>hundred percent smaller change than the addition of a small counterpoise
>system would make to the either antenna!
Tom,
Thanks for your input. Although most on Towertalk are quite knowledgeable,
I consider your input to be some of the most valuable. Of course, you are
correct, but I wanted to make a little clearer the constraints we were
operating under. I mainly did this before Field Day, to optimize the
dipoles we would use. We have a large number of pine trees, mostly 50 feet
tall. We were never able to do well on 80 with our 50 ft apex inverted
vee, except for very close stations, so I modeled dipole vs. inv-vee.
"Startling" might be a little strong, but I did find a significant
difference. Here are my figures:
Dipole, 50 foot height (broadside)
Elevation gain in dbi
20 .47
40 5.19
60 6.95
90 7.58
Inverted vee, 50 foot center height, 90 degree apex angle (broadside)
Elevation gain in dbi
20 -3.03
40 1.95
60 4.02
90 4.89
I am somewhat new to modeling, so could have errors in the process. I used
ground type real/fast, ground description 1.
The difference broadside is 2.69 to 3.5 db in favor of the dipole. On HF,
I would define this as significant.
On the other hand, the inverted vee was about 2 db better off the ends,
particularly at low angles. However, since we had dismal luck on 80 meters
before, we went for the dipole favoring the East-West directions (we are in
the middle of the country), and this contributed (we think) to a
significant increase in our 80 meter score.
>While this tiny change might be called "substantial", none of it would keep
>me awake at night or cause me to erect a second tower just so the dipole
>would be "flat"! While the flat dipole might look better and impress more
>visitors, I would simply add 20 feet of apex height to the 120 ft high
>dipole and save the few thousand bucks required to install a second tower.
>This is one case where money spent on the antenna results in a VERY poor
>return on investment.
Excellent point - but one of our circumstances is that we have plenty of
trees!
I agree - for home use, getting the center as high as possible and tying
the ends off far enough away to keep the apex angle reasonable should be fine.
Thanks again, Tom.
73,
Dennis
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