On Mon,10/12/2015 9:15 AM, Stephen Davis wrote:
I was indicating what my actual results are, regardless of what software
indicates. Also, if I had looked at software first, I might still have done
the NIVIS based on other practical experience noted by others in the research I
did.
Nearly all published assessment of the vertical patterns of antennas has
been on the basis of what is called take-off angle -- the vertical angle
at which the antenna produces maximum radiation. That description leads
to a VERY false result.
In the tutorial I referenced, I looked at both high angle and low angle
field strength (performance) as the height of horizontal dipoles was
varied from 30 ft to 135 ft. I looked at 5 degrees, 10 degrees, 15
degrees, and 70 degrees, picking data points off of a full vertical
pattern. I looked most at 80, 40, and 20M.
In general, the high angle field strength is greatest when it is about
1/4 wavelength high (65 ft on 80), and drops by only 1 dB if the height
is increased to 3/8 wavelength (90 ft on 80M) or decreased to 1/8 wave
(30 ft on 80). But at lower angles (30 degrees and below), which is
where most of the stations we want to work in a domestic contest come
in, the field strength increases by 4 dB between 1/8 wave and 1/4 wave
high, and another 2 dB going to 3/8 wave. High angle field strength does
not begin to drop much until you get higher than 3/8 wave.
SO, repeating my statement, a low antenna does NOT work better for NVIS
than a high one, and a high one works MUCH better beyond a few hundred
miles. All of this is shown in easy to understand graphs in
http://k9yc.com/VertOrHorizontal-Slides.pdf
73, Jim K9YC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|