[TowerTalk] Omnidirectional antenna for domestic contests. Re: TowerTalk Digest, Vol 154, Issue 18

[Jim Brown]

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