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Re: [TowerTalk] NVIS (not exactly towers, but HF)

To: towertalk@contesting.com
Subject: Re: [TowerTalk] NVIS (not exactly towers, but HF)
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Tue, 9 Jun 2020 12:23:03 -0700
List-post: <mailto:towertalk@contesting.com>
On 6/9/2020 9:38 AM, jimlux wrote:
There's an awful lot of hams running 75m and 40m on "low antennas" - Sure, most goes up, and that lets you do local comms, but it's not like there's NO low angle radiation.

Right. Several years ago, I did a modeling study using NEC demonstrating the fallacy of common thinking on the topic of antenna height and radiation at various vertical angles. It's here.

http://k9yc.com/AntennaPlanning.pdf

It shows that the optimum height for high angle radiation is around 0.2 wavelength, that high angle radiation falls by only 1 dB at 1.5x that height, and by 3 dB at 0.4 wavelength.

The root of the fallacy is that patterns are nearly always plotted without a reference for calibration of the amplitude from one antenna or height to another. The differences jump out at you when field strength for multiple mounting heights are plotted with the same amplitude scale. It's easy to accomplish this in EZNEC. A pattern is computed and displayed for each mounting height, and the plot is saved. Then plots are recalled as multiple overlays to a single display.

One of the concepts I developed from these plots was a "figure of merit" in dB for mounting height for the lower HF bands, based on radiation at multiple low angles. That analysis begins on page 10.

I think, though, that there might be some fruitful work on a more tactical basis - people with crank up/down towers might find that there are "better heights" based on propagation *measurements* and modeling. (whether the model is in the brain of an experienced DXer or on a computer, it's all the same)

This has been VERY well known by DXers and contesters for several decades. N6BV's work is likely rooted in his long experience as a contester. In contesting stations, Yagis are often stacked at various heights, both to be driven in combination for gain and separately to take advantage of propagation at different vertical angles. Likewise, it has been well known for decades that the optimum propagation between one point and another varies over time with conditions in the ionosphere.

73, Jim K9YC
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