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

[Jim Brown]

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