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

[jimlux]

On 6/9/20 12:23 PM, Jim Brown wrote:
> 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.
>

what's potentially new, different, and exciting is that we can start to 
"mechanize" the knowledge embedded in experienced contester's head.

  Whether that's good or bad is a subjective thing - kind of like 
whether a keyer or morse decoders like CWSkimmer are a good thing - or 
whether JT65 is better/worse than CW for moonbounce.

Or chess playing computers.