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.
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