Actually, the regulatory bodies are very interested in skywave
propagation -- it is the basis for nighttime interference protection among
AM broadcast stations. But you are correct that skywave is always
calculated, while groundwave is vertified by field measurement.
The only well-documented skywave measurement I know of was done many years
ago at WHO in Des Moines, IA. Their tower is a vertical collinear with a
phase reversal network at 1/4-wave height. Can't remember where, but I read
some recent notes that compared Method of Moments modeling with those
helicopter measurements, and they were remarkably close. Kudos to the old
timers who made very careful measurements.
The other part of the story is that the measured data agreed with theory, so
I am comfortable assuming that skywave radiation is predicted with
sufficient accuracy by modeling. Groundwave measurements can be used to
establish overall performance and skywave scaled accordingly.
> What I have never seen documented is actual measurements of far-field
> sky-wave changing with numbers of radials, along with the corresponding
> measurements at the ground. Sky-wave has always been by inference, such as
> how much field is theoretically "available" with a given power level at
> the feed, given some "ideal" situation. All had one assumption or another
> at root, with one's confidence in the assumption propagated to the
> inferred sky-wave.
> It always seemed to me that some very tall non-conducting structures, or
> the likes of a helicopter and GPS or some strict, accurate positioning
> method for the helicopter would be required to document the ground/skywave
> Since the commercial and regulatory interest at MF was millivolts/meter at
> the ground, there doesn't seem to have been much motivation.
> It's not the sky wave that I would suspect as quirky, but real ground is
> subject to such variation as might give some kind of non-linear result
> with increasing radials that would not appear in the skywave.
> 20 degree skywave at a distance of one mile (~10 wavelengths at 160m)
> would require an elevation of 1922 feet for the measurement.
> There may be some directly measured data out there, but I have never seen
> it, and would love to see the citation if it exists.
> 73, Guy.
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