Modeling programs are an excellent tool to calculate and predict some
properties of antennas - radiators.
What they do not "see" is the propagation mechanism after RF leaves the
antenna. "Space wave" can be "hugging wave".
While the high frequencies signals behave more like high frequency
sounds - propagating in the straight line, hitting the ionosphere and
then reflecting or refracting and continuing on.
Lower frequencies tend to propagate similarly as low frequency sounds -
"flooding" the medium. On 160 and 80 we see signals "flooding" the
medium with hardly any dead spots or zones, except some long haul path,
where some focusing and ducting comes to play. I tried to elaborate on
ways we propagate on HF in my article
http://www.k3bu.us/propagation.htm
I have observed that, say on 160 low angle signals, such as produced by
the salt water locations, seem to "hug" the Earth surface curvature,
appears to be ducting, refracting, rather than "bouncing" off the
ionosphere. We know that low band signals are "flooding", refracting,
ducting, signals without dead zones, unlike we do experience propagation
on high bands.
Polarization, its rotation in media, is another factor that modeling
doesn't capture.
Long haul and focusing is on the mercy of propagation conditions.
We have to look at the antenna (radiator, radials, etc.) as RF source,
analogous to light bulb.
Environment, ground conditions, surroundings, analogous to mirror(s)
affecting, participating in the beam shaping and efficiency of the
antenna. Modeling is an excellent tool for that.
Propagating media, air, ionosphere, as a "wire" carrying our signals.
Propagation modeling can do some help here.
Tailoring each of those factors and variables for the best fit gets best
results.
Then we have long delayed echoes that make us wonder what is going on.
I keep wondering if Earth - air - ionosphere - VanAllen belts - space is
some kind of giant vacuum tube or resonator that enhances RF signals?
Tesla calculated the capacitance and self resonant frequency of
Earth....
73 Yuri, K3BU.us
On Wed, Aug 13, 2014 at 09:34 AM, Richard Fry wrote:
> The link in the opening post of this thread shows an interesting,
animated analysis of the elevation gains of a monopole, based on a NEC
far-field analysis not including the surface wave.
I then posted this comment, "Reality is that radiation leaving the
monopole at elevation angles of at least 5 degrees decays at a 1/r
rate. Therefore that radiation is a space wave which propagates in a ~
straight line to reach the ionosphere, where (with suitable
conditions) it can return to the earth as a skywave."
Others have been skeptical that this low-angle radiation was a space
wave that could reach the ionosphere.
Recently I investigated this more closely, leading to the NEC4.2 study
linked below. It shows that the 3.8 MHz radiation existing on a
departure angle of 5 degrees at a distance of 3/10 of a mile from a
monopole continues on toward the ionosphere along that path, while its
value decays at a 1/r rate -- indicating that it is a space wave.
http://s20.postimg.org/6yau4m225/Monopole_Radiation_5_deg_Departure.jpg
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