Topband: Skywaves from Monopole Surface Waves

[Guy Olinger K2AV]

This may be the nub of it, and it comes apart with our two fairly separate
uses.

Hams are after sky wave, and local area or even regional groundwave could
matter less to most.  For those trying to compare local wire antennas
without going airborne in the next county, the broadcasting paradigm is
adopted.  That works because standing on the ground with a meter is all we
can do with precision.

NEC's accuracy at MF is optimized for the broadcasting paradigm, for
standing-man-with-meter, who is also
standing-man-with-meter-where-broadcast-customers-are.  That is no
coincidence and simply represents where the issues have taken us year in
and year out, and represents the driving force for anything done to NEC for
MF frequencies.  Going to HF for hams with yagi's, elevated horizontal
wires where ground interaction is not an issue, ground wave is a not
player.  Hams with vertical antennas at MF is a foot firmly planted in both
worlds.

Along comes Mr. Fry and others, and says that the SKY WAVE is affected by
surface waves escaping which will "fill in" the low elevation notch from
sky wave from NEC sky wave pattern generation.

There certainly is something to some stations really having better signals
on propagation modes attributed to very low angles.  And just as clearly
some don't have it.  NEC patterns don't show any kind of fill like that
unless you do it over extreme conductivity, e.g. salt water.

Mr. Fry supplies a monolithic, complete fill up to three degrees at some
unspecified frequency over unspecified ground out 2.8 km.  The difference
between that and the NEC pattern generated for 1/4 wave over 120 0.4
radials at three degrees elevation is over seven dB.  That is a LOT of
fill.

The degree of increase in signal over salt water at the beach using a
battery K2, standing-man-with-K2, and the stark increasing difference
walking away from the water is a strong indicator that the surface waves go
for vast distances over sea water.  But the decrease also introduces the
idea this phenomenon is severely moderated over dirt.

If one is going to try to make sense of some modification of the low angle
pattern notch for hams working transcontinental signals, insisting on local
area methods will never capture the range of it, however appropriate for
broadcasting.  Nor will it capture the situations where it is useful.

There are all kinds of indications that the ground approximation methods
used in NEC work for broadcasters may not be so good in other situations.
 Try as I might with NEC4, I cannot construe any kind of ground to get the
current on radial wires for 90 degree radiator over 120 x 0.4 wl radials to
resemble the MEASURED currents in the radials (only) in Brown, Lewis and
Epstein, figure 42, with it's dips and peaks.   Instead they follow the
gradual smooth shape of the TOTAL (wires plus ground) current of figure 41.
 The shape of the NEC4 calculated current on the radials is part of the
ground approximation method.

What broadcaster cares if NEC4 can't tell what the currents are everywhere
in the radials.  Anyone going to dig them up and check?  They want
standing-man-with-meter to be dead on predicted values down range.  They
want standing-man-with-meter to tell them when something is wrong.  That is
the number that serves every need.

Oh that blasted sky-wave.  I think Mr. Fry has some kind of a point.  But I
only want to know how it finally behaves at distances like 1000 km and
10,000 km.  Everything else is a means to that end.  We need something
carefully designed just for this problem, and measured at altitude a lot
farther out there than most broadcasters have any need to measure.

73, Guy.

On Sun, Oct 7, 2012 at 7:49 PM, Tom W8JI <w8ji at w8ji.com> wrote:

> On several occasions the author of EZNEC told me he never wanted to
> include TOA in his program because it would be misused. Since a competing
> program had it, he reluctantly included it. I may be mistaken, but I think
> he mentioned EZNEC calculated pattern at 50 or 100 wavelengths, not
> infinite distance.
>
> 73 Tom
>