I certainly agree with Richard Fry from a pattern perspective, although my
experience with 5/8th wave antennas and other low angle tall verticals over
the past 40 years (and I have had several antennas) is that really low
angles on 160 for extended groundwave contacts or DX are utterly useless.
If you want a dog of a performer that is good for stuff within 40 miles, use
a really low angle radiator on 160, especially one that puts a null at 20-40
degrees. At about 200-220 feet height with flat ground the overall
performance of a vertical starts to take a dive.
Consequentially, at least on 160 for practical uses, NEC far field is fine.
Reaching the ionosphere at a low angle that simply uses up the energy in
losses is not a good design goal, especially when the gain is so small and
significant energy is removed from more useful angles.
Tom
----- Original Message -----
From: "Richard Fry" <rfry@adams.net>
To: <topband@contesting.com>
Sent: Tuesday, October 01, 2013 6:38 AM
Subject: Re: Topband: 5/8 wavelength vertical is mo betta than
shorterversions??
The radiation toward an elevation angle of 5 degrees shown in the surface
wave plot continues in essentially a straight line, to reach the
ionosphere."
I'm still puzzled by these statements.
Its clear that a NEC far-field analysis over a real earth path omits a
significant amount of low angle radiation produced by vertical monopoles.
Such an analysis always shows zero radiation in the horizontal plane, and
not much more than zero at very low elevation angles.
But if that pattern was correct, then MW broadcast stations would have no
daytime or nighttime groundwave coverage -- which obviously they do.
However the NEC near-field analysis used to calculate the surface wave
does show that low angle radiation.
BOTH the NEC far-field and near-field analyses are required to describe
and understand the total radiation envelope of a monopole over real earth.
For background, I contacted Gerry Burke in January, 2012 when I was
researching the basis for the comments I have been posting here. Probably
most will recognize Gerry Burke as the co-author of NEC software, working
at
Lawrence Livermore National Laboratory.
I sent him the NEC surface wave plots linked below, and asked him,
"...would you expect the fields at elevation angles of 1 to 10 degrees in
these
plots to continue on to the ionosphere, and under the right conditions
be reflected back to the earth as skywaves?
His reply was (quoted with his permission): "The low angle 1/R fields
should
reach the ionosphere, although perhaps not accurately predicted by NEC,
since it does not include the effects of earth curvature and the
ionosphere."
G. Burke's reply should be conclusive on this subject.
BTW, the 2.46 V/m groundwave field shown at 1 km from the WLS tower for 8
mS/m earth in the NEC plots linked below is almost exactly the value
measured at 1 km by the newly-retired chief engineer of WLS, who is an
acquaintance of mine.
http://s10.postimg.org/xq4ngg4hl/WLS_Surface_Wave.jpg
RF
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