I learned the hard way with a 5/8 wave insulated self standing tower on
to band is basically useless for DX work compared to a standard 1/4 or
3/8 wave vertical radiator. As Tom so knowledgeably pointed sending
radiation close to the ground is not helpful except perhaps for a tall
tower AM station. If you have this kind of height available for
broadcast then a folded Franklin may make more sense. But one of the
basic rules of non-Maxwell physics, that so far no one has been able to
disprove, and remains an undeniable truth for topbanders: That which
work best on 160, works! Now try that one on for size.
Herb Schoenbohm, KV4FZ
On 10/1/2013 8:26 AM, Tom W8JI wrote:
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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