If you look at Ken Norton's paper from the December 1941 Proceedings of
the IRE "The Calculation of Ground-Wave Field Intensity Over a Finitely
Conducting Spherical Earth", he lays out the equations for the E field
from a vertical radiator for short distances (short enough to neglect
earth curvature). In these equations Norton explicitly shows three
distinct terms; a "direct wave" term, a "ground reflected wave" term,
and a "surface wave" term. If you neglect the surface wave term and add
up just the "direct wave" and "ground reflected wave" terms for the case
of psi=0 (i.e. zero elevation angle), the sum of the terms goes to zero
except for the case of infinite ground conductivity. In that case, the
surface wave term goes to zero and the sum of the direct wave field and
the ground reflected wave field is twice the direct wave field which
accounts for the 3dB gain at the horizon that Dick mentioned in one of
his previous posts.
The 1941 paper mentioned in the previous paragraph references another
paper by Norton that was published in the Proceedings of the IRE in
September 1937 - "Physical Reality of Space and Surface Waves in the
Radiation Field of Radio Antennas". In this paper, Norton discusses
whether or not the physical interpretation of the surface wave portion
of the E field solution is equivalent to a "Zenneck" guided surface
wave. At the end of the paper, Norton leaves the question open stating,
"Although the above evidence is illuminating the final establishment of
Sommerfeld's view that the surface wave is similar to a guided wave on a
wire must await further theoretical and experimental studies".
It sounds like we are asking the same questions as Norton asked in 1937
again in 2012. One possibility that I suggested previously, which would
reconcile the discrepancy between the 2.8 km elevation pattern with that
of the 50 km elevation pattern, is that the surface wave portion of the
field is truly a guided wave that propagates along earth's surface in a
manner analogous to the way energy flows down a transmission line. Since
real earth is lossy, you would expect that part of the field to die off
with increasing distance eventually (due to ohmic heating rather than
radiation into space) leaving only the field from the skywave terms in
the field equations. I think the argument that it is valid to sample the
elevation pattern of the vertical radiator at some modest distance (e.g.
2.8 km) and then project that field radially on to the distant
ionosphere, depends upon some implicit assumptions about the
corresponding amplitude, phase, and direction of the H-fields in that
moderate distance elevation pattern. Rules of thumb about far field
distance aside, what ultimately matters is whether the E-field and H
field are physically orthogonal and in-phase.
Interesting topic.
73, Mike W4EF...............
On 10/12/2012 10:37 AM, Guy Olinger K2AV wrote:
On Fri, Oct 12, 2012 at 7:27 AM, Richard Fry <rfry@adams.net> wrote:
There is little point in dissecting the far field tens of kilometers from
a vertical monopole to find the field remaining there at low elevation
angles, because that does not account for ALL of the fields radiated by the
monopole. In fact, that approach misses the existence of the greatest
contributor to low-angle radiation -- the fields of the elevation pattern
within 1 km of a 160m monopole radiator.
First, I would like to thank Richard for passing on the information from
the helicopter measurements. It was quite non-intuitive to me, and
certainly got my attention. Then my intuitive vast over-simplification of
the vector arithmetic was telling me that it should continue into the far
field, and there was possibly some serious missing stuff in a typical
far-field plot.
I certainly would have thought that the far field was formed with
sufficient accuracy out 2.8 km from the monopole. Experimenting with
various ground and frequency, results quite more indicated CONFORMITY
between NEC4 near field values and the helicopter measurements. This at the
time was advanced as proof of low angle energy that was MISSING in the far
field plot, that needed to be added in.
Recap: NEC4 running NEAR field analysis ALL THE WAY OUT to 2.8 km
duplicated the helicopter measurements. For myself as well, 2.8 km WAS all
the way out. So why would there be a discontinuity like that with the far
field plot? So I decided to see what the near field process is doing, very
carefully.
I started all over from scratch with my 1/4 wave over 120 buried 0.4
radials. I used that antenna so that I didn't take myself out of the gold
standard monopole + radial paradigm that has been validated over and over.
I got a very similar result at 3 km, noting some mild modification due to
1.825, but obviously begging the same intuitive question: Where did all
this low angle radiation go that is so clearly in the near field table at 3
km. First look score goes to Richard.
Running near field setups at larger and larger distances, until my eyes
ached, I looked for a point where NEC4 near field calculations fell off the
table, thereby invalidating anything beyond some distance, asking ugly
questions about NEC4, and so still leaving room for Richard's assertion.
Instead, what I found going very gradually between monopole and 50 km with
the NEAR field process, was a very smooth progression, which included all
Richard's graphs at particular distances, and the essentials of the
helilcopter measurement shape at 2.8 km that raised all the questions in
the first place. It was a very smooth progression all the way out to values
that at 50 km that match the FAR field plot. No discontinuity, no falling
off some processing cliff. Just inexorable millions of vector additions in
precision bookeeping.
***** ===== *****
NEC4 NEAR field processing firmly predicts that the FAR field notch at
ground DOES NOT EXIST at 2-3 km, and that the notch very gradually forms
until the NEAR field notch matches the FAR field notch in the region of 50
km.
***** ===== *****
There is little bit of a bow in the 3km curve at 0-500 meters, but the
center of the bow is about 250 meters up, and intuitively would be aimed
upward. It does not at all suggest the massive notch in the far field plot.
In hindsight, the bow is the first faint hint of the notch to come.
One thing NEC4 is very good at, and that is vector arithmetic. If NEC4 has
processing math good enough to correctly present the verified and rather
confounding low-angle-filling pattern correctly at 2-3 km, why does this
processing suddenly not pass muster at 50 km, absent it falling off the
cliff somewhere in a processing tangle.
Going beyond that, why would one assume that surface energy doesn't count
for anything in the pattern? It's just another vector value to add in. One
might better speculate that the surface energy relaunch is what keeps the
monopole vertical pattern from looking like the low angle notch in a plot
of a horizontal antenna at half wave height, which is truly severe.
DISproving the 50 km near field figures will be a hard task. NEC4 has
proven itself on the mark at 2-3 km, and shown that the same math says
familiar old low angle notch at 50 km. Anecdotes for superior sites and
why, are if anything revalidated by this.
73, Guy
_______________________________________________
Remember the PreStew coming on October 20th. http://www.kkn.net/stew for more
info.
_______________________________________________
Remember the PreStew coming on October 20th. http://www.kkn.net/stew for more
info.
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