Topband: Near Field/Far Field

[Guy Olinger K2AV]

On Fri, Oct 12, 2012 at 7:27 AM, Richard Fry <rfry at 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