Topband: Near Field/Far Field

[Michael Tope]

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 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
> _______________________________________________
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>
>