Topband: Modeling the proverbial "vertical on a beach"

[Richard Fry]

Guy Olinger wrote:
>"Just to mention that the prior opinion is controversial and not 
>universally agreed upon. Nor to date has anyone surfaced with actual 
>measurements made at the distances (25 to 50 km) and with span of altitudes 
>(0 to 10 km) to either prove or disprove either side."

Not exactly as described in the quote above, but below is a link comparing 
the real-world groundwave fields measured by a consulting engineer using a 
calibrated field intensity meter (bottom of that page) with fields 
calculated later by NEC for those same conditions (top of the page). 
Agreement is quite good.  The fields calculated by NEC include the surface 
wave, and do not go to zero in the horizontal plane as they would for a NEC 
far-field analysis excluding the surface wave.

>"Arguments on both sides remain basically intuitive."

In January 2012, Jerry Burke of LLNL (co-author of NEC2/4) and I exchanged 
some e-mail bearing on this discussion.

I sent him a NEC plot of field intensity vs distance similar to the one I 
linked earlier in this thread, and asked,  "Also, 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?"

J. Burke response:
"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."

"The near field in NEC (NE) and the surface wave (RP1...) include the 1/R
field that is given by the Fresnel reflection coefficients and goes to zero
at the interface and also higher order terms for the surface wave.  The
formulas that NEC uses are similar to the Norton formulas, but are derived
directly from an asymptotic analysis.  Norton has some terms that make them
more accurate at moderately close distances at the expense of fixed errors
for large distances, while NEC should get increasingly accurate as distance
increases (except for neglecting earth curvature)."

The NEC study I attached to his e-mail and the one I linked in my post of 
Sat, 9 Aug 2014 18:23:58 -0500 in this thread do not consider the 
reflections produced at/by the ionosphere, and the path distance is so short 
that earth curvature is nil.

(I requested and received J. Burke's permission to publicly quote his 
comments to me shown above.)

http://s20.postimg.org/oo0j2dur1/Measured_vs_NEC2_D_Fields2.jpg