Topband: Skywaves from Monopole Surface Waves

[Paul Christensen]

> Mr. Fry supplies a monolithic, complete fill up to three degrees at some
> unspecified frequency over unspecified ground out 2.8 km.  The difference
> between that and the NEC pattern generated for 1/4 wave over 120 0.4
> radials at three degrees elevation is over seven dB.  That is a LOT of
> fill.

I recently modeled the WLS-AM radiator, using the dimensions and ground 
system provided by my former supervisor, Warren Shulz, who recently retired 
from WLS after working for more than 40 years in Chicago broadcast 
engineering.

When using NEC 4 during the WLS modeling, the vertical profile also shows 
(as expected) an infinite notch at zero degrees elevation over moderately 
good soil.   What the far field model of NEC does not show is the 
significant radiation at low elevations.  For that, a surface wave tool is 
required like the one that comes with 4Nec2.

If we were to only believe in the NEC far field computation, AM braoadcast 
would have met its death in the early 1920s -- for nobody would be able to 
hear the stations except at night when the upper angle radiation could 
utilize the ionosphere.

To get a complete picture of the vertical profile over soil really requires 
a meshing of the two data.  So far, I don't think any of the modeling 
authors have expressed an interest in creating a hybrid analysis tool.  NEC 
is providing the raw data and its accurate.  But we need a better form of 
display that integrates the far field with the surface wave.

As W8JI pointed out, this is nothing new.  Academics like Terman, BL&E, et. 
al. were teaching it back in the early 1930s.  But we've become firm 
believers in the typical vertical profile field plots when the only accurate 
vertical profile result from a vertical radiator is produced from a vertical 
over a super-conductive surface like salt water.  There's nothing wrong with 
the far field plot, but we should realize its limitation in the context of 
the vertical profile, especially on topband.

Paul, W9AC