Topband: "Sea Effects" on MW/HF V-Pol Fields

[Richard Fry]

Recent clips from one Topband post:
>... EZNEC Pro4 can segment ground along a line into two arbitrary ground 
>properties, ... The brief summary of modeling results is there is 
>significant benefit at elevation angles <20 degrees towards the salt water 
>IF the antenna is less than 0.7 wavelengths (WL) from the tide line.  ... 
>Further than 1 WL from the tide line, there is essentially no low angle 
>gain benefit from the sea and the vertical pattern is whatever you have as 
>ground + radials. The results for azimuth and elevation gain and pattern 
>showed no fractional wavelength peaking, the values all smoothly trend out 
>to more than 1 wavelength from the tideline.

However the EZNEC Help file has the following advice about using the NEC 
implementation of two ground media:

\\ ... The second medium is used only for far field calculations. ... Near 
Field and Ground Wave calculations assume that the first medium is of 
infinite extent, and ignores the second medium. The effect of the second 
medium is taken into account only in a very simplified way. The vertical 
pattern is generated by tracing "rays" direct from the antenna and reflected 
from the ground. When a second medium is used, the ground reflection "ray" 
is determined by whichever medium it strikes the top of. The "ray" does not 
penetrate either medium, and diffraction or similar effects aren't 
considered. //

According to the Topband clips quoted above, NEC shows essentially no low 
angle gain benefit from the sea if the (vertical) radiator is sited more 
than one wavelength from the waterline.

Yet this is not the experience of AM broadcast stations with radiators 
located significantly more than 1 wavelength from the ocean waterline.  They 
generate groundwave and low-angle fields that are many times greater over 
the ocean than over "terra firma."

The groundwave propagation charts published by the FCC for the MW broadcast 
band will show the difference between the values for a two-media earth path 
and the conclusion reached by a NEC analysis of that path.

For example, consider a vertical monopole that for a certain radiated power 
on 1700 kHz generates an inverse distance groundwave field of 100 mV/m at a 
horizontal distance of 1 km.  The FCC chart shows that for a path 
conductivity of 5000 mS/m (sea water), the field from that system at 50 km 
is 1.99 mV/m -- which is very nearly the same as the inverse distance field 
for that distance (100/50 = 2 mV/m). So for a conductivity of 5000 mS/m 
there is a 34 dB difference between the groundwave field at 1 km and the 
groundwave field at 50 km (other things equal).
Now consider the parameters for that same system and radiated power when 
earth conductivity is 5 mS/m for the first kilometer along the groundwave 
path, and 5000 mS/m for the rest of the groundwave path to a total, radial 
path distance of 50 km.

Using the FCC chart for 1700 kHz again, the groundwave field at 1 km is 79 
mV/m.  We know from the FCC chart that the groundwave attenuation for a 
sea-water path over the span from 1 km to 50 km is 34 dB.  So a 79 mV/m 
groundwave field at 1 km will decay to a 1.57 mV/m groundwave field at 50 
km.

If the conductivity of that entire path length was 5 mS/m, then per the FCC 
chart the field at 50 km would be about 0.07 mV/m.  So in this example the 
relative field gain provided by the sea water part of this 50 km groundwave 
path is about 27 dB -- a much different conclusion than shown in the clips 
quoted above.

Note that this analysis using the FCC charts was made for a transmit system 
located about 5.67 wavelengths from the waterline.  Also note that the FCC 
charts do include the effects of earth curvature, whereas as NEC does not.

R. Fry