The other issue is the radiated surface wave! Vertical antennas (and
horizontal wires near the earth's surface) have, as a part of their total
radiated vertical electric field, a surface wave E-field component (Zenneck
wave). Assuming you are not a great distance from the antenna, this
component is significant (depending on the antenna geometry, frequency, and
soil conditions) especially on MF and 160! If a person makes a field
strength measurement at some distance r from the transmit antenna, they are
probably measuring a vector sum of a surface wave and a space wave
component! Here again, if the receive test antenna is orthogonal to the
earth's surface, we only measure an orthogonal component of the E-theta
space wave component summed with the surface wave (which is usually
orthogonal unless some tilt has begun to occur).
The version of NEC we used (NEC-3 and others I presume) does not include the
surface wave radiated component in the NEC output file when you run a normal
card for all points of theta and phi and some distance r! You had to run a
seperate surface wave card (using the Norton-Sommerfeld ground
approximation) to get this data! This is a significant issue when comparing
E-field data measured near the earth's surface!!! NEC-3 would not, as far
as I know, allow the user to run both cards and merge the data into one
vector sum field...... Therefore.... We had to run them seperatly and write
Matlab programs to extract E-normal from E-theta in point #1 above, and then
add that vector component back to the surface wave component generated by
the surface wave card. Point #2:
2. The surface (zenneck) wave must be accounted for when running a NEC
analysis, and then must be added back to the E-normal component of the space
wave results! Then you get the total "normal" field (vertical component)
near the earth's surface! Then you can compare it to what you measure,
assuming sigma (conductivity) and epsilon-r (relative dielectric constant)
have been measured accurately. Here again, this is only important if you are
making measurements near an interface where a surface wave can be supported.
Way out in free space at smaller angles of theta (high elevation angles)
there is probably no surface wave component.
When we straightened out all this mess, we were able to generate NEC-3 model
electric field magnitudes that were within 1.5 dB (some within 0.5 dB) of
what was measured on the Rohde &Schwarz calibrated receiver for the specific
test case low dipole (for various heights above ground). I'm not saying the
SRI ground constant measurements were absolutely correct, nor am I an expert
at how they were made. Nor am I claiming that our test apparatus was
absolutely without fault. But for our small test case, the results were
surprisingly close, and we learned how significant the surface wave
component was in comparing NEC data!!! It wasn't even close until we
accounted for it!
