>In my experience models tend to fall in line more on 40 meters and up
>with real-world results, while on 160 results vary the most.
>I believe that most models - certainly prop models like VOACAP -- specify
that they are not accurate below 2 MHz. I suspect the same is true for
NEC-2 and 4. Does anyone know for sure?
Propagation on the low bands is not completely described by current
models, especially below 2 MHz. I believe this primarily has to do with the
electron gyrofrequency (~.6 to 1.6 MHz) changing the rules below 2 MHz.
Polarization Coupling Loss (low ionosphere loss effect at low frequencies)
seems to partially explain why horizontal antennas don't perform as well as
NEC models predict at low frequencies, but that is an effect outside NEC's
definition of "far-field". NEC does not extend farther than direct near and
far-field analysis and no propagation or sky-wave effect is taken into
There is not a low frequency limit I'm aware of, although I suspect the NEC
model falls apart at microwave frequencies for other reasons.
Here is one of the best summaries of medium frequency propagation and
antenna modeling I've found:
NTIA Report 99-368
Medium Frequency Propagation
Prediction Techniques and Antenna
Modeling for Intelligent Transportation
Systems (ITS) Broadcast Applications
Their comments on NEC:
"The model selected for analyzing antennas makes use of extensive method-of
moments calculations and is implemented in a computer program titled the
Numerical Electromagnetics Code (NEC) . It is an accurate method for
analyzing antennas at these low and medium frequencies where the antennas
are small or comparable to a wavelength in size."
73, Bill W4ZV