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[Towertalk] 40M Dipoles (Was: Need EZNEC model 402CD..)

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Subject: [Towertalk] 40M Dipoles (Was: Need EZNEC model 402CD..)
From: (Bill Tippett)
Date: Wed, 10 Jul 2002 07:31:06 -0400
W8JI wrote:
>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.

N4ZR wrote:
>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

                          August 1999

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) [50]. 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

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