Re: NEC-LIST: Near field

> My comments were based on the following example.
>
> CM
> CE
> GW1,41,0,-7.5,1.0,0,7.5,1.0,0.002
> GE1
> GN2,0,0,0,14.0,0.0001
> FR0,1,0,0,10.0,0
> EX0,1,21,01,1.0,0
> NE0,90,1,1,5.0,0,1.0,0.5,0,0
> EN
>
> This is an input file of a half wave horizontal dipole placed 1
> meter above imperfect ground. The NE card specifies computation of
> the near field E 1 meter above ground and at distances of 5 through
> to 50 meters in the direction perpendicular to the dipole. The plot
> of 20log10(|E| X^2) versus distance X suddenly changes its behavior
> from oscillations with the amplitude of about 3.5dB to a straight
> horizontal line. It happens at a distance of one wavelength from the
> dipole independently on frequency. Of course it is a very special
> case, but it still worth thinking about.
>
> Stan Tarnavskii

Stan, I have just modelled a half wave 10 MHz dipole at a height 1 m
over average ground (5 mS/m, 13), and I computed the ground wave field
strength vs distance over the distance range 1 - 50 m. The FS falls
off SMOOTHLY from a value of 41.60726 V/m at 1 m distance (height 1
m), for 1 kW to 0.1190376 V/m at 50 m --- in the direction
perpendicular to the dipole.

If you are interested in near fields compute near field in the plane
of the dipole, at say 0.5 m height. You will then see changing
fields, rather spectacular fields as you near the end of the dipole.

To validate NEC we would like to have measured fields (E and H)
beneath the dipole.

But this exercise does not tell us NEC is wrong. You have to have
measured fields.

FYI the resonant length of my dipole was 14.388 m, impedance 69.27
ohms. I used 51 segments (#12 wire) for accuracy and to put the
source in the middle of the dipole.

Regards, Jack

_____________________________________________
John S. (Jack) Belrose, PhD Cantab, VE2CV
Senior Radioscientist
Radio Sciences Branch
Communications Research Centre
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2779
FAX 613-998-4077
e-mail <john.belrose_at_crc.ca>
_____________________________________________
Received on Fri Jun 23 2000 - 21:58:16 EDT