Re: NEC-LIST: Near field

Pascale Lutz wrote:

> I'm modelling radiation patterns of a dipole using the near field
> option of NEC-2 (NE and NH cards). The dipole is 1m long and is
> situated either in free space or above a ground (Epsr=3.5 and
> sigma=5mS/m). The frequencies are 150 MHz or 100 MHz.
>
> I wonder if there is a maximal distance from the dipole beyond which
> the calculated fields are inacurate. It's usual to consider that the
> near/far field distance is : D=2*L*L/l where L is the dipole lenght
> and l is the wavelenght (l=2*L, so D=L ?). However Turner ("Modelling
> antenna-ground interactions", 1994) calculated the field of a dipole
> 0.5m long until 10m.
>
> I would like to know what maximal distance from the dipole we can
> admit when modelling patterns using the NE card.

    I have solved similar problem at 10 MHz. A plot of electric field
versus distance had a step exactly one wavelength away from the
dipole. It looked like imperfect match of two different
approximations. The inaccuracy depended on ground conductivity and was
getting essential as the dipole approached the ground.

    Apparently, NEC-2 employs rigorous near field formulas within the
radius of one wavelength only. The field outside this region is
computed in the far field approximation. It means that the region of
possible inaccurate computation is between one and two wavelengths
from the dipole.

    Dr. Stan Tarnavskii
    Centre for Sensor Signal and Information Processing
    SPRI Building
    Mawson Lakes Boulevard
    Mawson Lakes, SA 5095
    AUSTRALIA

    Tel. (+61 8) 8302 5841, Fax: (+61 8) 8302 3124
    e-mail: stan_at_cssip.edu.au
Received on Wed Jun 21 2000 - 14:18:12 EDT