RE: NEC-LIST:Flat Elements

Don,

I'm not sure that NEC is such a good tool for highlighting subtleties of
wire shape. Here is some info on slot modeling that may point you toward
what you really care about, which is thin ribbon modelling. I don't know if
this will be of any help, but it might give you some idea about what to
expect.

        Dave de S.

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In [slot / magnetic dipole] mutual impedance calculations, the fields from a
dipole are assumed to be represented by the fields from a filament along the
dipole axis. For segments within a dipole the mutual impedance is obtained
by integrating the product of the electric and magnetic fields along a path
separated from its axis by an equivalent dipole radius. This equivalent
radius is nominally one-fourth the slot width for slots in thin ground
planes [2]-[5]. An important modification to the equivalent radius for
cases where the segment length becomes comparable with the slot width is
given by Imbriale and Ingerson [8]. This modification is necessary to avoid
divergence in the mutual impedance calculations for these cases.

     (3) equiv. dipole rad. (approx.) = (w/4)*[1-.40976*(w/4)/(L/2)] , where
w = slot width, L = slot length

It is important to check for convergence by varying the number of segments:
use of too few segments can cause errors due to inadequate representation of
the aperture field distribution, and use of too many segments can lead to
divergent admittance calculations caused by a breakdown of the cylindrical
dipole model.

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REFERENCES

[1] J. Galejs and T. W. Thompson, "Admittance of a Cavity-Backed Annular
Slot Antenna," IRE Trans. Antennas Propagat., vol. AP-10, pp.671-678,
November 1962.
[2] C. R. Cockrell, "The Input Admittance of the Rectangular
Cavity-Backed Slot Antenna," IEEE Trans. Propagat., vol AP-24, pp. 288-294,
May 1976.
[3] J. Galejs, "Admittance of a Rectangular Slot which is Backed by a
Rectangular Cavity," IEEE Trans. Antennas Propagat., vol. AP-11, pp.119-126,
March 1963.
[4] A. Hadidi and M. Hamid, "Aperture Field and Circuit Parameters of
Cavity-Backed Slot Radiator," IEEE Proceedings, vol. 136, Pt. H, No. 2,
pp.139-146, April 1989.
[5] S. Hashemi-Yeganeh and C. Birtcher, "Theoretical and Experimental
Studies of Cavity-Backed Slot Antenna Excited by a Narrow Strip," IEEE
Trans. Antennas Propagat., vol. AP-41, pp. 236-241, February 1993
[6] K. W. Leung and K. Y. Chow, "Theory and Experiment of the
Hemispherical Cavity-Backed Slot Antenna," IEEE Trans. Antennas Propagat.,
vol AP-46, pp. 1234-1241, August 1998
[7] C. A. Balanis, "Antenna Theory Analysis and Design," New York,
Harper & Row, pp. 285-304, 1982
[8] W. A. Imbriale and P. G. Ingerson, "On Numerical Convergence of
Moment Solutions of Moderately Thick Wire Antennas Using Sinusoidal Basis
Functions," IEEE Trans. Antennas Propagat., vol AP-21, pp. 363-366, May
1973

 
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-----Original Message-----
From: DonNJ2E_at_aol.com [mailto:DonNJ2E_at_aol.com]
Sent: Monday, October 29, 2001 10:46 AM
To: nec-list_at_gweep.ca
Subject: NEC-LIST:Flat Elements

I am planning on doing some comparisons of antennas used by amateur radio
operators for on foot directional finding (i.e., Fox Hunting). Many use
metal
tape measure refills for elements. I'm using NEC Win Basic (NEC2).
Considering it's limitations, can anyone suggest ways to build the elements,

such as a large wire or multi wires. The tapes in question are about 3/4"
wide and the frequency is in the two meter band (146.585 MHZ). Thanks Don
NJ2E

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