Re: NEC-LIST: Modeling loops

Roy, you wrote:

>Firmly-entrenched lore gives the perimeter of a square loop at resonance to be
>1005/f(MHz) feet. NEC-2, NEC-4, and MININEC all agree quite closely with each
>other, but none show this value. (MININEC requires very short segments at the
>corners for this analysis.) For AWG 12-16 wire at 10 MHz, the programs show a
>free space resonant perimeter length of 1031-1034/f. This of course
>changes with
>proximity to ground and loop shape, but I couldn't find any reasonable
>configuration which would bring the model resonant perimeter lower than about
>1020/f.
>
>While two or three percent is good accuracy, an error that size is more than
>enough to significantly affect the pattern of a parasitic array if the error
>occurs in a parasitic element.
>
>Does anyone know of any good measurements of the impedance of a nominally full
>wavelength perimeter loop, or of the pattern of a quad or similar parasitic
>array of loops, for comparison to NEC results?
>
>Thanks!
>
>Roy Lewallen

-----------------------

Roy,

I wonder what prompted your memo about modelling loops?

As you might know I have been modelling loop antennas for years: 1)
full-wave loops close to the ground and supported by trees and metal
towers; and 2) half-loops connected to the ground (using MININEC) and
elevated with elevated radials (using NEC-2) -- using EZNEC version of
programs.

I have two paper on press (upcoming edition(s) of QEX) on the subject [1,
2]; and a brief summary concerning elevated radials to simulate ground
connection is published in reference [3].

The "firmly entrenched lore" which gives the perimeter of a square loop at
resonance to be 1005/f(MHz) is a myth. I have for years (since late 70s)
been pointing this out to ARRL Technical Editors, but with no avail. For a
full wave delta loop the perimeter (measured full scale, modelled, and
numerically modelled) for resonance is about 1.05 x wavelength (in metres
or feet) --- which corresponds exactly with the equation you have quoted
(our value is 1033/f(MHz) feet).

I have numerically modelled loops in two ways: 1) equal length segments
around the loop (but a lot of segments, a hundred or more for an 80M loop);
and 2) particularly in the case of squashed delta loops I have used the
taper option in EZNEC (tapering to 0.001 wavelengths both sides of all
wires). I have used your split source option for corner fed delta loops.
The resonant frequencies come out about the same.

The perimeter for resonance, and gain depend on frequency, the height of
the loop, shape of the loop, ground conductivity, *and* how it is supported
-- tree or metal tower. And if a metal tower is used, on whether the tower
supports a yagi. If the tower happens to be approximately resonant, the
tower can end up carrying more current than the loop!!!

We have also experimentally modelled ground plane half loops, having
half-quad; half diamond; and half delta shapes, and measured and calculated
the impedance, measured on a 30 m diameter ground plane. The resonant
size is about one half of (1.05 to 1.07 x wavelength).

I have also modelled quad-beams [4], but the purpose of this study was to
see how well MININEC modelled shorted stubs, since the reflector used a
stub to optimize gain.

I am very confident on the ability of NEC-2 to model full wave loops. My
present difficulties, now and in the past (see reference [4]) is with
modelling multi-wire fan monopoles. The fundamental frequency comes out
right with respected to measured experimental models -- but the higher
frequency impedances are quite wrong.

73, Jack, VE2CV

References

1) Belrose, J.S., "Loops for 80M DX - Part 1: Vertical Ground Plane Type
Half-wavelength Loops", on press QEX.

2) Belrose, J.S., "Loops for 80M DX - Part 2: Full Wave Delta Loops", on
press QEX

3) Belrose, J.S., "Ground-Plane Type Antennas with Elevated Radial Wire
Systems"< Conference Proceedings for The 12th Annual Review of Progress in
Applied Computational Electromagnetics, Monterey, CA, March 1996, pp. 2-11.

4) Belrose, J.S., "Modeling HF Antennas with MININEC __ Guidelines and
Tips from a Code User's Notebook", ARRL Antenna Compendium Volume 3, pp.
156-164 (see p. 163)..

John S. (Jack) Belrose, VE2CV
Director, Radio Sciences
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2308
FAX 613-998-4077
Received on Thu Dec 05 1996 - 18:34:00 EST