Re: NEC-LIST: CFA Monopole Version

> John
>
> I have access to full 3D modeling software and am interested in the
> CFA purely from a curiosity aspect. I would like to model it but
> don't have sufficient data on the physical structure. If you could
> provide a drawing or a location of where I can get one I will model
> it. I will not be able to complete the modeling in a short period of
> time as I have other comitments but should be able to get it done
> over the next month.
>
> Regards
>
> Ashley Bicknell
> Broadcast Antenna Specialist
> Radio Frequency Systems
> Kilsyth 3137
> Victoria
> Australia

Ashley,

If you send me your FAX number I will FAX you a photograph of the CFA
that I modelled, and the print-out results of my analysis. It is a
simple antenna to describe, but not so simple to model.

The frequency is 1161 kHz.

The principal dimensions are:

        A Hollow Metal Cylinder (no top or bottom cap) fed against the
ground plane (it is in effect a fat monopole) 2 m diameter, 2.5 m
high, supported 0.6 m above:

        A Flat Metal Disc, 4 m diameter, supported 0.6 m above the
ground plane, and also fed against the ground plane.

The feed wire for the fat-monopole runs vertically up through a hole
in the centre of the disk, and when it reaches the bottom edge of the
cylinder (length 1.2 m), this wire is bent through 90-degrees, and
attached at one point to the bottom edge of the cylinder (total length
of this wire 2.2 m).

The feed wire for the disk runs vertically parallel to the feed wire
for the cylinder, and is attached at one point to the edge of the hole
in centre of circular disk (my hole was octagonal).

I have used feed wire diameters 25.4 mm, and for the wire grid, wires
2 mm diameter.

Brian Stewart has told us that his feed wires are horizontal (and so
cannot contribute to the vertically polarized radiated field), but I
do not have details of his version of the antenna (which I think is a
dipole version). However I can see clearly in the photograph for the
monopole version that I modelled that the feed for the cylinder runs
up the centre of the antenna, and Alberto Fassio, Italy, who has
purchased the newer flared version for this antenna, confirms that the
feed is to one edge of the cylinder. We are awaiting the results of
Alberto's "proof-of-performance".

Concerning my wire grid model, I am happy with my model for the
fat-monopole. But, the wire grid disk (viewed as a separate antenna
for clarity) has a very complex current distribution over the disk. I
can see the effects of the current around the perimeter due to the
octagonal shape of the hole --- there are eight humps of high current
around the outside perimeter of the disk.

The feed is off-centre (it has to be since we have a hole) and hole
should be circular.

There is no doubt that I would compute somewhat different results if I
used eight wires to feed my wire grid disk (one wire to each corner);
and (perhaps) several wires to feed the cylinder. In fact there
should be no hole in the disk, and the disk should be fed at the
centre. But the Hatley, Kabbary and Khattab [1991] paper shows that
the disk has a hole in the centre.

Notwithstanding I think the results of my analysis are about right.
The azimuthal patterns (ground wave and space wave) are perfectly
circular.

I will post this reply on the NEC-List, since others may have wondered
about the details of my model, and someone else may want to model the
antenna.

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 Apr 02 1999 - 06:44:49 EST