Re: NEC-LIST: CFA's hither and yon

>Jack,

>Now that CFA's are supposed to be operating on Isle of Mann and
>in Italy, what's new?
>//
> Forrest

Both are yet be erected and come-on-the-air --- as far as I know.

The only point that Burke, Stewart and I can agree upon, is that I
need to measure the performance of a CFA myself!!! See below. So we
are gearing up to do so --- an 80M band version.

Clearly there is a very-very (unbelievably) great difference between
characteristics that I calculate, and claimed performance. Brian
Stewart has sent me dimensions for a monopole version of the CFA for
the 80M band --- complete with details re phasing and matching. The
antenna described in the Internet Magazine "Antennex"
<http://www.antennex.com>. I have so far stayed at arms length from
that Magazine. There is apparently a better method to tune and match,
but that information is not yet available for public release.

In considering fabrication of a monopole version of the CFA, I find I
am at a loss re frequency/dimension scaling --- my original MF version
of the antenna compared with the present version for 80M band.

The CFA I have modelled extensively, for a frequerncy of 1.161 MHz,
has a cylinder diameter 2 m, length 2.5 m; and a disc diameter of 4 m.
Spacing between bottom end of cylinder 0.6 m. Spacing between disc
and GP 0.6 m.

The 80M version has a cylinder diameter of 20 cm, length 25 cm; disc
diameter 40 cm (with a hole 6 cm diameter). Spacings are 10 cm.

The 80M version is TEN TIMES SMALLER than the version I have been
modelling --- excepting for the spacings. So I changed my model for
spacings in accord with the 80M CFA. For an operating frequency of
3.75 MHz, my modelling frequency should be 0.375 MHz. I am not going
to make a new wire grid model. The port impedances and gain I
calculate start to become ridiculous.

I calculate the impedance for the monopole (E-plate) 659 - j 2414
ohms; the impedance for the disc (D-plate) -640 - j 1832 ohms --- and
the gain (antenna resonated, inductor Q = 300, no ground loss) is
dropping out of sight (-26.3 dBi).

Clearly the parameters given for the matching/phasing network for the
80M CFA do not resonate the antenna --- or my calculations are
meaningless.

I am temporarily out of touch with Brian Stewart (he is moving his
office), and Maurice Hately has yet to reply to my e-mail messages.

Concerning future of my numerical modelling, I am now adding a conic
section to the my cylinder --- I am only part way there. A cylinder +
conic extension for a height of 3.8 m brings the capacitive reactance
of the monopole into agreement with that for the disc. As far as I am
concerned all I am doing is top-loading the very small monopole.

73, Jack, VE2CV

*******************************************

Delivered-To: john.belrose_at_crc.ca
Date: Fri, 28 May 1999 09:09:39 +0100
From: Brian G Stewart <b.stewart_at_gcal.ac.uk>
To: John Belrose <john.belrose_at_crc.ca>
Subject: Re: CFA - Information Exchange

Dear Jack

I appreciate you comments very much on your modelling and the CFA. I
realise that you have put much time and thought into it and I
personally greatly appreciate it.

Jack my desire, is as you know to get the CFA measured by as many
people as possible. I will speak to Maurice today and we'll try and
get one sent out to you with a good phasing circuit that you can get
access to and alter/modify etc.

The CFA as you have quite rightly said is a complex antenna and we are
aware that what we "seem" to measure and also experience in radio
contacts (and I say "seem" because we want others to measure them too)
does contradict what we would all accept as standard electromagnetics
associated with antennas.

Jack, I'm sure both Maurice and myself would be happy to give you any
technical support that we can once you get the CFA - simply because
there are many more aspects to getting it to operate as desired which
we would like to share with you. For example, if a GP CFA radiates
many amateurs are also discovering that the main gain of the radiation
can be at a different angle (20-30 degrees) for the same antenna. We
would like you to try experimenting with this yourself. Perhaps there
is something we haven't appreciated - if so then great.

In relation to the GP of 1m x 1m for the 80m CFA yes it is too
small. I always use about a 2m x 2m for this size. The reason for the
GP version is as you would expect geared for broadcasting. The
original CFAs were the dipole versions. I don't really experiment with
these nowadays - and I haven't yet tried the new phasing circuits I
developed this lasy year on these types .

Unfortunately I'm moving office at work so I won't be able to access
my email till later on next week.

As I said, I'll speak with Maurice and we'll see what we can get to
you. I suspect that it shouldn't take more than a few weeks.

Best regards

Brian

_____________________________________________
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 04 1999 - 04:19:47 EDT