Re: NEC-LIST: CFA Presentation

> Dr. Belrose,
>
> Did you present your findings on the CFA at the AP 2000 Conference
> in April? I was wondering how it turned out. Can we now put the
> CFA to bed? Perhaps the NEC-LIST subscribers would also be
> interested. Sincerely,
> --
> John Wood

Hello John,

Thank you for your continuing interest.

The presentation of my paper went (I think) quite well. I was pleased
to have an audience. My presentation was on the morning of the last
day, and there were TWELVE parallel session, seven on antennas and
five on propagation. The CD-ROM for the meeting has all thousand or
more papers available to read.

But, I was only contacted by about six participants who had an
interest in this antenna.

The CFA should be put to bed, but it will not lie down.

Besides the poor radiation efficiency for this electrically small
antenna system, a major problem is a result of feeding electrically
small closely coupled antennas in phase quadrature. When two
electrically small coupled antennas are fed 90-degrees out of phase,
mutual coupling from one source induces a current in phase with the
other voltage, and this results in a large reactive power (large
compared with the transmitter power) surging out one port and surging
back in the other port.

In our experimental model when we added an additional pi-network
between the tuned and paralleled ports feeding the antenna, this
recycles the power --- and so our transmitter was happy --- low SWR
--- but this resulted in reduced radiated power. Recyling the power
out means that some comes back again, I do not know how many times ---
and this INCREASES the losses since our matching networks are not loss
less.

No one that has experimented with the CFA (as well as the inventors)
mentions this to be a problem. I consider this to be a major problem.

The following is abstracted from a recent note I have written re
present status of MF CFA antennas.

The original CFA antenna was a dipole (no GP needed). Then the
co-inventors had the idea to made a GP version. Since the antenna is
so small a relatively small GP is adequate. But all CFA antennas are
on the roof of buildings, excepting for the one at Barnis, Egypt, and
Kabbary insists on well grounding the wire grid GP. I told Kabbary
and Hately in 1991, I was there for the presentation of their 1991
ICAP paper, that they would probably realize more radiation from the
grounding wires, or from the feeder coax if the antenna was fed
without a balun, then from the CFA by itself.

The Tanta antenna is on the roof of a building, which is for all
purposes copper clad --- copper strap all over the outside of the
walls of the building. The Nile Delta is an area of very high
conductivity --- probably something like 25 mS/m. I have numerically
modelled this configuration. The building radiates better than the
CFA by itself, and, the antenna works better if the two ports are fed
in phase. But we deduce FS that are dBs down on a quarter wave ---
measurements by 5 Brazillian engineers show(???) that it better than a
quarter wave by 6 dB!!!

Feeding the antenna in phase eliminates the problem of reactive power
surging out one port and surging back through the other port --- a
major problem in my view. In this configuration we might just as
well throw away the disc.

Proven performance is still an open question. Re the MF CFA in Sidney
Australia, one set of measurements show it is 8 dB down on stations
employing conventional antennas [Steve Olney, private communications,
2000]. Kabbary [a note posted on the April 2000 issue of AntennEX
[http://www.antennex.com] retuned this antenna in November 1999 and
now claims it radiates as good as as a quarter wave* --- but the first
referenced measurements were made in February 2000. The CFA in the
Hamburg/Kiel area is reported working well, but my planned visit there
(early April), at their invitation, was cancelled by them at the last
minute. I tried to visit the San Remo site and see the CFA installed
there, but my e-mail messages (to Alberto Fassio) remain unanswered.
It is said to be working well. But, if CFA antennas are working so
well why are the project engineers unwilling to show and tell???

I intend to write a paper for the IEEE Trans on Broadcasting, but not
after another attempt to realize better performance experimentally ---
I plan to modify our experimental model by adding conical sections.
This will help, reduced antenna reactance --- but according to our
numerical modelling it is still a no good antenna.

It is still said that the long wave station on the Isle of Man will
employ a CFA [http://www.musicmann279.com/engineering.html] --- and, I
believe that measurements on a reduced size model, ground mounted, are
planned.

Regards, Jack

* I have no confidence in Kabbary's near field measurements.
Measuring fields in an urban environment where everything may be
reradiating is the problem. Since this roof mounted antenna is
connected to ground -- current on that wire radiates. In fact the
entire building is probably a part of the antenna system. Buildings
can be considered to a wire grid structure at MF frequencies --- the
grounded ground wires in the AC power system. Probably buildings next
door are a part of the antenna system. A FS Meter a few hundred
metres from this CFA installation is undoubtedly surrounded by current
carrying conductors.

Steve Olney's measurements were made in the far field, and absolute
FS measurement was not a problem. He referenced his measured FS to
other stations providing a similar service, employing similar
frequencies and at similar distances (but distance correction was
included in his analysis), and using conventional antennas.

_____________________________________________
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 Thu May 11 2000 - 03:55:47 EDT