NEC-LIST: The CFA a Very Efficient Radiator, Believe-it-or-Not

Ben,

Thank you for the copy of the Press Release on the CFA for the Isle of
Man.

In our search of the INTERNET Antennex magazine we have found a Table
giving "measured" field strength vs. the previously used quarter wave.

This is another set of results that I cannot agree with --- but
"measured" data has to be explained.

Here is my first go at it.

Regards, Jack

PS

This may be my last e-mail for a couple of months, since I will be on
holidays during August and September

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

CFA at Tanta, Egypt
Measured FS compared with Quarter Wavelength Vertical
________________________________________________

As previously noted, details about the concept, performance, and
advice on how to build a (relatively small) CFA for the amateur 80M
band are on INTERNET, published in the Antennex on-line magazine.
Part 4 of a series of articles contains a table showing field strength
(FS) measured at 12 locations, in various directions, for the distance
range 5 to 100 km, comparing for the Tanta Radio Station, Egypt, the
1995 version of the CFA with a 65 m tower.

Believe it or not the CFA shows gain at all locations over the
monopole, difference ranging from + 1 dB to + 9 dB, median value + 3
dB.

In spite of the fact that these measurements were made by a broadcast
engineer, and a scientist, I have to say I do not believe that
measured data represents the true performance of a CFA.

I do not have details on the Tanta antenna, other than its height (6.5
m), and the photograph clearly shows that it has the conical
extension. And, it seems to be roof mounted.

For my numerical model I have assumed that this CFA with conical
extension (my WEB site model with the top two sections deleted, it was
too high) is on a flat roof, height 4.3 m, it is fed against a wire
grid GP 10 m on a side, which is grounded at the corners to 3.6 m
ground rods. I have assumed very good ground for the Nile Delta, the
location of the station (15 mS/m, 30).

I have calculated only the near radiation field (1-10 km), for the
monopole, with a good ground system; and the CFA; for comparison, see
Table below.

Field strength dB microvolts/m for a transmitter power of 30 kW,
frequency 1.161 kHz.

________________________________________________________________

Distance CFA Monopole
   km
          Calculated Measured Calculate Measured
________________________________________________________________

 1 112.5 123.2
 5 98.4 105 109.2 102
10 93.3 103.2
18 98 89
32 94 88
38* 84 81
42 89 86
46* 83 82
47 88 85
_________________________________________________________________

The conical extension to the CFA and putting it on the top of a
building, with good connection to ground (currents flow on these
wires) has improved the performance of the CFA, according to our
numerical modelling, compared with a ground mounted basic model (+7.8
dB), but the performance of the CFA is still about 11 dB down on the
monopole.

It was noted that the FSs to the south (* values) for both the
vertical and the CFA are relatively less (5 dB, 5 dB, 5 dB and 3 dB
which is a significant difference) than northerly FSs, attributed (by
Kabbary) to a microwave tower. But in my view the difference is due
to the fact that both antenna systems (CFA and monopole) are a part of
the radiating system, regardless which is fed, since the antennas were
tuned, in the near field of each other, and the transmitter was
alternately switched from one to the other. I have numerically
modelled a CFA with a nearby tuned tower, and certainly one can find a
directional pattern, F/B ratio 3-5 dB.

My calculations for the Table assume that the CFA and the monopole
radiate in isolation.

Plot these field strength values out, if you are interested. One
wonders whether the measured values might be reversed, by mistake when
they were made (calculated values for the monopole agree better with
measured values for the CFA, and calculated values for the CFA agree
better with measured values for the monopole???), or because this is
the way things came to be because of radiation coupling between the
antenna systems.

John S. Belrose
22 July 1999

_____________________________________________
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 Sun Jul 25 1999 - 21:01:21 EDT