NEC-LIST: The CFA with Added Conic Sections

Brian,

My numerical modelling of the CFA is now complete. I now have three
models:

Antenna (1)
____________

The original cylindrical-shaped E-plate MF version, cylinder diameter
2 m, length 2.5 m; disc H-plate, diameter 4 m; spacing (between bottom
of cylinder and disc and between disc and GP) 0.6 m.

Antenna (2)
____________

The reduced size version for the 80M amateur band: cylinder diameter
20 cm, length 25 cm; disc diameter 40 cm; spacing 10 cm.

Antenna (3)
____________

Addition of conic sections to the cylindrically-shaped E-plate:
antenna height (GP to top of conic section) 8.183 m, diameter of top
conic section 14.338 m.

I may have overestimated the diameter of the top conic section --- I
have nothing to go by, only photographs.

The claim in your NAB'99 paper is that the addition of the conic
sections has the effect of confining the curved E-field lines in the
"interaction zone" to low angles, producing a significant increase in
ground-wave radiation and an accompanying decrease in sky-wave
radiation.

My view: The conic sections add top loading to the basic cylinder,
significantly decreasing the capacitive reactance of this element.
The circulating power is much less, the antenna impedances more
reasonable, but the gain is still very poor. The vertical plane
pattern is cosinusoidal, characteristic of a small antenna.

The no-loss antenna system impedances at 1.161 MHz are:

        Z(cylinder) = 65 - j 165

        Z(disc) = -48 - j 438

And, the gain (reactances cancelled, inductor Q-factor = 300) is -8.3
dBi. Return power (for a 1 kW transmitter power) 2300 watts.

For reference (re gain), recall that I have included no GP loss. The
gain of a 75 m monopole (which has an electrical length > 0.25
wavelengths) is + 4.8 dBi. So the ground-wave field strengths
(according to NEC-4D) will be more than 13 dB below that for the
previously used monopole.

A final comment Brian, based on my extensive numerical modelling, and
our extensive discussion about "real" antenna system impedances:

Do not consider that the resistive component of the antenna system
impedances that I calculate (and in my view the impedances you
measure) to be radiation resistances. These resistances are the real
part of the antenna system impedance. The radiation resistances are
very-very small, corresponding to the electrical size of the CFA
antenna, which is very small.

One can easily see that this is the case (for my numerical modelling),
by inserting loss resistances in series with feed. The gain decreases
significantly for small loss resistance values.

I do not intend to correspond further on my numerical modelling. We
intend to experimentally model Antenna (1) (scale factor 3.2) --- and
as I said before we will be pleased to determine the characteristics
of any antenna you might send us (tuned for the 160M or 80M amateur
band).

Regards, Jack

Post Script
For Those who may have Fabricated Antenna (2)
_____________________________________________

For Model (2) at 3.75 MHz, I calculate:

        Z(cylinder) = 651 - j 2301
        Z(disc) = -628 - j 1679

        Gain (antenna resonated, i.e. reactances canceled) = -27 dBi.
        Inductor Q = 300

This is ridiculous, particularly the reactances we have to tune out.
As far as I am concerned the antenna is too small, and the gain is
very-very poor. This is the antenna radio amateurs are constructing.
I wish them good luck.

A word of caution re performance evaluation by amateurs in radio: I am
very familiar with the performance of electrically small antennas. I
have used for several years an AMA 8 compact loop, diameter 1.7 m, on
the 80M and 160M amateur bands.

The measured radiation efficiency for this antenna for the 80M band is
4.7 percent (calculated radiation efficiency 5.5 percent). One can
use this antenna to communicate reliably with radio amateurs over
distances of several hundred kilometers during daytime; several
thousand kilometers (when radio noise levels are low) at night. I
have been heard on the west coast of Canada after local midnight ---
but not very well.

_____________________________________________
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 11 1999 - 19:54:00 EDT