Hello to all interested in the saga of the Crossed-Field Antenna.
Thank you to those who have commented on my CEM analysis of the CFA.
Apologies for adding confusion (wrong information) concerning the saga
of the CFA. All of us make mistakes now and again, but my present
mistakes were posted.
In my initial CEM analysis of the CFA (coarser grid for the disk),
note posted last Friday, I ended up by resonating the elements, by
including inductive loads. I had no hole in the centre of the wire
grid disk.
When I redid this analysis (note posted Tuesday) I forgot to remove
the inductive load associated with the fat-monopole cylinder element
--- this is why the impedance seen by that source was more-or-less
resonant!!!
But a major problem (?) I apparently ran into, not revealed until
further analysis, is that a SQUARE hole in the centre of the disk does
not work. Changing the shape of the hole to an OCTAGONAL shape made a
*very large* change in current distribution on the disk, and on the
feed wire, and a large change in calculated impedance of the disk
antenna, and in the CFA antenna's gain.
Clearly there is a most complicated current distribution over the
disk, large currents are found around the perimeter (this is almost a
study in its own), but at least it now looks like a very-very short
top loaded monopole. The current on the feed wire constant.
Please TRASH my e-mail notes sent last Friday and Tuesday. To avoid
confusion I have rewritten my technical note on my analysis of the
CFA.
Jack
CFA-CEM Modelling --- Revised
______________________________
At the recent 15th Annual Review of Progress in Applied Computational
Electromagnetics Conference, Monterey, 15-20 March 1999 I spoke in
open forum (Session 7 on Wire Models) on the CFA. I decided to do
this, since Broadcast consultants were asking me about the CFA.
I had noted before that in the February 15, 1999 National Association
of Broadcasters weekly newsfax for Radio broadcast engineers, in the
Radio TeckCheck Column, it is announced (the headline to column states
"Please Forward to the Engineering Department") that:
"AM broadcasters may be on the verge of an antenna revolution,
according to a paper to be presented at the 1999 NAB engineering
conference, with the advent of a new design known as the Crossed Field
Antenna. Currently in limited operation for the Egyptian Radio and
Television Union (ERTU), the CFA represents a fundamentally different
approach to medium wave (MF) antenna design."
It is further stated that one version of the antenna, operated at a
power level of 100 kW, frequency 603 kHz has a bandwidth of 48 kHz,
and while gain is not mentioned, the "flared version" is said to
produce a significant increase in ground wave radiation and an
accompanying decrease in sky-wave radiation.
You can see why broadcasters are excited about the possibilities for
this antenna.
The statement about the effect of the flare creates an additional
problem for a person like myself, familiar with conventional antenna
systems. In accord with conventional thinking there is no way such a
small flare can change the vertical plane pattern, since the contour
surface of the flare is such a small fraction of a wavelength.
The theme for my talk at the ACES Meeting was that there is a real
need to CEM model the antenna; and to measure the performance of the
antenna by (perhaps) someone other than its inventors.
So, on my return to my office I modelled the (original) monopole
version for this antenna --- the antenna described in the 1991 IEE
ICAP paper by Hately, Kabbary and Khattab. I should have done this
prior to my ACES talk.
The frequency is 1161 kHz.
The principle dimensions are:
E plate Hollow Metal Cylinder (which is in effect a fat
monopole) 2 m diameter, 2.5 m high, supported 0.6 m above:
D plate Flat metal disc, 4 m diameter, supported 0.6 m above
ground plane (wire mesh mat) 10 m square and thoroughly bonded to
earth. The ground-plane grid was on the roof of a building.
I have assumed that the antenna is sitting on a PEC ground --- lifting
the antenna off ground, and connecting it to ground by a wire or wires
will change the picture, since currents will flow on this wire, or
wires conecting to the earth, and on the outer surface of the coaxial
cable feeding the antenna system.
The only program I have for creating wire grids to model solid
surfaces, models the surface by many-many square or rectangular grids,
all wires connected. I initially chose to model both surfaces, the
cylinder and the plate as a wire grid, side length about 0.001
wavelengths (exact size chosen so that the grid models the
dimensions). My cylinder is a polygon with 20-sides, which looks
quite round; but my "circular" disc was a bit ragged.
So I revised my model for the disk: smaller grid size
(0.00047-wavelengths), so the disk looks more-or-less circular; and I
made (after some detailed analysis) an octagonal hole in the centre of
the disk (the photograph for this antenna shows a circular hole in the
centre (about 0.3 m diameter (?)).
For this analysis the current feeding the fat-monopole cylinder is 1
amp/+45-degrees; and the current feeding the disk is 1
amp/-45-degrees. The paper by Hatley, Kabby and Khattab [1991] shows
this phase relation. Changing the phases to -45 degrees and + 45
degrees respectively we calculate a gain of less than - 99.99 dBi!!
Most interesting.
The antenna's impedance at 1161 kHz follows:
Z (cylinder) = -221 - j 707
Z (disk) = +164 - j 405
and Gain = - 10.2 dBi.
If we resonate the antenna, the gain is less, even for inductor
Q-factors of 300 (Gain = - 12.7 dBi).
Colin Davis [1993] measured - 23 dBd for his experimental VHF dipole
version; but his antenna system was not resonant.
Please note: This looks like a pretty impossible antenna to feed
(according to NEC-4D); and the gremlin turning the crank to provide an
antenna with gain has gone to lunch.
I tabulate below the impedances (according to NEC-4D) as the relative
phase difference is changed from 90-degrees to 0-degrees. Notice the
systematic change in impedances.
_________________________________________________________
Relative Z(cylinder) Z(disk)
Phase
Difference
_________________________________________________________
90 -199 - j 707 192 - j 405
80 -186 - j 740 189 - j 438
60 -163 - j 802 167 - j 500
40 -121 - j 852 124 - j 552
20 -64 - j 885 66 - j 586
0 +0.17 - j 896 0.06 - j 597
_________________________________________________________
The impedance of each element of the antenna system in isolation is:
Z(cylinder) = 0.14 - j 727
Z(disk) = 0.014 - j 423
Is this reasonable? With respect to the disk antenna, I have no
reference for such a short antenna (a 0.6 m vertical heavily top
loaded by a 4 m diameter disk). But the results of our analysis seems
to be about right.
Clearly this is a most interesting complex antenna. My (understanding
to-date) comments follow:
1) This analysis shows that indeed there is a strong near field
interaction, dependent on the relative phase difference between the
source currents. While this is very interesting, the interaction
between the two electrically very small antennas (according to NEC-4D)
does not produce a practical effective antenna system.
2) Let us consider the practicality of resonating and feeding this
CFA. For a transmitter power of 1000 watts, the source current for
the wire feeding the disk plate is 11.6 amperes, and the associated
power is 26,150 watts; and for the fat monopole cylinder antenna the
source current is 11.6 amperes amperes, and the associated power is
-25,150 watts. Recall that a negative resistance is real (see below).
The minus sign indicates the direction of power flow, back toward the
source. The Egyptian ERTU are using transmitter powers of 60-100
kW!!! Clearly there should be problems in resonating and feeding this
CFA.
3) The power radiated, calculated from the inverse distance field strength
at 1 km, for a transmitter power of 1000 watts is 17.2 watts, and so the
radiation efficiency is 1.7 percent.
4) Do not trash my results because a radiation resistance is negative.
We frequently find a negative radiation resistance associated with one
(or more) towers in a closely-coupled multi-tower MF broadcast array,
if we force currents feeding the towers to have a particular current
magnitude and phase [c.f. Belrose, in The Handbook of Antenna Design,
Volume 2 (editors Rudge, Milne, Olver and Knight), published by the
IEE, 1983, pp. 614-616].
5) Finally, concerning impedance, Hatley, Kabby and Khattab [1991]
show a SWR = 1:1 for a 50-ohm source, for a CFA with *no* matching
(Figure 8), and, 30 kW of power feeding each antenna element (???).
Figure 3 of that paper shows no connection to ground (unless the
phasing unit is grounded to the elevated counterpoise mat). There
must be currents flowing everywhere: on the wire connecting the
elevated mat to ground; and on the outside surface of the coaxial
cable feeding the antenna; perhaps on the quarter wave tower
previously used, unless it was detuned.
6) Hatley, Kabby and Khattab [1991], see their Figure 4, show that the
relative output power for their CFA is very-very sensitive to the
relative phase being very close to 90-degrees --- for a phase error of
+ and - 0.1-degrees the relative power radiated drops to 60-percent.
We do not find that. The phase difference can change significantly
before anything much starts to change.
7) There must clearly be something the authors are not telling us. Or
NEC-4D is giving us crazy results --- and I do not believe that.
Making no change to the model, but changing the phase of the currents
leads to different results, as we have seen above, in particular when
the phases are zero (or the same) the impedances seen by the sources
are about what we expect for isolated antenna elements. That is not
much interaction.
I shall have a continuing interest in this antenna, even though it
does not seem to be a practical MF broadcast antenna, from an academic
point-of-view.
I would be pleased if someone who can model a solid disk with a hole
in the centre, and a solid hollow cylinder could check my findings.
John S. Belrose, VE2CV
31 March 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 Wed Mar 31 1999 - 19:15:00 EST
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