NEC-LIST: CEM Modelling of CFA Antenna Continued

Hello All,

My models now have so many segments it is taking a long time to run,
but finally we getting somewhere, if where we want to go is move in
the direction of agreement between simulation and measurement.
Unfortunately for the CFA all we have is handwaving, claiming that it
is an efficient antenna since the service area is the same (or better)
compared with the previously used 75m tower over an extensive radial
ground system. But this statement must be significant.

Please read with interest the continuing saga below.

If where we want to go is to determine the characteristics of the CFA
we have already been there. But we now need measurements for
comparison. But bear in mind that NEC is consistent with Maxwell's
equations, and since my wire grid model for the CFA is a very detailed
wire grid model, we have in fact (in my view) simulated the
performance of the CFA all by itself on a PEC ground plane.

I see in recent postings comments about comparisons been NEC
simulation and measurement. In my view if you simulate antennas in
there operating environment you will find pretty good agreement ---
within a dB or so --- but you have to make careful measurements. For
GP type antennas they can be calibrated by making measurements on an
open field site, a calibrated field site (reference to a standard
antenna, making measurements at 100m, 200m, 300m 400m and 500m). This
is needed for two reasons. So that the effect of ground conductivity
can be taken into account. And, besides looking for a ground
conductivity effect, multiple measurements means that we can determine
FS much more accurately than we can from a single measurement.

73, Jack, VE2CV
22 April 1999

CEM Analysis of the CFA Antenna in its Operating Environment
____________________________________________________________

Our studies to date have considered the characteristics of the CFA 0.6
m over a PEC ground. The antenna is all by itself.

The Egyptian CFA antenna described by Hately et.al. [1991] is mounted
on the roof of a flat building, over a 10 m square wire grid mesh mat,
which is thoroughly (?) bonded to earth. Site: - Moist soil, flat
land, Delta area of Egypt. The height of the building is 4.3 m. And,
one see clearly in the only photograph I have seen, a nearby tower,
undoubtedly the previously used 75 m tower over a very good radial
ground system.

Having arrived at my "best" wire grid model for the CFA, I have
configured it with this wire-grid mat (my grid size 1/3 metre). I
have grounded the mat by a thick wire (coax) connting to the centre of
one edge of the ground mat, since the photograph shows coax running up
the side of the building.

I still have a PEC ground (but I can include real ground and a ground
stake later). Call this Case Study- 2. Case Study- 1 is the CFA
antenna 0.6 m over a PEC ground.

Case Study- 3, which will follow, will include the previously used 75
m tower (initially we will consider that this tower is tuned). I do
not know the distance to the tower, but I can try different distances
---- initally I will choose a distance of 100 m.

All these Case Studies are for a PEC ground, which in fact might be
reasonable, since the tower is fed against the usual 120-radial wire
ground system --- and, the building may be sitting on the edge of the
radial wire ground system (?).

For the second CFA antenna model with its elevated counterpoise, Case
Study- 2, the antenna's impedances (compared with the impedances for
Case Study- 1) are:

        Z(monopole) = 203.3 - j 738 (187.0 - j 715.3)

        Z(disk) = -199.1 - j 509 (-179.1 - j 466.7)

And the FS (antenna resonant) at 200 m (Pt = 1000 watts) is 337 mV/m;
compared with 156 mV/m calculated previously, for a CFA antenna 0.6 m
over a PEC ground --- we have picked up 7 dB gain. The azimuthal
pattern is omni-directional. The current on the wire connecting the
wire-grid counterpoise mat to ground is 5 amperes, compared with an
antenna current of 10.9 amperes.

Now, I will add the 75 m tower (Case Study- 3). I resonated the
tower, since this might be the situation. It seems reasonable to me
that the promoters for the antenna did not think about an effect of
the tuned grounded tower in the near field of the CFA when they
installed it. It is more likely they just removed the feed to this
tower, and connected it to the CFA.

Clearly the tower is a part of the antenna system, I can see induced
current on it, and the azimuthal patter is no longer omnidirectional
(but the effect is small a dB or so). But, for this situation we have
not realized additional gain (7 dB gain over the CFA antenna by
itself).

As I said there is current on the unused (but tuned) 75m tower (it is
late in the day and so I cannot determine how much, since I have to
scroll through 2,620 wires).

More on this later. I will move the tower a bit closer to the
building. It looks closer in the photograph.

John S. Belrose
22 April 1999

PS

Have a good week-end --- I will be back Tuesday next

************************* end of message

_____________________________________________
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 Apr 22 1999 - 21:35:12 EDT