NEC-LIST: Egg on More Faces than One - 2

Hello All,

Please refer to my Case Study 3 (reference my TechNote posted last
Thursday), i.e. the Egyptian CFA antenna with its 10mx10m elevated
wire-grid counter-poise, and a tuned 75m tower nearby --- I made a
mistake.

A 75m tower is 0.29 wavelength high at 1161 kHz, and so its base
impedance (according to NEC-4D) is 102 + j 1292 ohms.

I connected (by mistake) a tower base load comprising an inductance
and a resistance, rather than a no-loss capacitor to "tune" the
parasitically excited tower. What a difference --- as there darn well
should be.

My model for the Egyptian CFA antenna in its operating environment now
has some directivity according to NEC-4D (about a dB front/back with
maximum gain in the direction away from the CFA toward the tower);
but, be prepared to read carefully:

The antenna system now has a space wave gain equal to + 2.9 dBi.
Since the space wave gain for the ground mounted CFA antenna by itself
is - 14.7 dBi the difference is + 17.6 dB!!!! Great, we now have a
radiating antenna system*.

Clearly the tower is the dominant radiator, and so we might just as
well throw the CFA antenna away, and drive the tower. But we could
now fine tune the tower, and change the directive pattern!!!

There is however a continuing need to model the CFA antenna by other
MOM programs, and by someone who clearly understands any limitation
that the NEC-4D program might have when modelling such a pecular
antenna (Jerry Burke according to his Friday's e-mail is modelling the
antenna so you will hear more on this later) --- because we have to be
absolutely sure that my modelling is right. I think it is, I feel
sure it is, but we have to be doubly sure.

For the corrected Case 3 model my source impedances are more nearly
equal (in magnitude): -200.1 ohms for the cylinder or barrel monopole
and + 200.9 ohms for the disk; but interestingly the power is now
circulating in the opposite direction comparing with the CFA antenna
by itself --- whatever there is sure a pile of power circulating ---
239,700 watts going out and 238,700 watts coming back. For the CFA
antenna by itself we had 15,970 watts going out and 14,970 watts
coming back.

I have never seen anything like this before.

In making this change to my model I moved the 75m tower a bit closer,
distance 75m --- perhaps a bit too close (?).

We now know why listeners in the service area can hear the station,
and the enigma of the CFA antenna is (in my view) now basically
solved. There had to be an explanation as to why the service area for
the real world CFA was identical to the service area for the
previously used 75m tower. After all if the listeners could not hear
the station, the Broadcaster would soon know.

73, Jack, VE2CV

* Recall that all my modelling to date is for the case where the CFA
antenna and antenna system in it operating environment is over a PEC
ground. I highlight this as a foot-note so that this point is
perfectly clear.

_____________________________________________
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 Tue Apr 27 1999 - 14:36:03 EDT