TR1992 said:Thanks for the answers guys! I must admit I am not much of an engineer, I found your math and calculations
to be very impressive and interesting. You opened a whole new world to me, being on the sales and marketing
side. I must admit that I'm shocked that the power would be that low. I would have guessed they would have been
in the .006kw to .020kw range. I would be very grateful if you could direct me to some sites to find out more information
on the calculations you did, as well as sites that would help me learn more about directional antenna systems. These are
definitely things I would love to learn more about. Years ago I wish I would have listened to my algebra teacher.
On the subject of WCPT 820 AM's tower setup. The tower is located in back of a store front(that used to be home to
WXRT/WSCR), between a crowded city street and an alley. In fact you can drive down the alley and practically reach
out and touch it. It is a very tall tower, as it used to be home to graveyard station WSBC AM 1240. It is still used as
a aux. tower for them. It is mounted to the roof of the one story building. I imagine the grounding isn't quite as good
as would be, if it was located in a more open area. How much effect does this type of setup have on a stations signal?
Sorry, but I can't point you to Web sites that offer tutorial material. Although I am a degreed EE and hold a PE license in one state, I learned everything I know about this subject by reading applications for AM upgrades at the FCC Web site, drawing conclusions from them (often incorrect), posting my thoughts, often getting shot down (hard) by other EEs who knew what they were talking about, studying some more, and getting shot down again--and again. Not efficient, but when it's your hobby, it's not difficult to justify the prodgious amount of time spent.
As for the WCPT tower, there are curves (somewhere) on the FCC Web site that show how the efficiency of a vertical radiator changes based on the ratio of the area occupied by the ground system to the area occupied by a standard ground system, which the FCC defines as 120 1/4-wavelength radials. I don't remember whether these FCC curves take into account the tower's electrical height. As I understand it, if the curves don't do this, they should, because a good ground system is more important to the efficiency of electrically short radiators than to the efficiency of electrically tall ones.
I believe that WCPT's tower is more than 150 degrees high (500' give or take at 820 kHz). With a standard ground system, a tower of that height would have an efficiency almost great enough to meet the minimum requirements for a Class A AM (362.2 mV/m/kW @ 1 km). CDBS gives WCPT's efficiency as only ~292 mV/m/kW @ 1 km--well over the Class C/D minimum of 281.7 mV/m/kW @ 1 km, but not even quite the efficiency of a 1/4-wavelength tower with a standard ground system (~300' @ 820 kHz, which would yield, IIRC, ~306 mV/m/kW @ 1km). The reduction in efficiency from ~350 mV/m (the value for a 150-degree tower with a standard ground system) to 292 mV/m, suggests (and your description confirms) that the ground radials are severely truncated.
As for the low value of the calculated maximum-allowable ND night power, I too was surprised. In part, I think, the reason the power came out so low is that WCPT's proposed night array appears to have been very conservatively designed--at least as far as the protections to WBAP go. It was prudent for the consulting engineer who designed this array to be conservative. The actual performance is likely to be considerably worse than the calculated performance, but the design margins are probably adequate to cover the largest imaginable discrepancies. I don't know how the latest FCC rules on proofs of performance on new AM DAs affect this stuation, but until the new rules went into effect (just within the last few months), the actual performance would have been established through field-strength measurements and the discrepancies would have been taken care of by augmenting the theoretical pattern.