"Bob Haberkost" wrote in message
...

But....I've seen (and fortunately NOT had to deal with) antenna systems
with very
high base impedances (one, if my memory serves me correctly, was 800 ohms!
Not much
current, but do the math...any appreciable power, like 3 or 4 kW, and
there's a real
danger of getting tangled in with some pretty high voltages). While it's
not a
scientific survey, I can tell you that those systems, watt-for-watt,
perform worse
than lower impedance systems, and that's not even counting the
difficulties in having
1kV base voltages!

At medium wave (AM Broadcast) many Class A stations (formerly "clear
channel") use antennas of about 190 or 200 degrees tall. The FCC requires a
minimum antenna effectiveness for that class which is higher than for the
other classes of stations. The base impedance of these sticks near a
half-wave tall is going to be pretty high - and all but one US Class A
station run 50 kW. One of the factors in deciding AM tower height is to
place the first null in the vertical pattern such that the nighttime skywave
interferes as little as possible with the ground wave toward the edges of
the groundwave coverage.

Of course Class A AM stations are a Big Deal and generally have very good
ground systems.

At medium wave (AM Broadcast) many Class A stations (formerly "clear channel")
use antennas of about 190 or 200 degrees tall.


A Clear Channel is a Clear Channel is a Clear Channel. Any of 540, 640, 650,
660, etcetera.

The average height over all ND-U Class As is 195 degrees.

The real goal here is to get 400 mVm/kW at 1 km, or better, without also having
high-angle radiation which could cancel the groundwave in the fringe area ...
that area where the primary service area ends and the secondary service area
begins.

Taller than about 200 degrees requires sectionalization to do this.

225 degrees is a real killer for a Class A, but is perfectly fine for a Class B
or C, which doesn't have a large primary service area, anyway.

The best performing Standard Broadcast radiator is 360 degrees tall, and
consists of a 180 degree bottom section, and a 180 degree top section.



The FCC requires a minimum antenna effectiveness for that class which is higher
than for the other classes of stations.


362.10 mV/m/kW at 1 km for Class A.

281.63 mV/m/kW at 1 km for Class B and D.

241.40 mV/m/kW at 1 km for Class C.

Of all lower 48 Class As, two don't have conforming radiators, and both of
these are in San Francisco.

Of all Alaska Class As, only one has a conforming radiator.



The base impedance of these sticks near a half-wave tall is going to be pretty
high - and all but one US Class A
station run 50 kW.


The only such Class A in the lower 48 is 1560 in Bakersfield, CA.

There are numerous such Class As in Alaska.