This is the AM Antenna Section of
The Broadcast Archive
Maintained by:
Barry Mishkind - The Eclectic Engineer
AM transmission systems have three major goals:
1. To cover the area of interest, with a
signal that overcomes manmade RFI
2. To put as much groundwave out as
possible, avoiding "fading" at the fringes of coverage
3. To reduce skywave interference to other
stations on the same and adjacent frequencies.
Since the electrical height of a tower controls, to a large
extent, the level of signal received at any given point, there is often a
balance between #2 and #3.
Today, a regional channel often needs 50 kW to be heard above the noise
floor, even locally.
In the 1960s, 5 kW, the Class III maximum, was good for a hundred miles or more,
under conditions of ground ground conductivity.
Today, just about every appliance uses switching power supplies, or variable
frequency three-phase motor drives. Lots of noise, and clothes washers or dryers
do not have an FCC label on them, even though these
appliances produce more AM band RFI than any personal computer.
Notice that a 225 degree radiator can produce 440 mV/m/kW at 1 km, but, the
225 degree radiator has exceptionally poor fading within the service contour.
This is not important to a Class C, but an absolute killer
for a Class A.
The Average for a US non-directional Class A station is 195
degrees.
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