I'm nitpicking but WHO has a license for class 1A (non-directional) 50,000 watt station not 40,000 watts. There are only a handful of those stations around (30 or so). Basically what that means is that they don't have to point their signal in any specific direction, and they don't have to power down. There are other stations that have as much wattage but they are directed in specific areas.
When they don't have to point the signal then a 50,000 watt station can end up just about anywhere. WHO for example (from what I've heard from the engineers at WHO when I worked there) sounds crystal clear in Dallas once the sun goes down. This is due to a effect called a sky wave. That isn't the limit though, when I interned there I had to respond to all of these people involved in a hobby called DXing. DXing is bascially listening to static and hoping to come into a signal from far away. I confirmed that people would listen to Iowa basketball broadcasts from all over the globe, its been awhile but I think the furthest away was from South Africa. I actually had a teacher who fought in Vietnam and caught WHO just by chance for a moment or two.
The original reason for these big signals was in the case of a national emergency before the widespread adoption of TV. They still exist today because the FCC grants a license for broadcast for life.
When they don't have to point the signal then a 50,000 watt station can end up just about anywhere. WHO for example (from what I've heard from the engineers at WHO when I worked there) sounds crystal clear in Dallas once the sun goes down. This is due to a effect called a sky wave. That isn't the limit though, when I interned there I had to respond to all of these people involved in a hobby called DXing. DXing is bascially listening to static and hoping to come into a signal from far away. I confirmed that people would listen to Iowa basketball broadcasts from all over the globe, its been awhile but I think the furthest away was from South Africa. I actually had a teacher who fought in Vietnam and caught WHO just by chance for a moment or two.
The original reason for these big signals was in the case of a national emergency before the widespread adoption of TV. They still exist today because the FCC grants a license for broadcast for life.
