PROTECTED SKYWAVE COVERAGE AREA FOR 50KW STATIONS

[radiotower40]

I was wondering how far the protected skywave coverage was for 50KW AM stations is. I believe it is 750 miles.The reason
I am asking is because I noticed quite a few 1watt -25watt stations on the FCC website that are more in the 400 to 500 mile range.
If it is possible to go 1watt at night inside the 750 mile protected contour, what is the closest distance possible to the protected
station that you can get a license to do so? Can it be done at distance of 200 to 400 miles for low power levels such as these?
Can it be even closer if you are only requesting 1 or 2 watts and does the FCC let these low power stations get permission from
the protected stations to broadcast or does it strictly go by the FCC'S rules? Thank you for any information you could give me.

 

[rfry]

I was wondering how far the protected skywave coverage was for 50KW AM stations is. I believe it is 750 miles. The reason I am asking is because I noticed quite a few 1watt -25watt stations on the FCC website that are more in the 400 to 500 mile range. If it is possible to go 1watt at night inside the 750 mile protected contour, what is the closest distance possible to the protected station that you can get a license to do so?

AM stations have different "classses," and only Class A stations have nighttime skywave protection -- which is defined by their 0.5 mV/m 50% skywave contour, not by a fixed distance. Class A stations are assigned on these frequencies, with a power of 50 kW, non-directional day and night: 640, 650, 660, 670, 700, 720, 750, 760, 770, 780, 820, 830, 840, 870, 880, 890, 1020, 1030, 1040, 1100, 1120, 1160, 1180, 1200, and 1210 kHz.

Some Class B stations also may operate with 50 kW day and night but usually are directional at least at night. They have no nighttime skywave protection, so other stations can be located closer to them than a Class A. For example WRKO in Boston runs 50 kW day and night, but is directional, with different patterns day and night, and has other 24-hr co-channel stations fairly close to them compared to Class A stations.

Can it be even closer (than 200-400 miles) if you are only requesting 1 or 2 watts and does the FCC let these low power stations get permission from the protected stations to broadcast or does it strictly go by the FCC'S rules?

The lowest daytime AM power authorized by the FCC is 250 watts. Even though some existing stations operate at powers of a few tens of watts at night under some conditions, a new station would not be authorized unless it operated with a daytime power of at least 250 watts, and that subject to the daytime groundwave protection of 50 kW stations (with or without their permission).
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[rfry]

(I missed the edit deadline for my previous post...)

The FCC also requires that a station licensed for 250 watts daytime power must use an antenna system capable of generating a groundwave field of at least 241 mV/m at 1 km for 1 kW of radiated power.

Typically that requires a fairly tall (in fractional wavelength) monopole and a good radial ground system.

The inverse-distance groundwave field at 1 km over a perfect ground plane from a 1/4-wave monopole with 1 kW of radiated power is about 313 mV/m.
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[kevtronics]

The FCC also requires that a station licensed for 250 watts daytime power must use an antenna system capable of generating a groundwave field of at least 241 mV/m at 1 km for 1 kW of radiated power.

Stations must be originally licensed to meet that standard, but they can then subsequently apply, and be approved, for lower power and less coverage.

For example, 1170 WWTR in Bridgewater, NJ (originally WBRW) was originally 500 watts directional, but after several years of being off the air in the '90s their towers and building were torn down, so when they rebuilt in 1997, they applied for a change to non-directional operation from a single folded unipole tower, and that resulted in a power of 243 watts, with a coverage of 141 mV/m at 1 km.

WBRW/WWTR actually came within serveral hours of having their license permanently deleted for being "dark" for too long. There was a deadline of around February 12th, 1997, of when all silent stations were to have their licenses cancelled, and they made it on the air around noon that day!

 

[rfry]

For example, 1170 WWTR in Bridgewater, NJ (originally WBRW) was originally 500 watts directional, but after several years of being off the air in the '90s their towers and building were torn down, so when they rebuilt in 1997, they applied for a change to non-directional operation from a single folded unipole tower, and that resulted in a power of 243 watts, with a coverage of 141 mV/m at 1 km.

But then they more than met the minimum acceptable inverse distance field at 1 km for a station running 250 watts with the minimum acceptable radiator efficiency, which is 121 mV/m.
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