AM band signal propagation is enhanced at night, due to the shifting and combining of the atmospheric layers of the planet. This phenomenon creates a serious radio interference probability.
To alleviate this problem, the Federal Communications Commission restricts most AM stations, to specific modes of operation, which follow a rigid annual schedule. The main purpose of these mode changes is to offer the necessary protection from interference, to stations operating in other regions of the country on the same or nearby frequencies.
These different modes create a reduction in the amount of energy radiated, in the critical direction of the protected station(s), as compared to the normal day-time operation. Common methods to achieve these reductions are:
1. Decrease the amount of power in the antenna.
2. Use a directional array (multiple towers)
3. 1 and 2 combined
Like many stations, WNTA must also protect other stations during the day, with the use of a directional antenna. Our two-tower array is adjusted to produce the “radiation pattern” below:
Maps will enlarge
As you can see, when power is reduced at night, the pattern shape remains the same, while we shrink the pattern size. This accomplishes WNTA’s mandate to eliminate interference on the AM dial. They are referred to as contour maps. All points along each individual contour are of the same signal intensity. As the pattern indicates, the station radiates less in some directions, and more in others. So if you travel outside of the “distant” contour, your reception will be become marginal.
These necessary mode changes coincide with the rising and setting of the sun. While it’s true that the sun rises and sets at a different time every day, the FCC uses a consistent time throughout a given month, based on the stations latitude and longitude. Many stations also benefit from an additional authorization. Pre-Sunrise Authority allows for the use of an intermediate power level, during the fall, winter and spring, when it is still dark at 6:00AM.
WNTA – Transmitter Output Power
Pre-Sunrise Sunrise Sunset Zone
(500 Watts) (1000 Watts) (91 Watts)
January 6:00 AM 7:30 AM 4:45 PM CST
February 6:00 AM 7:00 AM 5:30 PM CST
March 6:00 AM 6:15 AM 6:00 PM CST
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March 6:00 AM 7:15 AM 7:00 PM CDT
April 6:00 AM 6:15 AM 7:30 PM CDT
May - N/A - 5:30 AM 8:15 PM CDT
June - N/A - 5:15 AM 8:30 PM CDT
July - N/A - 5:30 AM 8:30 PM CDT
August - N/A - 6:00 AM 8:00 PM CDT
September 6:00 AM 6:30 AM 7:15 PM CDT
October 6:00 AM 7:15 AM 6:15 PM CDT
November 6:00 AM 7:45 AM 5:30 PM CDT
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November 6:00 AM 6:45 AM 4:30 PM CST
December 6:00 AM 7:15 AM 4:30 PM CST
- Asfarez Eino
