RadioTechTalk

Why is AM reception worse at night?

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

------------------------------------------------------------------------------------------

                    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

-----------------------------------------------------------------------------------------

                    November        6:00 AM               6:45 AM                4:30 PM       CST
                    December        6:00 AM               7:15 AM                4:30 PM       CST

 -  Asfarez Eino

"I'm hearing a different FM station today."

Tropospheric Ducting Explained

FM broadcast band signals are just one of the many radio services that can be affected by this summer-month ‘Ducting’ phenomenon.  The lowest region of the earth’s atmosphere is called the Troposphere.  This is the region where most weather occurs, and where VHF & UHF communications normally travel and die out, shortly beyond the horizon.

However, usually in September and October, (I’ve noticed it as early as mid-July) a condition develops, allowing radio signals to be heard hundreds of miles further than intended.  Gulf coast states see it often, Atlantic seaboard, Great Lakes, and Mississippi valley areas see it occasionally.  Although these ducts ordinarily form over water, they can form over land, as well, creating a great deal of listener confusion, and broadcaster dismay.

A weather anomaly called a Temperature Inversion is at the root of the problem.  Normally air temperatures & pressures are highest at the Earths surface, and then decrease up and through the atmosphere.  But, when a stable high pressure system of cooler air, becomes overrun by a warm air mass, a Tropospheric Duct is a result.  Radio waves can become trapped below the warm air, traveling great distances through the cooler air, with little loss.

Other stations, from way out of the area, on or next to our frequency, that you could normally never hear, come banging in on top of us, with the intensity of a local station.

The degree and duration of the interference will vary.  So, if you experience this symptom, and it is summer, think "Ducting".

- Eimite B. Wright