Background
As you are likely aware, conditions observed
on the AM band change drastically between daytime and nighttime.
This concept is important in order to understand why the
FCC and other organizations impose certain power and antenna pattern guidelines.
Propagation
During the day, signals are received primarily
by groundwave. Signals propagate through the Earth's surface,
to the destination. Ground
conductivity is the primary inhibitor to the distance propagated.
On low frequencies and in
areas with good ground conductivity (such
as the north central plains), signals can reach 300-500 miles in the day,
although weak.
At night, however, skywave propagation becomes the primary factor. During the day, skywave is inhibited because the D region of the Earth's ionosphere absorbs medium frequency wavelengths. As the sun lowers in the sky, the "fuel" that ionizes the D region begins to dissipate, allowing for the free electrons and ions to recombine. The D region has a very rapid recombination rate, meaning that it usually dissipates quickly around sunset. Thus, the absorption of MW signals disappears.
The dissipation of the D region allows for other parts of the ionosphere to "reflect" radio waves after dark. Most MW propagation is supported by the E region, and some is supported by the F region of the ionosphere. Both of these regions have much slower recombination rates than the D region, meaning that they will persist throughout the night.
Skywave propagation usually supports distances in the 300-1600 mile range with the greatest efficiency, but certain conditions, and stations of high power, can occasionally reach much greater distances.
Frequency Allocations
Originally, the FCC wanted to insure structure
to the AM band, and wanted to insure that certain cities and regions of
the country had adequate levels of service. For this reason, they
identified some frequencies as "Clear
Channels", "Regional
Channels" or "Local
Channels". Clear channels had a few dominant 50 KW stations designed
to serve much of the USA. Regional Channels consisted of many smaller
5 KW stations serving a state or two. Local channels had dozens of
1 KW stations meant to serve a single town or metro area.
Taking the clear channels as an example, usually just 1 or 2 stations had priority on a given frequency. This meant that any other station operating could not interfere drastically with the coverage areas of the primary stations. The FCC would grant licenses to stations on clear channels, but the license would either limit the operation of the station to daytime only, or require a drastic power cut and/or antenna pattern change at night.
Since the characteristics of propagation change so rapidly around sunrise and sunset, it was decided to make sunrise and sunset the time at which stations were forced to end their broadcast day or change facilities to protect the primary occupants of the frequency.
These FCC guidelines are still in affect today, and are also used by Mexico and Canada.
How do I take advantage of facility
changes and sunrise/sunset DX'ing?
It would be very difficult for radio stations
to change facilities (i.e. change antenna pattern or change power) right
at sunset or sunrise every day. Instead, it was decided to take the
sunrise and sunset time for the station in question on the 15th of each
month, and round it to the nearest 15 minute increment. This would
be the time to change facilities for the month.
For example, a station in Phoenix, AZ is granted a daytime only license. For the month of November, the facility change time would be based on the sunrise and sunset times for November 15. Sunset is at 5:26 PM on the 15th, so that would be rounded to 5:30 PM. For the month of November, the station would have to leave the air at 5:30 PM.
Recalling the propagation overview above, the D region has a rapid recombination rate. This means that the D region starts to dissipate even before sunset, allowing some odd conditions to exist around the period of sunrise and sunset.
A DX'er can develop target stations based on knowing when the target will change powers and patterns. For example, at sunset, you would check for a station in the period of 5-10 minutes before it is scheduled to drop power or go off the air. At sunrise, you'd check right at the time it is scheduled to increase power or come on the air.
As you become more experienced, you will notice that you can take advantage of the 15 minute rounding as well. Using the Phoenix example, recall that for the entire month 5:30 will be the sign off time. However, on November 1, the actual sunset time is 5:36 PM. This means that the station will sign off before the sun has even set, lessening the likelihood of it being heard by people at a great distance. On November 30, the sunset is 5:20 PM. The station will get to operate for 10 minutes after dark on the last day of the month, making it a better target then.
In other months, this effect is more drastic
(September, for example, has sunsets ranging from 6:54 to 6:14 PM in
Phoenix).
Sunrise and Sunset DX Review
The National Radio Club sells a USA/Canada station listing that identifies day and night powers, hours or operation, and other useful data to help identify target DX.
These links are useful for identifying
sunrise and sunset times:
FCC
Calculator
Sunrise/Sunset
Calendar
Worldclock
- current time and SR/SS times by major city
