Why do I lose AM radio reception in the evening?

Question

I live in the vicinity of Portland, OR and every night around 4:45-5:00 am radio reception goes to cr*p. What could be the cause of that? It is winter now and that is about the time the sun goes down. ?

Answer 1

Part of the problem has to do with changes in the Ionosphere and Stratosphere as the sun goes down, and due to FCC regulations.

When radio signals are transmitted from the antenna it radiates 360º in all directions. This means the signal goes into the upper levels of the atmosphere.

During daylight hours the atmospehere is heated and its density is reduced. The radio signals pass through the atmosphere into space.

At night the atmosphere is cooled and becomes denser. The radio signals will bounce off the atmosphere back to earth. This is also called SKIP.

When you listen to an AM radio station in your area, on a certain frequency, it is not the only AM radio station using that frequency. Many other AM stations around the U.S. and world are using it.

During daylight hours, due to a less dense atmosphere, the AM signal becomes more of a Line of Sight Signal like FM. The signal is transmitted in a Radius around the antenna for a distance determined by the output wattage.

Once the sun sets and the atmosphere becomes denser, the radio signals start bouncing back to earth. The signal from your AM station, that might have had a 50 mile range during daylight, will now have a range of hundreds to thousands of miles a night. This is due to the reflection of the atmosphere. The signal from a NYC station may be received in California because of this. The rest of the country between NYC and Calif. may not receive that signal due to many factors pertaining to the atmosphere and angle of the reflection.

Because of this reflection, other AM stations using the same frequency are received by your radio. This is why you might be hearing your station and then another station overrides that signal and you hear it. Then that signal fades and you get your station again.

So, you can blame part of the poor reception, at night, on MOTHER NATURE.

The rest of the blame can be heaped upon the FCC. This is where it becomes a little more complicated.

At Official Sunset (where you live) the radio station is required to change the signal transmission pattern from its antenna, and sometimes to reduce its transmission strength (output wattage). This is determined by the location of other AM radio stations using that frequency.

Lets say you live in Nebraska. You live 50 miles south of the AM stations antenna and have great reception in daylight and little to none at night. The FCC probably requires your AM station to change its transmission pattern so the signal goes north of the antenna. The reason for this is could be another AM station on the same frequency in Kansas or Oklahoma. The transmission pattern change is required so your AM stations signal doesn't interfere with the signal of the AM station to the south.

Doing this does not mean the people in KS or OK will not get the signal from your AM Station. The SKIP will ensure they do get some of that signal, but not nearly as bad as it would be if the pattern wasn't changed.

AM radio is on its death bed. I know, we have been hearing this since the invention of FM radio. Now, though, it is true. The Feds have decided the AM frequencies are needed for other uses. It won't go away tomorrow, but will be eliminated in the next few years.

Answer 2

Two things happen at sunset that affect AM radio. One is that the ionosphere becomes reflective at the wavelengths of the AM band, sometimes causing interference from more distant stations. The other is that, to reduce the amount of this interference, most stations are required to reduce their broadcast power after sunset.

Answer 3

because all the AM radio stations stop playing stuff in the Afternoon, i think around 5, or 6 o'clock