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Name: Daniela
Status: student
Age: 13
Location: N/A 
Country: N/A
Date: 9/6/2004

How does the position of the Sun affect the reception of the radio?

The sun makes our ionosphere, which does three things in Earth's radio-wave environment:

1) being conductive, it reflects low-frequency radio-waves back to earth.

2) being only partly conductive, it absorbs radio-waves near the cutoff frequency for such reflection.

3) it makes radio-wave noise ("static") at low and medium frequencies.

The ionosphere's cutoff frequency varies depending of the intensity of light from the sun. During the day, the ionosphere thickens up, and during the night, it fades to weaker. The cutoff frequency can go up to 25MHz at peak times, and decline to maybe 5 MHz at night. The usual daily variation isn't that large, more like 2:1.

I have an old Radio Shack timecube/weather-radio. Lets me listen to boring WWV time announcements on 5, 10, and 15 MHz. Which of the three frequencies sounds best depends entirely on the sun. 5 at night, 15 at blazing noon. WWV-Colorado is 500 miles NE of me, WWV-Hawaii, 3000 miles W. Sometimes I can almost choose which I want to hear better, just by which frequency I listen on.

AM radio, at about 1 MHz, is low frequency. Day or night, the ionosphere always reflects those waves back down to earth. How far it goes, how many times it can bounce between earth and sky without being absorbed, depends a significantly on the sun. If I recall correctly, at this low frequency, reflection is lousier during the day. Also at night your receiver will hear lower a lower background noise level (between and "behind" channels), so you can hear stations farther away.

Shortwave radio, at 2-10MHz, is in the most interesting range for day/night cycles and sun position. It's just below the cutoff frequency. The sky is always a mirror, and more sun just squirts mud between it and you. In this range, it is actually possible that with the sun in your west (afternoon), you can hear stations to the east farther away. That is because it is night over where the station is, and the waves survive better in dusk or at night. A similar thing happens in the morning.

10-50 MHz, sunlight is all good, needed for getting around the curvature of the earth. More is better. This range is just above the cutoff frequency. Even on the good days, signals might go only 500 miles, as opposed to shortwave where they reach thousands of miles. But then, you may not have any radios to listen to in that band. "Hams" use this most.

FM radio, are about 100MHz, is a factor of 2 too high to ever be reflected by the ionosphere. The sun matters little to it, usually. (Except when they turn down the transmitter power because the FCC told them to, or they want to go to bed.) If you are in the country and you have a big directional roof antenna pointed at a distant city, then some noise-level variations might be apparent to a diligent listener. Especially if the sun gets approximately in front of that antenna at sunrise or sunset. But you'd have to see for yourself. You might see no change at all.

In higher frequencies, 100MHz to 2.4GHz and up, the ionosphere is transparent, and the sun itself is a noisy hot spot up in the sky. Somewhere above 10GHz atmospheric chemistry can absorb microwaves in certain frequency ranges. Temperature, humidity, and ozone and fluctuations due to day/night might matter if you are looking into the sky. But point-to-point on Earth should not change much.

I am not sure exactly which radio experiences you are asking about, but this is the outline of your answer. If you get a shortwave radio and hang out a 30-foot wire antenna, you will soon be personally familiar with the effects.

Jim Swenson

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