Radio signals bend in the ionosphere soup
 Q: I have always wondered why AM radio signals coming from a distance are much stronger at
night, especially cloudy ones and why FM signals seem stronger early mornings. --Dennis and
Treen
A: Earth's atmosphere changes how radio waves travel. Out in space they zip along at light speed and
travel vast distances: from Earth to Pluto, for example.
Right: [U.S. Navy] Wave travel: line of sight and bouncing off the ionosphere
Here on Earth, when a signal travels in a straight line from transmitter to receiver, Earth blocks the
signal at the horizon. Viewed from eye level, the horizon is about 2.5 miles away but, from the top of a
500-foot transmission tower, it's about 100 miles distant. Line-of-sight signals, thus, go about 100
miles.
When we pick up an AM signal from farther than that, a cloud-like layer of ionized particles in the air
(called the ionosphere F-layer) bent the radio wave down to reach us. See figure. When the radio wave enters the F-layer air, it slows
down and therefore bends to a new direction. A soda straw in a glass of water appears bent due to the same phenomenon.
The ionosphere bends signals best at night because the Sun is no longer ionizing the atmosphere then. That's why you pick up distant
AM signals at night. An AM signal can hop all the way around the world at night, bending down from the ionosphere and reflecting
back up from Earth: hopping in that fashion and ultimately going vast distances.
On the other hand, FM radio waves go right through the ionosphere. Consequently, most FM signals are line-of-sight.
Water in the atmosphere weakens all signals but the higher frequencies more than the lower ones. For this reason, water reduces FM
strength much more than AM since FM frequencies are about 100 times greater than AM. In fact, AM is not much affected even by
rain. This argues against AM signals being stronger on cloudy nights. However, it may explain why you pick up FM signals better in
the morning.
"FM repeater stations, which rely on long distance pick up of an FM signal, often experience reduced signal level during the hotter
summer months..." says Lewis Downey, engineer at KUER FM in Salt Lake City, Utah. This may be due to more moisture in the air
during summer, which weakens FM signals. The morning, however, has cooler temperatures and lower humidity. Consequently, the air
contains less water to disrupt the FM signal. Thus, the FM signal is somewhat stronger in the cool of the morning.
(Answered by April Holladay, science correspondent, January 23, 2002)
Further Surfing:
USATODAY.com: Humidity
KUER FM90: Tuning tips
BBC: Better reception
US Navy: Propagation of waves
New Zealand Amateur Radio: Proagation of radio signals
Clouds in a glass of beer by Craig F. Bohren, Wiley Science, 1987, pp. 10, 11.
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