MadSci Network: Physics
Query:

Re: How can an FM or AM broadcast be heard simultaneously nationwide?

Area: Physics
Posted By: Steve Czarnecki, senior technical staff member, systems engineering, Lockheed Martin
Date: Wed Jun 11 11:11:30 1997
Area of science: Physics
ID: 863446456.Ph
Message:
Your question applies as well to television programs... the answer
is that the program material is distributed through some alternate
channel.  This may be leased telephone lines or, increasingly, satellite 
links.  The telephone lines that are used provide better signal quality
and bandwidth than ordinary dial-up lines.

Back in "olden days", circa WW-II, a combination of telephone lines
and shortwave links were most likely used as the distribution medium.

By the way, it's not the transmit power of shortwave that allows 
continent-hopping or even hemisphere-hopping links; it's the fact that
at shortwave frequencies, the so-called "high frequency" or HF band 
(3-30 MHz), the ionosphere is able to refract signals back towards the 
earth.  That is, shortwave signals bounce off the ionosphere and can be 
picked up far beyond the transmitter's horizon.  Using Morse code, for 
example, it's possible to communicate thousands of miles with tens or a 
few hundred watts of power at shortwave frequency.  By comparison, some
FM broadcast stations transmit 50,000 watts but have an effective 
range of about 100 miles.  Some TV stations transmit up to 5,000,000 
watts with no more range!

Finally, did you know that AM broadcast stations have tremendous range
at night (thousands of miles) but relatively short range during the
day?  When the sun goes down, the lowest layer of the ionosphere 
disappears, which tends to absorb AM radio waves

I think there are still a few "clear channel" AM stations that can be
heard nationwide at night.  They're called "clear channel" because 
they are the single authorized transmitter on the continent at that 
frequency.  This eliminates interference from other transmitters on the
same frequency hundreds or thousands of miles away.  For a surprise,
try tuning through the AM broadcast band at night.  Even a car radio
with the engine off (to eliminate radio noise from the ignition system
and alternator) will often give surprising results.  Better yet, invest
in a cheap shortwave radio, or ask around and find a licensed amateur
radio operator.

There's an amazing world out there beyond the internet!





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