I’m doing a project on radio, and i wanted to find the transmission ranges of AM and FM radio, or the distance at which their respective signals are still useful.
Welcome to the magic of "propagation".
SHORT answers:
FM radio signals are normally limited to a maximum of about 50–100 miles in crowded radio markets, and about half again farther in areas where interference from other stations wouldn’t be a problem. The FCC sees to that in order to avoid interference issues that would degrade reception.
AM radio signals can cross the country, and in fact, a very few are encouraged to try to do just that at night with their high transmitter power, antenna design and frequency allocation.
LONGER ANSWERS:
Especially for AM, there’s no *easy* answer to this question since so MANY variables come into play. While AM and FM broadcast signals propagate quite differently (due to the difference in their frequencies) and their distances are affected by the following to varying degrees, here’s an interesting list:
For AM especially, what is the time of day? Makes a HUGE difference due to what is known as skip (see link below). AM can make it cross-country at night if there isn’t too much …
… interference. This can come from other stations on the same frequency or from artificial sources. Certain AM frequencies were once (and still are to some degree) called "clear channel" since a limited number were allowed to operate on the same frequency and with high power at night.
Power makes a big difference. A 250 watt transmitter and a 50,000 watt transmitter will certainly have different coverage areas. See "Time of Day", above. Because AM stations can have such greater coverage at night, those on many "regional" and "local" frequencies are required to lower their transmitter power at night (the FCC sees to that) to keep their range in check to avoid interference with other stations. A station might be 10,000W during the day, and only 500W at night.
Antenna design — antennas can be directional by nature, and squeeze more of their power into certain directions. That’s called "antenna gain". If you were on the west coast and didn’t care about the folks in the Pacific Ocean, why not try to direct all of your energy in an easterly direction? It’ll go a whole lot farther if it doesn’t have to try to cover a 360 degree circle.
Antenna height — especially for FM, which tends to be a more or less "line of sight" proposition in the first place. Get the signal up there, and you can get an FM signal to reach out a good bit more.
Sunspots — yeah, sunspots. Every 11 years, the sun goes on a tirade, and it does some very interesting things to the upper atmosphere. That "something" also makes a big difference to radio wave propagation.
The list of variables IS huge, and there’s just no one answer to your question as a result. Engineers spend a lot of time estimating (and then measuring) the propagation of a broadcast TV, FM or AM station… and are often still surprised at the results.
Here’s a couple of links that discuss this in more detail:
http://en.wikipedia.org/wiki/Radio_propagation_model
http://en.wikipedia.org/wiki/Radio_propagation
Some people make a hobby of trying to pick up distant AM (medium wave) stations, and a few try for long distance FM or TV. It’s called DXing (DX being the abbreviation for "distance". A couple more sites out of hundreds of possible ones devoted to this hobby:
http://en.wikipedia.org/wiki/MW_DX
http://en.wikipedia.org/wiki/E-skip
February 23rd, 2010 at 9:08 pm
Welcome to the magic of "propagation".
SHORT answers:
FM radio signals are normally limited to a maximum of about 50–100 miles in crowded radio markets, and about half again farther in areas where interference from other stations wouldn’t be a problem. The FCC sees to that in order to avoid interference issues that would degrade reception.
AM radio signals can cross the country, and in fact, a very few are encouraged to try to do just that at night with their high transmitter power, antenna design and frequency allocation.
LONGER ANSWERS:
Especially for AM, there’s no *easy* answer to this question since so MANY variables come into play. While AM and FM broadcast signals propagate quite differently (due to the difference in their frequencies) and their distances are affected by the following to varying degrees, here’s an interesting list:
For AM especially, what is the time of day? Makes a HUGE difference due to what is known as skip (see link below). AM can make it cross-country at night if there isn’t too much …
… interference. This can come from other stations on the same frequency or from artificial sources. Certain AM frequencies were once (and still are to some degree) called "clear channel" since a limited number were allowed to operate on the same frequency and with high power at night.
Power makes a big difference. A 250 watt transmitter and a 50,000 watt transmitter will certainly have different coverage areas. See "Time of Day", above. Because AM stations can have such greater coverage at night, those on many "regional" and "local" frequencies are required to lower their transmitter power at night (the FCC sees to that) to keep their range in check to avoid interference with other stations. A station might be 10,000W during the day, and only 500W at night.
Antenna design — antennas can be directional by nature, and squeeze more of their power into certain directions. That’s called "antenna gain". If you were on the west coast and didn’t care about the folks in the Pacific Ocean, why not try to direct all of your energy in an easterly direction? It’ll go a whole lot farther if it doesn’t have to try to cover a 360 degree circle.
Antenna height — especially for FM, which tends to be a more or less "line of sight" proposition in the first place. Get the signal up there, and you can get an FM signal to reach out a good bit more.
Sunspots — yeah, sunspots. Every 11 years, the sun goes on a tirade, and it does some very interesting things to the upper atmosphere. That "something" also makes a big difference to radio wave propagation.
The list of variables IS huge, and there’s just no one answer to your question as a result. Engineers spend a lot of time estimating (and then measuring) the propagation of a broadcast TV, FM or AM station… and are often still surprised at the results.
Here’s a couple of links that discuss this in more detail:
http://en.wikipedia.org/wiki/Radio_propagation_model
http://en.wikipedia.org/wiki/Radio_propagation
Some people make a hobby of trying to pick up distant AM (medium wave) stations, and a few try for long distance FM or TV. It’s called DXing (DX being the abbreviation for "distance". A couple more sites out of hundreds of possible ones devoted to this hobby:
http://en.wikipedia.org/wiki/MW_DX
http://en.wikipedia.org/wiki/E-skip
References :