04 How much power should I use to get a range of X miles? (Low Power Broadcasting)
Description
This article is from the Low Power Broadcasting FAQ, by Rick Harrison raredata@geocities.com with numerous contributions by others.
04 How much power should I use to get a range of X miles? (Low Power Broadcasting)
This is the most frequent question, and many newbies want a simple
answer. There is no simple answer. The distance at which your signal
can be heard will vary from listener to listener, depending on their
equipment and location. In fact, many elements affect your range:
* the transmitter power
* the transmit antenna's height and effectiveness
* local terrain (hills, large buildings, etc.)
* interference from distant stations on the same channel
* splatter from local stations on adjacent channels
* the height and effectiveness of the listener's antenna
* the sensitivity of the listener's receiver
* amount of "static" at the listener's location
predicting the range of an FM or TV signal
On FM and TV broadcast frequencies, antenna height puts an upper limit
on your range, regardless of power levels. (It is true that
diffraction can extend your range slightly, and signals sometimes
travel greater distances when atmospheric conditions are just right,
but we will ignore these factors for the time being.) The distance
from your antenna to the radio horizon is determined by this formula:
distance in miles = 1.415 times the square root of the antenna height
in feet.
Now you see why FM and TV stations go to the expense of building
antenna towers that are hundreds of feet high, or locate their
antennas on mountain-sides that overlook the cities they want to
serve.
Of course, the formula assumes that the terrain is relatively flat.
If the transmitting antenna is located at the top of a hill, its
range might be better, but this depends on the direction in which the
antenna radiates its energy (i.e. its vertical radiation angle). If the
antenna shoots a lot of energy up into the sky (as some types of antennas
tend to do), placing it in a higher location will not help much. You
can see the effect of terrain on coverage by following this link to some
low-power FM coverage maps ( http://www.bcradio.net/fmsrmap.htm ).
[The web version of this FAQ contains more information on this topic.]
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