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LPFM Radio Engineering
This section of the website deals with the issues of LPFM engineering. Basically, the FCC's LPFM decision allows the licensing of low power FM facilities throughout the FM band (88.1-107.9 MHz) using powers of 1-10 watts (the so-called "microradio" service) and 1-100 watts (that which we generally think of as "LPFM").
The LP 100 stations will be applied for first, then the LP 10 stations will be "dropped in" where there's space available to do so. Both types of stations will operate with an antenna height above average terrain (normally abbreviated as "HAAT") of 30 meters (98.4 feet). For all intents and purposes, one can say that a LP 10 will operate with an ERP (effective radiated power) of ten watts and a HAAT of 30 meters. Similarly, an LP 100 will operate with an ERP of 100 watts and a HAAT of 30 meters.
If you have a tower site in mind, or a building, or some other place to put your antenna, remember that if you exceed the height limit of 30 meters, then your output power (ERP) must be reduced so that your predicted 1 mV/m (one millivolt per meter) contour does not exceed what would be produced with 100 watts (or 10 watts) and 30 meters HAAT. If you have the software to do this, then the calculations aren't difficult. If not, there are now, and most likely will be many firms offering engineering services so you can prepare your application in plenty of time before the first LPFM filing window opens.
It's generally thought that using the normal 100 watt, 30 meter rules, your 1 mV/m contour will go about 3.5 miles from the antenna. This contour is considered good service, but in some cases it will be less, and in other cases it will go farther than you would think. Over the past 50 years it's been known that FM coverage generally exceeds predicted contours, and as FM receivers continue to improve, FM stations' coverage generally improves. And, keep in mind that if you provide a service people want to hear, and they go to lengths to get it, you could be heard many miles away-certainly further than what a coverage map would depict.
SAMPLE COVERAGE MAP
What you see in the image below is a computer-generated FM coverage map showing the FCC's City-Grade (3.16 mV/m) and Service (1.0 mV/m) contours. We use a fictional site in a general location. Please understand that this is for demonstration only, and in no way implies that any particular channel will fit at the site we've chosen for this demonstration.
As you can see, the 100 watt, 30 meter HAAT operation puts a "city grade" (70 dbu, or 3.16 mv/m) signal over all of the city limits. The service contour (60 dbu, 1.00 mv/m) signal extends beyond the city limits and would most likely provide good mobile coverage in the outskirts of the city.
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