Converting From mV/m To Decayed mW (ERP)

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The radiated power needed to generate a given, free-space field strength of course depends on the length of the propagation path to that value of field strength.

Re-arranging the FCC equation to solve for the radiated power needed to generate 562 mV/m when distance in kilometers is known gives

P = D^2/0.1552

So if there is a need to limit the field strength to 562 mV/m at a location

Another way of doing it is to massage this "classic" equation.

Field Strength in dB above 1 microvolt/meter = 104.77 + ERPi - 20 log (D)

where ERPi = Effective Radiated Power in dB referenced to 1kW from an isotropic radiator D = Distance in kilometers

It gives the same answer as the simpler equation above.

I think this will answer your question, if I understood it right.

RF

If I understand what you're asking, your question really centers around how to calculate Free Space Loss. The Friis Equation will allow you to do this, and it is a relatively simple matter to derive Free Space Path Loss in decibel form from the Friis Equation.

I suspect what you really want to determine is more complicated...(?) Shoot me an email if you like and I can walk you through this, or give you some excellent references for radiofrequency / antenna modeling, and propagation analysis in real-world situations.

-Michael

I'm not sure: Is this the same (or at least related to) W/m^2? The problem I see with these is that, as distance approaches 0, KW approahes oo and what I'm looking for shouldn't be greater than the transmitter's erp. What got me on this idea, is a few weeks go I was visiting up in the NH White Mountains and was checking out the AM/FM bands (I'm a radio geek! P=). When I got home, I went to the FCC site to create a bandscan of the area. The designation used was the radius from a point. I quickly realized that this was less than ideal, as--using 100 km radius, as an example--100 W stations at that distance would be listed along with 50 and 100 KW! What would be much more useful would be a listing of stations that provide--st least the theoretical baseline value (i.e., not considering intervening local terrain or one's receiver sensitivity and antenna gain)--a given mV/m or mW at that point (thus listing 100 W stations out to 15 km, whereas 50 KW stations out to 75 km and 100 KW to 130 km would be listed). Think of it this way: If you turned on a 100 W lightbulb in the middle of a field at night and held a book page right up to it, you would be able to read it clearly; if you moved about 10 feet away (with the book page still facing the lightblb, unobstructed), you would have difficulty trying to read it, as only about the equivalent of (e.g.) 50 mW of light may be hitting the page.

~Kaimbridge~

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Using the known ERP from the transmit antenna it is possible using various algorithms to calculate the field strength that will exist at a distant point for some defined condition(s). The FCC has done this for AM/FM/TV broadcast stations, and the techniques are available to the public via the FCC's website. Various broadcast-oriented commercial software programs also are available to do this, although they are expensive.

For AM stations, the distant field is a function of the frequency, ERP, path length, ground conductivity, terrain roughness, the ionosphere, and the time of day.

For FM/TV stations, distant field is a function of ERP, the heights of the transmit and receive antennas, reflections, atmospheric conditions, path length, and the terrain profile along the path.

These are not simple relationships. For example, in order to provide essentially equal signal strengths over most of their useful coverage areas, the ERP needed from an FM station with a "high" antenna can be much less than if the antenna is lower. Here is a plot showing this...

Unfortunately, there is no simple solution answering your queries.

RF

Unless i am more mistaken than usual, the basic reference distance for radiated power is 1 meter from the phase center of the antenna. Of course depending on wavelength / frequency this could mean sub-near field measurements to normal (semi-far to far field) measurements.