scwis

by Jonathan Smick
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It takes about 8-10mW T.P.O (Transmitter Power Output) into a 0dBi antenna (like a 1/4 wave groundplane) to get in the ballpark of 250 uV/m at 3 meters (one wavelength at FM Broadcast Frequencies)…

by Jonathan Smick
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It takes about 8-10mW T.P.O (Transmitter Power Output) into a 0dBi antenna (like a 1/4 wave groundplane) to get in the ballpark of 250 uV/m at 3 meters (one wavelength at FM Broadcast Frequencies)…

The higher the gain above 0dBi the less transmitter power required to reach legal limit. ERP, EIRP,and TPO are not the same thing.

EIRP (Equivalent Isotropically Radiated Power) is the amount of energy, which when integrated over a sphere radius of R(assumed isotropic or point source)equals the total energy density radiated by a gain antenna in its favored direction.

EIRP when integrated over a sphere (assuming true isotropic)at Part 15 Legal FM power level is in the nanowatts(can’t recall the exact figure, but suffice it to say, minuscule).

ERP (Effective Radiated Power) is the total energy density (degree of concentration of radiation, or collimation) radiated by any antenna in it’s favored direction (if it has gain) verus a true (theoretical) isotropic or point source.

Thus 0dBi means a perfectly (or close enough to it)spherical radiation pattern, energy radiated equally in all directions. Think “lightbulb”.

Actually, a good analogy, since light is simply very high frequency EM waves, essentially “radio we can see.”

Another example: An antenna has 6dBi gain. This means in it’s favored direction, it radiates 4 times as much power as an isotropic radiator (0dBi) would AT THE SAME TPO.

Put another way, the isotropic source’s TPO would have to be increased by a factor of four to equal the energy density of the gain antenna in the gain antenna’s direction of maximum radiation.

UHF TV stations have ERP’s in the multi-megawatt range, but their TPO is only 10’s or at most a couple of 100’s of KW. TPO is simply the actualpower output at the antenna terminals of the transmitter, in watts (or fractions thereof).

Jon
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Thanks to our contributor for bringing one of Jon’s articles back from the past. Jon operates an awesome Part 15 station list site at The Master List of Part 15 Radio Stations of North America