[BC] Part 15 FM Power/Radiation Limits

[Richard Fry]

"John" (WLOYPROD at loyola.edu ) wrote:

> ... 250uV/m @ 3m for FM, under 15.239 - works out to about
>  18mW into a conventional 32" whip...
____________________

Sorry, John, but that conclusion is a bit off.

Although only field strength matters here, the input power for unlicensed 
FMs of ~11.5 nanowatts (NOT milliwatts) into a 1/2-wave dipole has been 
posted here already--see the thread "FM and AM/slight drift" on the June, 
2005 BC list.  Depending on the length/configuration of the tx ground lead 
(if any), using a 32" whip antenna might allow increasing that antenna input 
power by a fraction of a decibel without exceeding the field limit at 3 
meters, but nowhere near to reaching 18mW.

Here is the applicable 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

RF