[BC] Part 15 FM Power

[Richard Fry]

Harold Hallikainen wrote about "legal" Part 15 FM tx power:
>100mw sounds a bit high. Back in 1993, I calculated that 18.7nw
> into an isotropic antenna would hit the FCC limit for FM
______________

That's the right answer for an isotropic radiator, according to equations 
in FCC publications (and antenna textbooks).

For a 1/2-wave dipole it is  ~18.7/1.64  =  ~11.4 nW.

It is curious that  most commercially built "Part 15" FM transmitters are 
rated for at least 10 mW, which is more than 900,000 times more power than 
a 1/2-wave dipole needs to generate the maximum legal Part 15 FM field 
strength.

Part 15 FM systems are certified by field strength measurements and not tx 
power rating.  Most certified Part 15 FM txs use an attached vertical whip 
around two feet long.  The peak gain of such an antenna in the FM band is 
somewhere between that of an isotropic and a 1/2-wave dipole, ie, between 
1X and 1.64X.  So a certified system must have been adjusted for extremely 
low power into the antenna if an accurate certification was done.  A 10 mW 
PA would have to be cranked back close to 60 dB.

The link below leads to a plot of field strength vs distance over the range 
3 to 1,610 meters for the maximum legal Part 15 FM field, and several other 
radiated powers commonly used, all using 1/2-wave dipoles.

    http://rfry.org/Software%20Download/Part%2015%20FM%20Plots.pdf

RF