[BC] Part 15 FM Power/Radiation Limits

[WLOYPROD WLOYPROD]

I understand the calculations, but I was pulling the 18mW from
recollection of testing various "kit" transmitters and Ramsey units and
things with a conventional car-type replacement antenna (32" was the
length I recall).  I'm using long fried brain cells, but IIRC that was
the approximate power that could be used with that type of antenna, NOT
a dipole.  I was referring to the common configuration used by the
various tiny transmitters that are often incorrectly setup with 100mW
for FM, when they are at the max around 18mW in their typical
configuration.  Those type of units often use conventional car whips as
their antenna (sometimes they use even shorter ones) or whatever is
handy at a Radio Shack.  I didn't mean something that was built more
conventionally ;)

John

>>> rfry at adams.net 07/07/05 9:05 AM >>>
"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 



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