erichmadison wrote:
Newbie here with a few questions regarding low power FM transmission.
Please excuse my possibnle improper use of vernacular. I just purchased
a 50mW FM transmitter for playing music in my home. It was built from a
kit and sold to a first owner, then sold to me... if that matters.

Since the FCC now uses the guideline of "250mV @ 3 meters" power
limitation, one needs to know what their transmission output is. What
sort of device would measure this? If I am pushing too much power, can
I shorten my antenna (currently 2 lengths of wire coming from the back
of the unit), in hopes to lessen the range? The unit is in my basement,
which reduces the effective range and possibility of interference with
licensed users, but it may still be exceeding the part 15 power output
limitations.

(snip)

Perhaps the information on the FCC Website is old, but it currently reads

"Sec. 15.239 Operation in the band 88-108 MHz.

....
(b) The field strength of any emissions within the permitted 200 kHz
band shall not exceed 250 microvolts/meter at 3 meters. The emission
limit in this paragraph is based on measurement instrumentation
employing an average detector. The provisions in Sec. 15.35 for
limiting peak emissions apply."

So, rather than 250 mV/meter you are allowed only 250 uV/meter.

It takes less than 20 nanowatts of power into a practical antenna to get 250
uV/meter at 3 meters. Conversely, your 50 mW into a realizable antenna will
result in a little under 400,000 uV/meter at 3 meters.

Good luck.

John