Most FM antennas are vertically polarized. You can lose up to 20dB by going horizontal, thus
cross polarizing the TX and RX. AM is far more forgiving of polarization. I've heard of great results on AM by tuning into the rain gutter of a dormitory.
Part 15 limits are
ERP. If you want to be
legal, use
up to 100mW and don't exceed about 20 micro Volts/m; at 30 meters.
WRONG!!!!!!!!!
while this post relates to a specific transmitter, the data is the same regardless when using a part 15 am transmitter
A center loaded 10 foot radiator may also be used. This involves splitting the radiator into two sections and adding an inductance to bring the radiator to resonance at the desired frequency. Approximately enough inductance is needed to resonate with the self capacitance of the top whip antenna. For 1600 kHz this will be on the order of 400 to 1000 microhenries, depending on whip length and diameter, as well as exact frequency. A good RF ground system is required, and antenna bandwidth of 10 kHz is typical. Radiation resistances of 0.1 to 0.3 ohms are typical, and the radiation efficiency of a system such as this will be a few percent at best, assuming ideal grounding, and 0.5 % for the typical home experimenter setup. However, experience at 1880 kHz with 160 meter Amateur mobile operation, using 10 watts AM and a center loaded 8 foot whip mounted on an automobile, shows that 2 way contacts at 50 to 100 km (30 to 60 miles) are possible and fairly common. Extrapolating this data based on theory, with 100 mW, (20 db below 10 Watts) therefore, ranges of 5 to 10 km. (3 to 6 miles) would seem possible without violating FCC rules. However, noise and interference will be the main limitation. Its all in the location and antenna system, and how well everything is tuned and matched.
http://www.northcountryradio.com/Articles/part15.htm
WRONG!!!!!!!!!!!!!
YOU GET 20 micro Volts at 30 meters. PERIOD. Reception will depend on the sensitivity of the receiver!
FURTHERMORE, the author is off by a factor of 10. In his theory, he should expect .5 to 1 km with .1 W. Propagation with AM isn't that predictable, so it doesn't matter.
If you're
not trying to be legal, use as much power as you want into the best antenna you can get: to accomplish your goal.