Rich said:
And even less, if your antenna has gain, right? Because it still has to
be < 200 uV/m at 3 meters, looking right up the throat of the antenna,
right?
You are correct about antenna gain. The FCC spells out many of the test
conditions.
So, what's the deal with those RF modems with thousands of meters' range?
Are they, like, pre-licensed or something?
I cannot answer your question about RF modems without much more study of the
regulations. Most likely they are manufactured to meet some other part of
the regulations and have probably been submitted to the FCC for approval. In
some of these cases, it is illegal for the end user to tamper with any part
of the device, including attaching an external antenna.
And will a signal that weak reach an RC airplane that's, say, 300m away?
Presumably you mean 13 nW. I estimate that the signal strength would be
about -115 dBm at the receiver (isotropic source in free space) at 300
meters. I wouldn't try it.
That band is regulated in other parts of the rules. I do not remember the
appropriate section. I am certain that higher power is permitted for those
specific frequencies.
See (a) above. There are exceptions listed elsewhere in part 15. For
example, they allow 100 mW input power to the final stage in a transmitter
on the AM broadcast band (with not more than 3 meters of antenna, lead-in,
and ground combined). In the 88-108 MHz (commercial FM) band, 250 uV/m at 3
meters is allowed. The OP asked about the 144-146 MHz band which is alloted
to licensed amateur use (covered by Part 97 rules). Unlicensed operation in
that band is limited to 150 uV/m at 3 meters per the above table.
It is important to wade through all applicable regulations to discover what
is permitted.
Cheers to you, too.
John