You've gotten some good advice some others. I'll just add that most Part
15 devices are specified in terms of field strength at some distance
from the antenna, depending on frequency, and not in terms of power or
ERP. There might be some sections with other criteria, but if there are,
field strength specification is by far the most common. The FCC does cut
some slack in testing for home-built devices (not marketed, not
constructed from a kit, and built in quantities of five or less for
personal use), in section 15.23. My copy is nearly ten years old now, so
I suggest checking a newer copy of Part 15. It's likely on the Web these
days.

Roy Lewallen, W7EL

Liam Ness wrote:
I've been homebrewing some simple part 15 transmitters and have always
thought that I was safely within part 15 by controling the RF output.
I use a spice program to estimate my output levels. I just read a web
page that suggests a antenna can increase the RF output power and I
wanted advice if that is true. It was suggested that output could be
increased from 30milliwatts to 60milliwatts by using this antenna. I
understand how you could increase voltage with a decrease in amperage
and vice versa, but I was under the assumption that you couldn't
increase total power without adding more power. I thought it would
violate one of the laws of thermodymanics otherwise. They didn't seem
to be talking about more effieciently radiating the transmitters
power, but actually increasing it above what is present at the antenna
port.

Could someone confirm whether it is posible to increase the power
output of an RF transmitter above the total presented to the antenna.
If it is, I'd appreciate any pointers to information about this. I
don't want put myself out of part 15 by a poor antenna choice. (even
though I still can't believe that it is possible, it sounds to much
like perpetual motion)

TIA