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.

Yes, IF the present antenna presents a bad impedance match
to the transmitter. Two factors come into play: 1) maximum
power transfer occurs when the source (transmitter) and load
(antenna) impedances match; and 2) the efficiency of the output
stage of the transmitter may be compromised when connected to
a mismatched load, resulting in less RF power produced.

But I doubt whether that was the point. I think that all they
were trying to say was that you would get more effective
radiated power. And the claim of doubling the power raises
suspicion. If memory serves me, part 15 radiations are severely
limited. That may preclude 3db gain antennas.



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