Hi,

Firstly, I am not familiar with Part 15 and any comments that follow
do not take into account system losses or inefficiencies.

It is generally NOT possible to increase the power output of a
transmitter just by changing the antenna connected to it. A transmitter
of 'X' watts connected to antenna 'Y' would produce exactly the same
radiated power if it were connected to a completely different antenna
'Z' instead. The exception to this is if changing the antenna causes the
transmitter to generate more power because of changes in its operating
conditions such as in the impedance match presented to it.

However every antenna, other than an isotropic radiator (a
hypothetical device that radiates equally in all directions), will have
gain in some direction at the expense of that in others. The product of
the gain and transmitter power is called the EiRP (equivalent isotropic
radiated power) and this is what is generally quoted as the limiting
power allowed.

So, if a particular antenna is stated as having a gain of 2 (3dB)
over an isotropic radiator then, along the axis of the antenna that has
the maximum gain, it will appear that the transmitter power has been
increased by 3dB (twice the power output) although the transmitter
itself has not in fact produced any more. This 'extra', of course, comes
from not radiating power in all the other directions.


Cheers - Joe