On Mar 10, 5:03*pm, b wrote:

By antennas with gain, are you referring to the physical structure or
to some kind of amplification circuit?

It's the antenna shape and all the nearby grounded or reflective
elements that creates 'antenna gain'. A metal-sheet reflector, for
instance, one half wavelength parallel to your whip antenna,
can give 3 dB of gain. This is the result of directing the antenna
output power, not of increasing it, but the effect at the receiver
is just like a higher power output.