Daniel wrote:
Hello,

Let's say I have a transmitter with output power T dBm using an
antenna with A dBi. Then we have a receiver, with gain B on the
antenna. How does this affect the received signal strength? I guess
the field strength at a point (in the transmitter antenna lobe) would
be something like T+A-PL where PL is the path loss at the point.

Yes, that's correct. But remember that what counts is the
signal-to-noise ratio at the receiver. At VHF and above, the majority of
the noise comes from the receiver, but at HF and below, from the
atmosphere. So at HF and below the receive antenna gain magnifies both
the signal and noise in the same proportion and doesn't help the S/N
ratio. The transmitter antenna gain, however, does, because it magnifies
only the signal and not the noise.

But what does the receiver get when it uses an antenna? Is there a
notion of reciever gain of an antenna?

Yes. It's exactly the same as the gain the same antenna has when used
for transmitting. This principle is called "reciprocity".

In my mind I can't see that
anything else than antenna area would be relevant. I mean, a reciever
antenna shouldn't be able to suck in radio waves from the sides...
Can anyone sort this out?

Sure, it's been sorted out for more than a hundred years. Antennas do
indeed suck radio waves in from the sides, unless you're talking about
antennas with dimensions of many wavelengths on each side. Waves aren't
little tiny things like BBs, but big things that spread out over a large
amount of space and interact with antennas in complex ways. Passing
waves induce currents in an antenna which then creates waves of its own
that interact with the original field. Ordinary intuition doesn't work
well for thinking about this, unless you took physics in high school and
got to play with a ripple tank. Lacking that, spend some time at a
harbor and see how water waves interact with pilings and docks. Of
course, there's always the option of reading some books on antenna theory.

Don't get hung up on an antenna's physical area, unless you're dealing
with antennas that are many wavelengths across, like parabolic
reflectors and horn antennas. An infinitesimally short, lossless dipole
has nearly the same aperture ("effective area" or "capture area") as a
half wavelength dipole. A fair size loop is about the same as a dipole.
Making a dipole's wire diameter several times larger makes no
significant difference to its aperture. Aperture or "capture area" is
simply an alternate way of stating gain -- if you know one you know the
other.

Roy Lewallen, W7EL