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

On 6 Mar, 20:31, Roy Lewallen wrote:
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".

Ok, at least that makes the definition of receiver gain clear!

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.

I *have* done one experiment as a kid: I placed a dipole permanent
magnet under a piece of paper and poured iron filings on the paper to
see the pattern created when the filings arranged itself in the
magnetic field. So I kinda have some understanding how field phenomena
can behave. I just didn't connect it to antennas. Is it more correct
to say that a field is established between the two antennas rather
than saying that something is transmitted in a direction from one end
to the other? (Of course on another level, *information* is
transmitted in a direction, but I am referring to the level of the
electromagnetic field).


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.

Ok, good to know!

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.

As far as I can understand, the notion of "effective area" is quite
artificial and only meaningful if one wants to fit a complex wave
phenomena into a form where one can think of it in the naive way I
have expressed in my earlier posts... Anyways, thanks for a great
reply!

Regards,
Daniel

Daniel wrote:
. . .
I *have* done one experiment as a kid: I placed a dipole permanent
magnet under a piece of paper and poured iron filings on the paper to
see the pattern created when the filings arranged itself in the
magnetic field. So I kinda have some understanding how field phenomena
can behave. I just didn't connect it to antennas.

Your magnet experiment showed you one thing about a *static* field.
Time-varying fields behave much differently, so it's a mistake to think
that you now understand radio waves.

Is it more correct
to say that a field is established between the two antennas rather
than saying that something is transmitted in a direction from one end
to the other? (Of course on another level, *information* is
transmitted in a direction, but I am referring to the level of the
electromagnetic field).

No, it's not. One antenna creates a field whether or not the other
antenna is there. The field propagates at the speed of light away from
the transmit antenna.

. . .

As far as I can understand, the notion of "effective area" is quite
artificial and only meaningful if one wants to fit a complex wave
phenomena into a form where one can think of it in the naive way I
have expressed in my earlier posts... Anyways, thanks for a great
reply!

Yes and no. "Effective area" or "effective aperture" describes the cross
section of the impinging field which contains the amount of energy the
antenna captures and delivers to its load. It's most useful in the
analysis of antennas that are very large in terms of wavelength, such as
parabolic reflectors and horn antennas. For those, the effective
aperture is on the order of, and closely related to, the physical area.
When dealing with simpler and smaller antennas, though, there isn't any
direct relationship between effective aperture and physical size, which
leads to a lot of misunderstanding.

Roy Lewallen, W7EL