Your summary seems reasonable, though I'd add one more thing. I
didn't mention the polarization in my first post, to keep things
simple. And I didn't mention that you can discriminate also based on
direction of propagation. So a highly directional Yagi or parabolic
reflector antenna or such can greatly enhance the signal/noise ratio,
if the noise is not coming all from the same direction as the signal.
And if the noise is coming from only one direction, it's possible to
put that direction in the null of a simple antenna with a dipole or a
cardiod pattern, such as an electrical dipole or a loop (magnetic
dipole).

With regard to 377 ohms, it falls nicely out of the physical constants
for freespace and Maxwell's equations. If you were embedded in a
large block of polyethylene, however, you would see a different ratio,
just as the impedance of a coaxial line changes if you change its
dielectric from air to polyethylene.

About polarization: that works well for signals in freespace. But
when you are close to a perfectly conducting plane, the electric field
is constrained to be perpendicular to the plane at the surface of the
plane. Though it's not a perfect conductor, the surface of the earth
tends to keep the electric field vertical in the region where we'd
generally put our medium-wave and long-wave antennas...that is, within
a fraction of a wavelength of the earth. That's why it's important
for a loop antenna to have symmetry about a vertical plane: the
so-called "shielded loop" should have its gap at the top (or the
bottom, but it's usually easier to have it at the top). This is all
explained nicely in King, Mimno and Wing, "Transmission Lines,
Antennas and Waveguides." The loop antenna explanation in Johnson and
Jasik is also quite reasonable. (There's more to be said about EM
wave polarization and the earth, and I suppose others will pipe up and
say much of it...)

Cheers,
Tom

Toni wrote in message . ..
Ok, so trying to summarize your answers, it seems that

- Electric and Magnetic fields, effectively, go together, but
their ratio may vary, especially at the antenna's near field.

- Cecil Moore says in free space this ratio is the 377 ohms
characteristic impedance. I don't know how to mathematically
check this, but seems reasonable to me.

- It is clear that polarization is an important factor in
rejecting noise. I never doubted that.

So, apparently, it would be clear that

- a reception system capable of discriminating electric/magnetic
fields should be able to reduce unwanted noise from near sources

and

-adding the capability to discriminate different polarizations
would also help with noise from both near and far sources.

Correct?

73,
EA3FYA - Toni