Toni wrote:
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.

SNIP

Zo = SQRT[u/e]

where u = permeability of free space
where e = permittivity of free space

Toni wrote:
"Then I read about mag-loops having better RX noise immunity than normal
antennas because they react to magnetic fields, whilst man-made noise
occurs mostly on electric fields."

A small loop requires high capacitance for resonance. Thus it has a
narrow-band response and this provides rejection of off-resonance
interference in addition to directional rejection of energy which is not
in the plane of the loop.

Read about small loop antennas (mag-loops) on page 7-24 of ON4UN`s
"Low-Band DXing".

Best regards, Richard Harrison, KB5WZI

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

Hi,

I have always been taught that electrical and magnetic RF fields
went always together. You couldn't produce one without the other
and vice-versa.

Then I read about mag-loops having better RX noise immunity than
normal antennas because they react to magnetic fields, whilst
man-made noise occurs mostly on electrical fields.

??????

If both fields go together, how can noise be greater in the
electrical field than in the magnetic one? Does that mean that
you can effectively produce one without the other?

Taking it further, if you can _receive_ one while rejecting the
other, then, by the reciprocity law, you should be able to
produce one without the other.

What is (where am I) wrong?

EA3FYA - Toni

Hi Toni,

The Magnetic field near a magnetic loop is stronger than the Electric field.
A few meters away from the loop the magnetic field and electric field
produced by the antenna get more equal. The antenna is less sensitive to
pick up man made noise that is mostly produced in the electric field and
often comes from the building close to your antenna. But also noise from
industrial areas and other sources can cause trouble.

An other thing is the high selectivity of the magnetic loop. It acts as a
filter and
suppresses unwanted signals dramatically. I have quite some information and
pictures on my homepage about magnetic loops in practice and some technical
explanations about how they work.

I have several loops working here.

73, Norbert (PA7NR)
http://www.qsl.net/pa7nr/index.html

En Norbert va escriure en Thu, 18 Mar 2004 15:40:56 +0100:

I have several loops working here.

73, Norbert (PA7NR)
http://www.qsl.net/pa7nr/index.html

Very interesting, thanks.

Would you know if I3VHF has a web page? I have tried googling for
it with no luck.

73,
EA3FYA - Toni

I have several loops working here.

73, Norbert (PA7NR)
http://www.qsl.net/pa7nr/index.html

Very interesting, thanks.

Would you know if I3VHF has a web page? I have tried googling for
it with no luck.

73,
EA3FYA - Toni

Toni,

The internetsite of I3VHF is:
http://script.dan.it/mazzoni/

One of the resellers is the Wimo company in Germany:
http://www.wimo.de

check for HF antennas, magnetic loops. They have an interesting page about
this antenna.

73, Norbert (PA7NR)
http://www.qsl.net/pa7nr/index.html