Roy Lewallen wrote:

Bill Ogden wrote:
OK, let me display my ignorance once again.

There are many construction articles about ferrite-core antennas for the low
bands. (Not to mention all the ferrite-core antennas in AM receivers.) Are
these not H-field antennas, to a large extent?

Only very locally, and only to a limited extent.

When a signal originates far from an antenna, the response to E and H
fields is in the ratio of about 377 ohms, the impedance of free space.
This is true for *all antennas*. In other words, all antennas have the
same relative E and H response to signals originating far away.

Very close to a small loop antenna, response is greater to an H field
than E field. It does respond to both, however, as all antennas must. As
you get farther away from the antenna, the response to the H field
decreases in relation to the E field response. At around an eighth
wavelength distance from the antenna, the response to E and H fields are
about the same as for a distant source. Beyond about an eighth
wavelength, the response to the H field is actually *less* than the
response to an E field compared to a source at a great distance. The
ratio of E to H field responses then decreases to the distant value as
you get farther from the antenna.

In summary, the antenna responds more strongly to the H field if the
source is within about an eighth of a wavelength from the antenna.
Beyond that, it actually responds more strongly to the E field relative
to the H field than a short dipole or many other antennas -- you could
more properly call it an "E-field antenna" in its response to signals
beyond about an eighth wavelength. The difference in relative E and H
field response among all antennas becomes negligible at great distances;
for antennas which are small in terms of wavelength, the difference
becomes negligible beyond about a wavelength.

Now, suppose you could make a magic antenna which would respond only to
the H field of a signal originating at any distance from the antenna
(which is impossible).

"A system for determining the modulation imposed on
a curl-free magnetic vector potential field.":
http://jnaudin.free.fr/html/tepvppl.htm

Other 'magic' antennae:
http://rugth30.phys.rug.nl/quantummechanics/ab.htm

Robust OP AMP Realization Of Chua's Circuit:
http://citeseer.ist.psu.edu/kennedy92robust.html

What advantage would it have over a real antenna?

I read the main reason was less electrostatic interference
but with less immunity to strong nearby stations. Does
the magnetic field really have less noise than the E-field?

Polarization is also an interesting component.

Remember that the E/H ratio of any signal originating very far away is
377 ohms, regardless of what kind of antenna or source it came from.

I seem to recall this had something to do with the
speed of light not being infinite.


Roy Lewallen, W7EL