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). What advantage would it have over a real antenna?
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.

Roy Lewallen, W7EL

There seems to be a number of commercial antennas
described as H-field antennas intended for LORAN
application. Most claim improved immunity to
precipitation static. Is there a theoretical basis
for such claims?

Thanks.

Chuck

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