Hello John,

On 3 mar, 13:30, "J.B. Wood" wrote:
On 03/02/2011 03:56 PM, Wimpie wrote:

If in your opinion there do not exist antennas that generate a
dominant magnetic or electric field (in the near field), then you are
contradicting yourself, as you can't transfer energy with a magnetic
field or electric field only. So your transformer also involves
electric fields. Maybe you should look into the Poynting theorem.

Hello, and that is correct. The Maxwell equations apply in all these
cases. When solving such problems, especially when dealing with
antennas, the total E-M field contains both reactive
(electric/capacitive & magnetic/inductive) and radiative components,
although certain components predominate depending on distance from the
excited structure.

When dealing with A.C. circuit problems where dimensions are a fraction
of a wavelength, one can usually ignore the the radiative/propagation
components. Why solve a problem with a sledgehammer when a small claw
hammer is adequate? Wouldn't you rather use Ohm's law in such case
rather than dealing with E and H fields? For example, the behavior of
A.C. power power distribution lines operating a 60 Hz can certainly be
modeled using transmission line equations but unless they're very long
(implying a propagation delay), a lumped-element/circuit approach is
much more easily dealt with (lumped lines. And yes, I'm intimately
familiar with the Poynting theorem and its derivation. (The designers of
the CFA obviously weren't).

Whoops, we have to be careful to not getting involved in a new
discussion, but I agree on your statement regarding that "special"
antenna and the statements regarding whether or not to use distributed
versus lumped circuit approach.


When a noise source is about 5..10m away from an 3.6 MHz antenna, the
coupling of that noise source towards a "magnetic" loop antenna may be
different from the coupling towards an "electric" antenna, though
both antennas may produce the same far field radiation. This is not
from a textbook, but from experience (I am also working in power
electronics).

There's no such thing as "magnetic" and "electric" antennas.

That is why I added the word "dominant", as you can't transfer energy
with H or E only and we were discussing small antennas as shown on
Norberts website.

One may imagine the electrically small magnetic loop antenna as the
primary of a transformer where there is no secondary coil in the
reactive field. The RF leakage (far field radiation) by accident hit
the antenna of another amateur.

The
marketing departments of antenna vendors or others can call these
whatever they want but they can't change the laws of physics. Now, if
one dimensions a loop antenna or dipole antenna small enough (compared
to a wavelength, one obtains a magnetic or electric dipole,
respectively. Such dipoles (note the absence of the word "antenna") are
a theoretical concept but can be applied in practice to those structures
having electrically small radiators/interceptors.

I fully agree with you on the far field statements, but when you live
in an apartment (where significant spurious emission from home
equipment are in the near field of your 3.6 MHz antenna), a so-called
magnetic loop antenna may behave different (w.r.t. a short "electric"
dipole). It can be worse or better. Many radio amateurs know this from
experiments, without knowing the EM theory behind it.

Hey, I'm a fellow Ham and well aware of the contributions over the years
by hams to antenna design. Many times, however, established
electromagnetic theory is distorted to match the perceived observation.

I agree on the above. Let I mention the two letters "EH" in addition
to your three-letter combination to avoid a new discussion...

In the case of noise immunity I would discuss the size of the victim
antenna, the antenna type (e.g. loop or dipole), antenna dimensions,
orientation and proximity wrt the offending noise source, and whether
the victim antenna is shielded and balanced. Unless one is referring to
an electrically small antenna treated as a magnetic or electric dipole,
typing an antenna as "magnetic" or "electric" is meaningless.


With kind regards,

Wim
PA3DJS
www.tetech.nl