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Re: How many joules --> e.m. waves?



On 22 and 23 Apr 1997 Bob Sciamanda wrote:

The E field has a null along the normal to the plane of the coil.
The radiation E fields of both magnetic and electric dipoles go as the
sine of the angle measured from the direction of the dipole moment
vector. The two kinds of dipoles have identical fields, except for an
interchange of E and H fields, and one change of sign.

True. The reason for my surprise was that for an electric dipole it means
the maximum intensity in any direction perpendicular to wires while for
the magnetic dipole it means the maximum in the plane containing the wire
loop. I knew that a common roof TV antenna must be positioned to be "open-
armed" toward the station and I inferred from this (incorrectly) that a
circular antenna would have to be oriented with the axis toward the source.
A misconception "by analogy", if you wish.

Thanks Bob for helping me (us?) to learn about the radiation resistance and
about the directional patterns of antennas. And to Brian for reminding that
the f spectrum of a short burst of waves is broad, even when a high Q
circuit is resposible for the oscillation of charges.

A radio astronomical connection was interestig but I still doubt that a
small fraction of 84 pJ, probably much less than 1%, can be detected above
the noise level of a common radio set. The mutual induction effect (via M
between the wire loop and the radio set antenna) is likely to be the main
mechanism by which we hear clicks while discharging capacitors. But this
remains to be demonstrated.
Ludwik Kowalski