NEC-LIST: Radiation

Hi David,

> Hold on. Aren't the Q and I out of phase? The Q peaks at the end as
> the I goes through a zero. There is transport of charge such that
> the ends of the dipole are alternatively +/-q and -/+q. Thus
> radiation results. Two charged pith balls (one +q and the other
> -q), stuck them to the ends of an insulating stick and twirled it
> around you'd get radiation too (it would have higher order
> multipoles than just dipole though). No standing wave needed.

>
> -David Fluckiger
>

In the time domain, where it might be easier to visualize, a wire
excited impulsively at its center, produces two counter-proagating Q/I
pulses, one going towards each end of the wire. The current pulses
are of the same sign, and the charge pulses are of opposite sign, with
the quantiative relationship between them being I = Qxv (Q is the
linear charge density), with v being the charge speed. Away from the
feedpoint and ends v is given appoximately by ąc. Of course, at
the ends, the charge piles up and the current goes to zero, i.e.,
their motion is no longer in phase. It's this direction reversal of
the charge motion at the ends of the wire that is a principle cause of
radiation due to the charge acceleration.

The pith-ball example can be conducted using only one and whirling the
charged ball around in a circle by a string attached to a center
point. The radiation that's produced is known as synchotron
radiation, although to be at all significant I believe that the speed
must be nearly c.

A very simple picture of radiation can be visualized as being caused
by charges being moved in such a way as to cause their E-field lines
to "wiggle." This is much like the transverse wave that's produced on
a string by moving one end up and down. A semi-quantitative model for
radiation can be based on the finite propagation speed of EM fields
(waves) and the continuity of E-field lines. A graphical construct of
the fields around a point charge based on these two observations leads
to what I think is called the "field-kink" model of radiation (I
haven't been able to find that term in any book, but I do think I read
it somewhere). I first became aware of this simple idea from a
computer program for the Mac computer developed by a physics professor
at Stanford University, Blas Cabrera. I diskette containing this and
other field programs is still available for the Mac. The radiation
module is especially in that various kinds of charge motion can be
selected and the program produces a real-time "movie" of the field
lines that very nicely shows the outward propagation of transverse
field lines and the radiation of energy. But it seems difficult to
connect this simple idea with a solution produced by a computer model
like NEC.

Best wishes,

Ed

--
Dr. Edmund K. Miller
3225 Calle Celestial
Santa Fe, NM 87501-9613
505-820-7371 (Voice & FAX)
e.miller_at_ieee.org