Re: NEC-LIST: E-M demos/simulations/animations

Hello all,

> there still seems to exist the wrong picture, that one electron
> travels from one end of a 1/2-wavelength dipole to the other one
> during 1/2 period. If this would really be the case, then the
> acceleration was independent of the field strength, which would
> contradict a = Q*E/m.

I'm not sure that I understand your comment about the acceleration,
but I don't think that I've ever said that "one electron travels from
end ...." The external E-field seems to be terminated along the wire
by a continuous succession of excess negative or positive charge, but
where the terminating charges don't move far their rest positions.
Our picture of equivalent current and charge needs never to deal with
physical electrons in a real metal; once perfect conductivity is
assumed, infinite mobility follows and these charges response
instantaneously to the external fields.

> Instead, each charge oscillates around its center position. This
> oscillation is nearly synchronous along the antenna (standing wave)
> and the polarization of the whole dipole is the sum of these
> simultaneous displacements. Excitation and radiation of the charges
> contribute to the standing wave which is guided by the wire and
> reflected at the ends of the dipole, where the radiation takes
> place.

I'm sorry to disagree with this picture of oscillating charge once
again. The standing wave on a simple dipole is really comprised of
two counter-propagating waves moving at constant speed. The apparent
oscillation is a result of summing the two waves mathematically. If
the charges actually oscillated all along the wire, then it would seem
that the radiated power would grow in proportion to the length. This
occurs for a spatially uniform current, but not for a sinusoidal
current whose power grows asymptotically as log(kL).

> To avoid confusion, we first should answer the following
> questions before forming suitable thinking models and pictures:
>
> 1. Is it in time-domain (e.g. pulse propagation) or
> in frequency domain (stationary solutions)?

A time-domain solution can often be more easily understood than a
frequency-domain result. A Q/I pulse propagating down a wire in the
time domain moves at essentially light speed, but its amplitude slowly
decays with distance, a phenomenon that I interpret to mean that its
slowly shedding energy. This equivalent phenomenon can be seen in the
frequency domain as a slow decay of a current with increasing distance
from the feedpoint for a long-enough antenna.

> Attached is a figure of the power flow (real part of the complex
> Poynting vector in the stationary case)in the near field of a small
> transmitting patch radiator without excitation (complex
> eigensolutions). It is nearly equivalent to a dipole on a grounded
> dielectric layer. The results have been obtained using a
> finite-difference procedure in space domain (frequency domain).

This figure is interesting and reminds me that when I was at LLNL, in
the mid 1970s, we developed a time-domain movie of the near E and H
fields and the instantaneous Poynting's vector around a half-wave
dipole by letting the time phasor rotate. The results were somewhat
informative, but not very clear concerning what happens along the
dipole between the feedpoint and ends. This was probably because
radiation from along the dipole's length is of low amplitude compared
with that from the feedpoint and ends. Concerning the question of
radiation from along the wire's length, I again refer anyone
interested to my 1999 April and June columns "PCs for AP" in the AP-S
Magazine in which FARS is discussed. Some further results will appear
in the June 2000 column.

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