Re: NEC-LIST: Low angle radiation?

Hi Roland,

> . . . As energy is
> continually being introduced into this standing wave, it exits the
> antenna in the form of a radiated EM field. If the energy of the EM
> field exciteing this antenna were of a nonresonant frequency, the
> standing wave would not be set up on the dipole elements. Since the
> dipole elements are located in space, it's fields in space begin to
> propagate since the unbound energy can't remain around the elements: a
> time varying magnetic field will generate a time varying electric
> field and visa versa. The entire length of the dipole is used to
> create the propagating EM fields.

Certainly, if there is a net power provided to some structure from a
connected transmission line, that power either has to be radiated or
lost in some resistance. Assuming there is no resistive loss, i.e.,
the object is a perfect electrical conductor, the antenna must somehow
act as a "transformer" between the TL and the external medium. If,
however, the structure were to be located within a PEC envelope,
obviously there is no radiation and the time average power from the TL
goes to zero. This would be true regardless of how big that envelope
is; after equilibrium is established, no net power is provided to that
overall system by the TL. Thus, the external medium can have a
profound effect on the behavior of that transformer.

Suppose instead of time-harmonic excitation, the same object is
impulsively excited, say by a Gaussian voltage in time, and for
simplicity we make the TL of zero length to move the excitation
directly to its surface. During the time the exciting voltage is
non-zero, energy is being provided to the object, some of which can
immediately radiate and some of which will be contained in the near
fields. Normally, all of the energy provided to the object will not be
radiated away by the time the exciting voltage decays back to zero, so
there will still be non-zero near fields. But, since we've assumed
the object to be a PEC, the Poynting's vector normal to its surface is
everywhere zero. This being the case, how can it radiate away any
further energy? Well, I have a sort of hand-waving explanation for
that (for futher discussion see an article in the November 1980 IEEE
Proceedings by Miller and Landt on time-domain modeling).

My model of the situation is that the "sloshing" around of the near
fields over time converts some of that stored energy to radiated
energy. The outward flow of radiated field, or a non-zero normal
Poynting's vector, is balanced by an inward normal Poynting's vector,
or flow of stored energy from the near fields. Thus, the object can
eventually radiate away all of that stored energy even though there
can be no net flow normal to its PEC surface.

I'd be interested in hearing any responses to this model.

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