sci.physics.electromag

"Bill Miller"
>>> "Szczepan Bia³ek" <
>>>>>>
>> You often wrote about antenas. What electrons are like in antenas? Water
>> or gas. (In water only the pressure is changing. in gas also density -
>> imagine what it means)
>
> Actually, in antennas, it seems that electron flow has nothing to do with
> radiation.

If electron gas is oscillating at the ends of the antena appear periodically
huge charges. It must something radiate.

> Instead, the metal seems to "guide" the ready-to-radiate EM energy to
> where it can do so. An excellent example of this is a microwave "horn"
> radiator, fed with waveguide. The "horn" can be formed by slowly expanding
> the dimensions of the waveguide until the Zo of the guide matches that of
> free space (377 ohm). At that point, the microwave energy radiates from
> the open end of the waveguide into free space.
>
> A similar phenomenon exists in non-radiating structures, but that is the
> subject of another -- and MUCH longer discussion. This was understood in
> the 19th century and first (?) espoused by Prof. Poynting in his seminal
> paper that introduced us to what we now call the Poynting Vector. Please
> see:
>
> /~mcdonald/examples/EM/
>
> As Poynting explains, current flow is *always* associated with fields. The
> same cannot be said for fluids.

I agree that it is the subject of another topic.
S*