Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx> At 07:55 PM 2/10/2007, Tesla list wrote:
Original poster: <davep@xxxxxxxx>
>...rectenna...
I shorthanded, perhaps confusingly.
As noted, the rectenna, itself is quite
conventional. I referred to the downlink source.
>inverse square for a 'well collimated' beam.
Skipping for the moment the engineering detail of how to,
I tend to wonder where the energy goes, in the _specific_
hypotehtical, case.
How does a well collimated beam differ, eg, from a
waveguide guided beam?
(skipping, for discussion purposes, absorption losses in
waveguide case...)
The waveguide is a "guided wave" (I know that sounds like a tautology).. but the key is that the waveguide is a conductor.. there are currents induced in the walls of the waveguide. For that matter, coaxial cable is a waveguide too. And, you can have a guided wave in a material that differs in propagation from the material around it (optical fiber).
The precise mechanism of waveguides depends on the propagation mode (Transverse Electrical, Transverse Magnetic, or some combination of the two), which in turn depends on the relative size of the waveguide and the wavelength (microwave waveguide is <1/2 wavelength across, for the usual mode).
Because the walls are participating in the propagation, there is loss in a waveguide (linear with distance). And of course, the "beam" in the waveguide can't spread (because of the walls).
The well collimated beam would be propagating in free space (so there are no walls or nonuniformities for "many wavelengths"), so it can spread, however little.
Well collimated beams are used at high microwave frequencies (millimeter waves) as well with light. Two cases I know of off the top of my head are the "Beam Waveguide" 34m antennas in the Deep Space Network used for Ka-band at 32 GHz, and the 94GHz quasioptical transmission line in the Cloudsat satellite that was launched a year or so ago. In both cases, the system was designed using optical techniques to eliminate the need for waveguide and it's losses.
Or we may be getting too far OT?
Feel free to drop.
best
dwp