Physics question

>
> > > >
> > >
> > Apart from the structural mass involved in increasing the
aperature
> > size, what additional high-tech, low mass components would be
needed
> > to increase aperature size?
> >
> There are a couple of different concepts for antenna design.
>
> Essentially:
>
> 1) A parabolic reflector with a single huge transmitting
element. The
> technical challenge is to keep the accuracy of the parabolic
surface to
> within about a 1/10th wavelength over the entire surface

At the moment I'm holding the opinion that the majority of the
structure of the SPS would be constructed in orbit of lunar/NEO
material, but that the high tech components would have to come from
earth.

Do you foresee any possible way that the antenna you described in
option #1 could be constructed on earth and launched to the SPS? The
inference I make from reading your description is that the orbital
assembly of such a large parabolic surface from discrete components,
would introduce intolerable errors.

Perhaps the transmitting element is launchable?

Could vacuum vapor disporition in orbit provide the accuracy to the
parabolic form? Perhaps an inflatable mold from which the parabolic
shape is lifted?

1/10 the wavelength is pretty steep isn't it? How close are the
tolerances on telescope mirrors?

>
> 2) A flat phased array, with zillions of equally spaced
transmitter elements,
> usually klystron tubes. Again, the main challenge is to keep the
surface
> perfectly flat within 1/10th of a wavelength over the entire
surface.

Could these be assembled as components on earth and the assembled as
a whole in orbit. Perhaps jigs could be used to achieve the
necessary tolerances? Would the vibration of launch ruin the
tolerances of the component assemblies?

>
> As the aperture increases it becomes more difficult to maintain
the required
> surface accuracy.

Understood.

>
> So lots of little SPSes would be easier to build than one
humungous one.

Does the above statement assume one transmitting attenna per SPS?
Could not a SPS incorporate multiple antenna modules, so that the
one SPS can beam to multiple rectennas and perhaps achieve load
balancing on the SPS itself between the different trans. antennas?

In this way, if the SPS is of solar thermal design, could engines be
run in cycles, so that as more power is needed more engines are
brought on line. Other times, engines could be shut down for
servicing or swapping out, without shuttting down all of the power
transmission to the various rectennas?

As you pointed out in a previous post, the beam efficiency varies
depending on the angle of capture at the rectenna. With the limited
amount of "slots" in GEO wouldn't it make sense to make a super-
sized SPS, attach multiple antennas, and beam to multiple rectenna
farms over a land area stretching 10-20 degrees longitude?