[AR] Re: space based solar
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Wed, 20 Mar 2019 23:14:05 -0400 (EDT)
On Wed, 20 Mar 2019, Norman Yarvin wrote:
...the solar array and the transmitter array typically want to point in
different directions...
The pointing direction is much less important for the microwave output
than for the solar input, though, at least if you're steering the
microwaves via phased-array.
Unfortunately, phased arrays are not magic, especially if you want them to
produce needle-sharp beams -- which this application demands, since the
rectenna is circa 40,000km away and only a few km across, and essentially
all the beam power has to go into it. With a really big transmitter array
and good phased-array control, that's practical over a modest range of
angles. Alas, for the GSO case, where the power beam is more or less in
the plane of the Sun's motion around the sky, the angle between sunlight
and power beam has to sweep around a full circle, essentially 360deg per
day. I don't think there's any way to embed a transmitter array which can
do *that* in the flat surface of the solar array.
(Ian Cash's latest design concept, which Keith mentioned, can do it. But
it's a twisted three-dimensional shape, not a flat plane, and I think it
accepts a certain amount of inefficiency for its simplicity.)
It's routine in radios to have a very directional beam produced by an
antenna that has a number of radiating elements straight in line with
the direction the beam is to travel, the Yagi antenna being a common
example. I'm not sure what the downsides are, numerically, for emitting
edge-on from a planar array of microwave transmitters...
My understanding is that the beam-forming performance deteriorates rather
badly as the beam direction approaches the plane of the surface, which is
why phased-array radars usually have multiple arrays when really wide
angular coverage is needed.
Henry
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