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From: eder@ssc-vax.UUCP (Dani Eder)
Newsgroups: sci.research
Subject: Re: Microwave power beaming  (and airplanes).
Message-ID: <1281@ssc-vax.UUCP>
Date: Sat, 6-Jun-87 14:00:25 EDT
Article-I.D.: ssc-vax.1281
Posted: Sat Jun  6 14:00:25 1987
Date-Received: Thu, 11-Jun-87 01:03:39 EDT
References: <6693@allegra.UUCP> <1115@oliveb.UUCP> <5893@brl-smoke.ARPA> <1541@uvacs.CS.VIRGINIA.EDU>
Organization: Boeing Aerospace Corp., Seattle WA
Lines: 31
Summary: real hard

In article <1541@uvacs.CS.VIRGINIA.EDU>, mac@uvacs.CS.VIRGINIA.EDU (Alex Colvin) writes:
> It strikes me that the great potential of a solar microwavepower station is
> as a weapon.  Properly refocused, of course.
> I can see why other countries might be leery of having us orbit one.
> And vice versa.

     The problem is how to focus thmicrowaves.  In the canonical SPS
design, the microwave wavelength is 12 centimeters (2.45 GigaHertz)
because (a) it is efficiently transmitted through the atmosphere, and
(b) electric to microwave conversion can be done with high efficiency
at this wavelength.  At higher frequencies (shorter wavelengths), both
of these conditions get worse.

     Now, the resolving power of any optical element is approximately
lambda/D, where lambda is the wavelength and D is the diameter of the
element.  In the case of the SPS, the microwave transmitter is about
1.2 kilometers in diameter.  Thus lambda/D=0.0001 radians.  At the
19000 km altitude of a synchronous satellite, the minimum spot size
on the ground is then 1.9 km in diameter, which makes for a lousy
weapon.

     If you want to make a weapon of an SPS, you want to go to a
much shorter wavelength.  Light, for instance.  Our electric-to-visible
light converters are presently of low efficiency, but there is much
money being spent on improving them (free electron lasers for SDI).
here the wavelength would be 500 nanometers (yellow), and the
aperture diameter would be, say 10 meters.  Then lambda/D would be
5x10E-8, giving a minimum spot size of 1 meter.  This is much more
useful as a weapon.

Dani Eder/Boeing/Advanced Space Transportation/ssc-vax!eder