[AR] Re: space based solar
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Mon, 18 Mar 2019 23:56:20 -0400 (EDT)
On Tue, 19 Mar 2019, Jordan Trewitt wrote:
efficiencies can easily go above 80% with class E and class F GaN amplifiers
... that's still going to have to radiate more than 100 kW away from the
spacecraft... at a 90% efficiency case and 250kW for 80% efficiency...
One advantage of the microwave sandwich concept (solar array = transmitter
array) is that the transmitter heat generation is distributed over the
whole array. Even very good solar cells put out 2-3x as much waste heat
as electric power, so the further loss of a small fraction of the electric
power does not add much more. Keeping the solar cells cool is definitely
an important design consideration, but this way that deals with all the
thermal problems -- there is no separate issue of transmitter cooling.
Which is just as well, because one disadvantage of microwaves is that the
transmitter array size tends to be set by distance and requirements for
beam sharpness, and doesn't scale down -- a megawatt is much too small for
a useful (high-orbit) microwave powersat, because it needs to be *big*.
Regarding the rectennas, you can power divide the antenna and have each
antenna element have a lower power amplifier on it (might have to watch to
phase closely for interference patterns).
The rectenna is the ground receiver, actually (rectenna = rectifying
antenna). Yes, a distributed transmitter antenna makes all kinds of
sense, not least because it lets you build the antenna as a phased array,
ideally controlled by a pilot beam from the ground. Really big antennas
are inevitably flexible structures anyway, so active phase control is
almost a necessity, and basing it on a pilot beam from the rectenna
automatically gets the pointing right and has the huge political advantage
that the beam cannot focus on a noncooperative target.
The biggest issue that I see that could happen with a really high power
microwave satellite is multipaction...
I'm not a microwave guy, but my understanding is that multipaction
problems scale with RF field strength, again favoring a distributed
transmit antenna which spreads the RF generation out over a large area.
Henry
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