Plasma Antenna For Beaming Microwave Power

[richalex]

Space-based solar power transmitters that transmit power to Earth using microwaves need antennas that are about a mile in diameter to be efficient. This is related to the wavelength of microwaves. Such a large antenna is not practical, so no one plans to build these sort of devices (the work is looking into the use of lasers as an alternative, as lasers are much smaller and lighter transmitters). <br /><br />Plasma antennas may be formed when gas is turned into a plasma, and a signal is applied to it. The plasma is electrically conductive, and functions as a conventional metal antenna. It might be possible to use a plasma antenna to project the microwave beam of a space-based solar power transmitter. Perhaps it would be practical to make a plasma antenna that is large enough to be efficient. But, there is the possibility that the gas would contaminate areas of space if it were not contained. <br /><br />It might be possible to contain the plasma antenna in a magnetic field. Such fields on a smaller scale are under research for space-based applications, such as the collection and storage of antimatter and for radiation shielding of space craft. <br /><br />Perhaps a rotating magnetic field could be stretched out by the plasma it contains, until it is large enough to serve as an efficient microwave power antenna. Such a structure would be impossible on Earth, but might work in space.

 

[billslugg]

The basic problem with the concept of a plasma antenna in space is a fight between small and large.<br /><br />The antenna wants to be large in the x/y directions with small surface errors in the z direction. A cloud of plasma wants to be large in ALL directions. Bringing the z direction down to smallness with a magnetic field is difficult. Gas pressures tend to be large and magnetic field pressures tend to be small. Any device capable of generating a magnetic field strong enough to contain a plasma would be heavier than the equivalent metal antenna. <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>

 

[eniac]

Instead of a plasma, how about a cloud of a million microsatellites, each with a solar panel and a microwave transmitter. They would cooperate as a giant irregular phased array and convert sunlight directly into a sharply focussed microwave beam. No large structures needed, and a proof of principle would be quite cheap to launch.<br /> <div class="Discussion_UserSignature"> </div>

 

[billslugg]

Yes, a wonderful idea! You have to figure out how to do it without requiring attitude control or position control. You just throw 'em up there in about the right spot, and they know exactly where they are, they get their light make it into electricity and they beam it where required. <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>

 

[eniac]

Since these would be flying in very close formation (no more than meters apart) you could perhaps do attitude and position control using near field electromagnetic or electrostatic forces. You'll need some navigation, or else the swarm will disperse quickly. Propellant could also be an option, as not much will be needed, and empty satellites could be simply replaced.<br /><br /> <div class="Discussion_UserSignature"> </div>

 

[MeteorWayne]

I would suspect any such EM or ES control would interfere with the antenna function. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p>I would suspect any such EM or ES control would interfere with the antenna function.<p><hr /></p></p></blockquote><br />Since we are talking static fields for attitude control, there will be no RF interference. If using a coil to generate a magnetic field, it must be subdivided into little pieces using low-pass coils to make it transparent to microwaves. Electrostatic electrodes can be small to begin with.<br /><br />The solar panels presumably also need to be made from small cells separated by low-pass coils for microwave transparency. I do not know enough about microwaves to judge how hard it will be to keep the microsatellites from disrupting the very beam they are working to generate, but my gut says it should be possible.<br /><br /> <div class="Discussion_UserSignature"> </div>

 

[eniac]

<blockquote><font class="small">In reply to:</font><hr /><p><br />Yes, a wonderful idea! You have to figure out how to do it without requiring attitude control or position control. You just throw 'em up there in about the right spot, and they know exactly where they are, they get their light make it into electricity and they beam it where required.<p><hr /></p></p></blockquote><br />It occurred to me that for beaming purposes, the microsats would not even have to know anything. They could have a receiver for a half-frequency pilot beam sent up from the location of the receiving rectenna, and would simply phase-lock their transmitters to that signal. This would eliminate any complexity associated with beam steering, and leave on-board computers free to worry about stationkeeping. It would also make the system inherently safe from wandering beams, as beams can not form except towards a pilot beacon.<br /><br />Of course, such a system could lead to a black market in "heat your home for free!" devices... ;-)<br /> <div class="Discussion_UserSignature"> </div>