Optimum Microwave Frequencies

This article http://monolith.caltech.edu/Papers/ParkinLauncher.pdf
talks about using microwaves to power a hydrogen fueled launch rocket
(see discusion in space.tech).

It talks about using microwaves in the 140 - 245 GHz range.

SSPS concepts usually talk about 2.45 GHz? Should we be looking at
much higher frequencies to make the rectanna much smaller? What are
the imoplications for the power reception? Smaller, more dense
rectanna? Higher beam intensity? Would this only work for high
altitude, dry climate rectannas?

Also, what is the highest frequency at which microwaves be efficiently
produced and captured? I was thinking for beaming from a powersat to a
lunar base, without the inefficiencies of lasers, whilst trying to
keeop the rectenna small enough?

Alex Terrell wrote:
This article http://monolith.caltech.edu/Papers/ParkinLauncher.pdf
talks about using microwaves to power a hydrogen fueled launch rocket
(see discusion in space.tech).

It talks about using microwaves in the 140 - 245 GHz range.

SSPS concepts usually talk about 2.45 GHz? Should we be looking at
much higher frequencies to make the rectanna much smaller? What are
the imoplications for the power reception? Smaller, more dense
rectanna? Higher beam intensity? Would this only work for high
altitude, dry climate rectannas?


For a launcher, you really, really care about reciever area.
As drag goes with area, an area goes with the inverse of frequency
squared (assuming transmitter antenna is same size) then high frequency
is good.

Also, what is the highest frequency at which microwaves be efficiently
produced and captured? I was thinking for beaming from a powersat to a
lunar base, without the inefficiencies of lasers, whilst trying to
keeop the rectenna small enough?

For launch vehicles, you really, really care about wavelength.
For a given size of transmission antenna, at a given distance, the
area of receiving antenna varies as the inverse square of wavelength.
And as drag is proportional to area, you want to keep this small.

I see a reference on the web to 35Ghz 60% rectennas actually fabricated.
Let's say that 100Ghz at 60% is possible in the near term.

How large a transmitter would be needed to hit the moon?
Let's say we want 10MW on the moon.
Cooling is a problem, so say we don't want more than 1Kw/m^2 of waste
heat.
This means around 3Kw/m^2.
Or 3000m^2.
Call it 60m diameter.

The moon is 4*10^8m away, so the angle the array covers as seen from
earth is 1.5*10^-7 radians.

Invert this to 6*10^6 waves, times the 1.22 fudge factor = 7.3*10^6,
times the wavelength (3cm) = a dish diameter of 2.1*10^5m, or 200Km.

Even upping this to 300Ghz, the dish still needs to be 66Km.
If a 'reasonable' dish (phased array) might be 5Km across, then you need
to raise the size of the reciever to around a kilometer to get most of
the beam.

Magnetrons are used to generate long microwaves. Low frequency
magnetrons are more efficient than high frequency magnetrons.
Microwave ovens have magnetrons which operate at a frequency
of 2.45 GHz and have efficiency of about 70%. Magnetrons
operating at 915 MHz frequency have efficiency of about 85%.
Magnetrons cost about $0.1/W.

Gyrotrons can produce short microwaves ( 3 mm) which are easy
to focus into a narrow beam, but their efficiency is low
(15%-60%). The maximum frequency is about 170 GHz. Gyrotrons
cost about $1/W. The best gyrotrons are made by a Russian
company named Gycom.

Heating one kilogram of hydrogen from nearly 0 K to 1200 K
consumes 17 MJ of beam energy. If the beam has the power of
1 MW and the rocket absorbs all its power, the maximum
payload size is about 1 kilogram.

A system that launches 1-ton payloads would cost about
$10 billion. The environmental impact of the hydrogen
propellant on the ozone layer may be severe.

Atmospheric transmission of microwaves:
http://www.submm.caltech.edu/cso/weather/atplot.shtml

The best CATS web site: http://www.islandone.org/LEOBiblio/

Ian Stirling wrote in message ...
How large a transmitter would be needed to hit the moon?
Let's say we want 10MW on the moon.
Cooling is a problem, so say we don't want more than 1Kw/m^2 of waste
heat.
This means around 3Kw/m^2.
Or 3000m^2.
Call it 60m diameter.

The moon is 4*10^8m away, so the angle the array covers as seen from
earth is 1.5*10^-7 radians.

Invert this to 6*10^6 waves, times the 1.22 fudge factor = 7.3*10^6,
times the wavelength (3cm) = a dish diameter of 2.1*10^5m, or 200Km.

Even upping this to 300Ghz, the dish still needs to be 66Km.
If a 'reasonable' dish (phased array) might be 5Km across, then you need
to raise the size of the reciever to around a kilometer to get most of
the beam.

Thank you Ian - I was thinking from a L1 powersat to Moon (4 E7m
distance). If we want Earth to Moon (or L4/L5 to moon), then Lasers
would be the way to go.

Scaling the distance by a 10th, using 300GHz, dish needs to be 6.6km
across, with a 60m diameter lunar rectenna.

Optimum size assuming equal construction costs would be if dish and
rectenna were both 630m diameter. This solution would work up to power
transmission of 1GW. However, the rectenna size is larger than I'd
hoped for, especially if the base is at high latitude.

How expensive/complex/massive are the rectenna, and the dish? Are they
something that could be made of lunar materials?

Alex Terrell wrote:
Ian Stirling wrote in message ...
How large a transmitter would be needed to hit the moon?
Let's say we want 10MW on the moon.
Cooling is a problem, so say we don't want more than 1Kw/m^2 of waste
heat.
This means around 3Kw/m^2.
Or 3000m^2.
Call it 60m diameter.

The moon is 4*10^8m away, so the angle the array covers as seen from
earth is 1.5*10^-7 radians.

Invert this to 6*10^6 waves, times the 1.22 fudge factor = 7.3*10^6,
times the wavelength (3cm) = a dish diameter of 2.1*10^5m, or 200Km.

Even upping this to 300Ghz, the dish still needs to be 66Km.
If a 'reasonable' dish (phased array) might be 5Km across, then you need
to raise the size of the reciever to around a kilometer to get most of
the beam.

Thank you Ian - I was thinking from a L1 powersat to Moon (4 E7m
distance). If we want Earth to Moon (or L4/L5 to moon), then Lasers
would be the way to go.

Scaling the distance by a 10th, using 300GHz, dish needs to be 6.6km
across, with a 60m diameter lunar rectenna.

Err, no.
The initial figure was assuming 100Ghz, at 300Ghz it's still 66Km.

Optimum size assuming equal construction costs would be if dish and
rectenna were both 630m diameter. This solution would work up to power
transmission of 1GW. However, the rectenna size is larger than I'd
hoped for, especially if the base is at high latitude.

How expensive/complex/massive are the rectenna, and the dish? Are they
something that could be made of lunar materials?

The dish can be pretty light.
1mm aluminium would be just fine.
I don't know about the rectenna.

Ian Stirling wrote in message ...


Scaling the distance by a 10th, using 300GHz, dish needs to be 6.6km
across, with a 60m diameter lunar rectenna.

Err, no.
The initial figure was assuming 100Ghz, at 300Ghz it's still 66Km.

Sorry - I meant scaling by 10 because Moon - L1 distance is about
40,000 km, and Moon to Earth is about 400,000 km.

Optimum size assuming equal construction costs would be if dish and
rectenna were both 630m diameter. This solution would work up to power
transmission of 1GW. However, the rectenna size is larger than I'd
hoped for, especially if the base is at high latitude.

How expensive/complex/massive are the rectenna, and the dish? Are they
something that could be made of lunar materials?

The dish can be pretty light.
1mm aluminium would be just fine.
I don't know about the rectenna.

Alex Terrell wrote:
Ian Stirling wrote in message ...


Scaling the distance by a 10th, using 300GHz, dish needs to be 6.6km
across, with a 60m diameter lunar rectenna.

Err, no.
The initial figure was assuming 100Ghz, at 300Ghz it's still 66Km.

Sorry - I meant scaling by 10 because Moon - L1 distance is about
40,000 km, and Moon to Earth is about 400,000 km.

Sigh.
Sorry, got hung up on the whole 'moon' thing, and diddn't read L1
properly.