I actually think that this idea (10Billion small transmitters in a phased
array) is pretty cool !

The technical challenges of keeping so many transmitters in phase for the
super-narrow
beams we want is obviously a technical challenge of the first order. But
technical challenges
can be solved in time, with intelligence. And micro-transmitters will reduce
in cost with
Moore's law (OK maybe a little bit slower, but any way).

One thing will remain unchanged : You need a lot of real-estate for
micro-wave beaming !
I did not do the calculations, but the size of your array is probably close
to a small continent....
And you would need to build a small (200MW) power plant to provide the
power, because it will be
in a very remote area. With all respect, I'm not sure if any civilisation
will want to afford that.

But here is an idea : If we were to spread these billions of small
transmitters across
the planet, would be theoretically be able to create a planet-sized phased
array transmitter ?
If the technical difficulties are overcome, would it essentially be possible
that anyone
who would like to join in beaconing can buy and connect a small
transmitter/antenna in his/her/it back-yard ?

Just like all of our SETI@homers now analyze signals received, we could have
a TETI@home
(Transmit for ETI) program where you hook your transmitter to internet, and
join in the global transmission !
Might even use less energy than SETI@home uses now...

Are there any theoretical limitations to that idea ?

Rob

"Louis Scheffer" <lou@cadence.com> wrote in message
news:40aa456b$1@news.cadence.com...

"Rob Dekker" <rob@verific.com> writes:

"Louis Scheffer" <lou@cadence.com> wrote in message
news:4095e404$1@news.cadence.com...

Andrew Nowicki <andrew@nospam.com> writes:

[...]  As a first guess, cover the planetary zones
of the nearest million stars or so, with beams that are on
all the time  [Think of the beams as looking like a pincushion].

This is something we can do with existing technology for about
$200M, for a beam bright enough that we ourselves can detect it.
And this cost will come down further as a consequence of Moore's
law.

    Lou Scheffer

How did you get to just $200M for a system which cover 1M stars;
semi-continuously ?

Here's how I got that number.  Assume you build a large phased array

consisting of

many small (10mw) transmitters.  To be detectable by ourselves, you need

about a

10^12 watt EIRP.  1M of these implies a total EIRP of 10^18 watts (it

turns out with

phased arrays the total EIRP can be split up with almost no penalty).

This means

you need 10^10, or 10B transmitters.  We can build today a 10mw

transmitter plus

antenna for about $0.20 (actually we can build a chip with 100 of these +

a PC board

with 100 antennas for about $20, but for our purposes this is the same.)

This gives a total cost of $2B for the transmitters.  However, you are

ordering

100 million identical PC boards and chips, so you should get a substantial

volume

discount.  How much?  I'm guessing a factor of 10 since it would be

reasonable to

set up a dedicated production line for these volumes.

How much power do you need for the transmitters ?

About 200MW is required for 10^10 10mw transmitters at 50% efficiency.

Which cost/W do you use ?

At $0.08/kwh this is about $140M/year.  However, you could also make the

array

solar powered to cut the operating expense (though this reduces your

coverage

since you cannot transmit at night, and introduces more site selection

limitations).

You need about 32 watts/m^2 of low voltage DC - a great match to solar

cell technology.

And how large is the antenna that you need ?

At about 3.5cm wavelength, the antennas need to be about 2.5 cm apart, or

1600 per

square meter.  This yields a surface about 2.5 km on a side - big, but not

that

much bigger than the proposed square kilometer array.  And it's a lot

lower

technology.

No matter how I try, I cannot get this cost number to below billions of
dollars. Per year!
Either way, doesn't your cost go down if you increase your frequency,
and start pulsing rather than transmitting contiuously ?

Yes, increasing the frequency can be good, especially if your costs are
dominated by the area of the transmitter.  However, this is not always the
case.

What if you go to infrared or higher transmitters ? Wouldn't cost go down
many orders of magnitude (Because you can make ns pulses )

The costs look similar since antennas (telescopes) are much more expensive
per unit area, since they must be much more precise, and the photons
are much more costly compared to the backgound noise.  Overall, it's hard
to tell if short pulses in the IR/visible are better than microwaves.

That's

why the SETI 2020 report recommended looking for both.

and 'antenna' size reduces to a small telescope...

Yes, but one of the two ends (the receiver or the transmitter) needs one
(meter class) telescope per star.  If both the transmitter and receiver
just look at one star at a time, the odds are the reciever won't be

looking

when the sender is sending. One of the two has to be looking (or sending)
all the time at all the targets to have any appreciable chance of success.
And 1M telescopes are currently much, much more than $10K each, so you are
talking one end or the other spending at least $10B dollars.

It is more likely to find a beacon signal at the frequency, bandwidth,

pulse

rate and pulse width which is most cost-effective for ET to produce and
detectable for us...

This is a very good question, but turns out to quite sensitive to

assumptions

about ET technology.  So it's hard to find a good answer.

Did anyone make a formula for this already ?

The most serious attempt is in 'SETI 2020', chapter 5.

    Lou Scheffer