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

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. 

Actually, we have plenty of technology for this already.

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....

Nah, just a few km on a side.

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 ?

Sure, there is no physics barrier to this.  We make planet size receiving
telescopes quite frequenntly.

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 ?

Actually, a next generation or so cell phone would be about right.  
If it contained an improved GPS such as Galileo, the location could be known
to a few cm.  If then the phase was adjuseted properly, a large
array of cell phones could make a very effective transmitter.

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 ?

No, it's quite possible....