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

"Louis Scheffer" <lou@cadence.com> wrote in message
news:40ab046d$1@news.cadence.com...

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

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.

Yeah, but these are interferrometers. They provide resolution, but also have
massive amount of side-lobes, so they dont have the sensitivity.

You are confusing two issues here, and neither affects the transmitting
problem.  The bad side lobes are a result of a non-optimum pattern of
antenna locations, or just not enough antennas.  These sidelobes  
hurt the ability to do imaging (you can't see dim things if you 
have all sorts of spurious echos of the bright things) but affect the 
forward gain very little.  Even a truely horrible sidelobe
of -20db only siphons off 1% of the power.

The sensitivity (to point sources) is determined only by the total area
of all the antennas, which does not depend on the configuration.  This is
the analog to the forward gain we are trying to maximize.  Of course the
radio astronomers also want sensitivity to extended sources, which does
depend on the configuration, but this does not matter when sending a beacon.

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.

Mmm. nice !
However, if the transmitters are scattered around the globe, and not
clustered on your few square km, then we have the same problem as
with interferrometers : side-lobes. The more scattered the transmitters,
the narrower the beam(s), but the more uncontrollable side-lobes too.

Doesn't that waste energy beyond belief ?

No, it's a very small effect.  See above.

(Or if used as a receiver, we receive noise from uncontrollable directions).

It's more of a problem when receiving.  In particular there can be sources
in the non-beam direction (such as satellites) that are much stronger then
the desired signal.  These can sneak in through the sidelobes and cause
all sorts of problems.  This is a serious problem in current radio
astronomy already, and only likely to get worse.

    Lou Scheffer