In article <4096cbd5$1@news.cadence.com>,
Louis Scheffer <lou@cadence.com> wrote:

Louis wrote:

because the EIRP goes as N^2, so for constant EIRP, the power per module 

                            ^^^

Yes.  There is nothing magical.  The array with more modules is physically
bigger.  This is exactly the same as replacing a dish with another dish
with N times the area, and illuminating it with 1/N of the power.  The

                                                  ^^^
There's nothing magical about scaling gain with N, but you were 
claiming that the gain scales as N^2, if you use a discrete array
(see above quote - monospace font assumed).

To do the maths more precisely.  If you divide the power by N, you
get Nth of the power per element.  This corresponds to sqrt (N) in
terms of the E or H field (loosely the voltage).  E and H fields
superimpose linearly, so that in the aiming direction, the array of
N elements will produce N / sqrt (N) E field relative to feeding the
whole power to one element, which equals sqrt (N).

Converting back to power, one gets N (not N^2) increase in the effective
isotrpic power.

system gets physically bigger, the beam is physically smaller, the EIRP 
is constant, the total power required goes down.

But as 1/N, not as 1/N^2.

Arrays are even more cost effective for transmitting than receiving.

Another factor is that it is difficult to justify a transmit only
antenna for this purpose.

The need for cryogenic cooling is what drives the SKA to use dishes at

Interesting, because the SERENDIP receiver was swapped for one without
cryogenic cooling, but with a better noise figure, during S@H.

current technology, you can put about 100 small transmitters on a single

But the chip needs 100 times the power of a single tranmsitter.  Also
the feed cables and all the joints on them probably represent the 
main source of failure modes (at the high end of frequency range
you might be able to use strip-line construction for the whole
10 by 10 sub-array, though - maybe a 1 metre square one for 21cm
is not too unrealistic).

Also, whilst 1,000,000 elements would give the same feed point power
as Arecibo (although the optimal array, at 21cm) would be smaller than
Arecibo, although probably comparable to its effective area) you could
only produce 1 Arecibo equivalent beam for 1 watt per element; each
additional beam would mean adding another watt to the output of each
transmitter (more as input power).  Forming 100 beams would require
that your, 100 tranmitter, chip could handle 10kW (assume a fairly high
efficiency and one is still talking about a single chip dissipating as
much power as heat as a portable room heater.

chip on a single PC board.  There are only 10,000 of these, so any
reasonable MTBF will do, since one dead board won't cripple the system.

100,000 hrs is still multiple swapouts each day.

Incidentally the individual antennas couldn't be omnidirectional, as
such antennas are impossible to realise. They couldn't realistically be
ideal dipoles, for a ground based transmitter, as that could only be
achieved by putting the array over a 377 ohm per square ground plane,
which would waste half the power.  Putting dipoles above a real ground,
as well as producing a more directional per element beam, would produce
an array whose characteristics changed drastically after a rain shower.
I think, therefore, that you would need explicit reflectors/ground
plane, to make the behavour more repeatable and minimise losses.
(Anything except a metallic reflector, or a cryogenically cooled (<<12K)
377 ohm absorber would make the array unsuitable for reception.)

(There have been passively phased arrays - therefore one beam only -
that have used patch antennas, which have a natural ground plane.  I 
believe the squarials, briefly marketed for a DBS service, that failed,
in the UK, where like this.)

Agreed that's what people are using now.  However, if a permanent beacon
is ever set up, I don't think it will use this technology.  It will use a

I think that is quite likely in the longer term.  It is a reasonable 
assumption to make about the source of an incoming signal, although 
that doesn't matter that much as we can only monitor the transmission
in one direction.