david@djwhome.demon.co.uk (David Woolley) writes:

This seems intuitively reasonable, but is not true.  In a phased array,
replacing modules with smaller cheaper modules saves power.  This is
because the EIRP goes as N^2, so for constant EIRP, the power per module 

No.  There is nothing magical.  

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

In fact, unless the array elements are far enough apart (outside
each others' "capture areas") the gain won't even scale as the number
of elements, but rather as the effective area covered by them).

True, but the spacing required is rather small (1/2 lambda) for dipole
type elements.

Note that even for reception, the 1 square km array uses a hybrid
configuration with directional antennas at each array node.  That's
because the current break even in a receive configuration occurs when
some of the gain is done by passive reflectors.  Arrays are becoming more
cost effective for reception because the electronics cost is reducing,
and passive reflector costs scale more like the cube of the diamter than
as the area.

Arrays are even more cost effective for transmitting than receiving.
The need for cryogenic cooling is what drives the SKA to use dishes at
each node.  For transmitting, a huge number of tiny uncooled antennas
is more cost effective.  (They may even do this for the low end of the
SKA array, where the noise is so high cooling the receivers is not
necessary.)

Incidentally, it would be more or less impossible to keep a million
element array operating on all 1 million elements; you would need
elements with enormous MTBFs.

But you don't need all the elements working all the time.  
A few non-working modules have little effect, and they can
(if need be) replaced while the antenna is in operation (or batch them
up and replace all the defective modules every few days).  Also, with
current technology, you can put about 100 small transmitters on a single
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.

True, but no rational SETI transmitter would use vacuum tubes any more,
since they don't last very long and represent a significant cost.

Current planetary radar uses vacuum tubes, as does the Evaptoria
transmitter for Cosmic Call.  

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
phased array of solid state elements.

    Lou Scheffer