Let me try to answer a number of points. First, Alan wrote:

Tom,

Have we given up on the idea of using DSP techniques to do the phasing? 
If each antenna/amplifier had its own RF generator controlled by
separate (I & Q) DACs, then it would be easy to control the phase of
each element precisely with "infinite" interpolation between steps.

The same goes for the amplitude.  So you could, for example, taper the
amplitudes of the elements near the edges to reduce sidelobes.  It
should be possible to get a fully-symmetrical beam pattern to eliminate
spin modulation.

The full DSP approach has not been discounted, but it comes with a lot of pain. First of all, it would make sense to do the phasing at IF and then up-convert to microwave with a single, coherent LO. The problem is that we would need 40-ish D/A converters, and this would cost a lot of power. It wouldn't even be necessary to do the phasing in DSP -- programmable NCOs could be used. If you think about it, the phase modulation pattern needed for all these LOs is a sinusoid, with the phase (with respect to the spin rate) and phase amplitude being a function of that particular patch's geometry. And yes, even if an array is off the spin-axis, the spin modulation can be removed too.

Franklin is right in noting that the element amplitude does not need to be tuned. We will always be in the mode of desiring maximum gain, and hence all individual PAs should be running at maximum efficiency. If we have too much TX power, we simply turn the unnecessary elements power off, saving valuable watts and making less heat. The presence of sidelobes is of no concern. We want sidelobe levels to be controlled, but that's just to prevent wasting power beaming it at the Venusians.

The problem with the phase shifters Jon mentioned is that their spec shows a nominal=4 dB, max=10 dB insertion loss. I can't find info in the spec sheet about power consumption. Again, if discrete phase shifters are to be used, doesn't it make sense to apply them at IF where power is of less concern?

Regardless (just speaking of the TX side), for N (a number like 40 ±4) patches in an array, we need N ~1 watt high efficiency amplifiers which will have at most 15 dB of gain. Somehow, we need to generate N drive signals thru some sort of power splitter. If we split at microwave, this will introduce at least 16 dB of loss, even if the splitter is perfect. Hence we will need a few watts of RF just to get enough power to the PAs at the array elements. And if we use John's lossy phase shifters, we need even more watts in the driver.

Back on Feb 13, I circulated an idea for doing the phasing in something like a Butler Matrix (if you don't have that message, I can resend it -- the subject was "Eagle Microwave Antenna Concepts"). It suggested a way to do the power splitter and array phasing using a Butler matrix (or something like that). That concept included the concept that, if the Butler array had ports for all possible beams, then beam steering could be done with something like a crossbar switch. I ended that message with a set of 7 questions I am still pondering:

So here are some challenges to make something like this work:

Is there some better way to do the phasing that using Butler matrices?

The antenna array phase needed to point at a given location is a simple linear phase gradient. If we don't use a Butler combiner, is there a simple way to invoke a linear phase "tilt" on a bunch of elements.

How complicated is it to fabricate the 2N Butler matrices that we will need in a microstrip structure?

The current baseline calls for 36 elements, arranged as either a 6x6 square or as a 37 element "bee hive" hexagon. What do Butler-like matrices look like for the cases where the array does not have 2^N elements?

Can we invent the microwave widget that allows us to linearly interpolate power between 2 (or preferably 4) of the (N-1)² beams?

Remember that we need a similar structure for the S2 receiver, with LNA's instead of PA's at each antenna.

Have I gone off the deep end with these ideas?

73, Tom