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# RE: variable phase delay / phased arrays

```Doug Wrote:

> Question to the group:
>
> Suppose I have an "n by n" patch array at my ground
> station.  Suppose I want to receive the satellite as
> it moves across the sky using my "n by n" patch array.
>
> Instead of phasing the LO to the mixer for each patch,
> could one instead use a single LO that is fed
> to each of the mixers, and do all of the phase
> processing on the signal output from the mixer (say at
> audio with DSPs) instead of transforming the phase
> of the LO? Wouldn't this be equivalent?

In short ... Yes!

What you are doing is beamforming, or spatial filtering.  Each patch
represents a sample in space.  As a plane wave comes in, a line of constant
phase passes each patch at a different time.  If you put in appropriate time
delays after each patch, you could effectively line up the phases from each
patch and add up the contributions to maximize the signal from the direction
that the plane wave came from (ie steer the beam).

For a single frequency, the time delay is equivalent to a phase shift.  When
we are talking about bandwidths as narrow as several 10's or even 100's of
kHz at a couple of Gigs, you can safely use the single frequncy
approximation and deal with beamforming as the application of appropriate
phase weights to each channel (ie patch, or spatial sample).

Now that we're dealing with the phase of the signal, we can apply the phase
shift anywhere in the signal chain.  As you pointed out, we could make our
job easier by applying the shift at as low a bandwidth as we need to acquire
our signal of interest (need to be at least wide enough to be able to
compensate for Doppler after the beamformer).  You can mix all the way down
to baseband, as long as you do it coherently (ie you must maintain the phase
relationship between the spatial samples ... er, I mean patches).  A common
quadrature LO (or single LO with an I/Q splitting mixer)would do the trick.
```

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