[eagle] Re: Eagle Microwave Antenna Arrays -- mechanical concepts

Alan Bloom n1al at cds1.net
Fri Mar 23 11:42:07 PST 2007


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.

This technique would allow full flexibility in antenna placement.  The
optimum phasings and amplitudes could be calculated before launch for
all beam angles (every few degrees) and stored in a table.  Software on
the satellite would then interpolate between the table values.

Alan Bloom

On Fri, 2007-03-23 at 07:52, Tom Clark, K3IO wrote:
> Grant Hodgson wrote: 
> > Tom 
> > 
> > Don't forget to claim back the expenses that you've incurred for
> > these models... 
> > 
> > More seriously - is the intention to have a separate phase shifter
> > for each element? 
> Grant -- there are several basic ideas for doing the phasing:
>      1. A scheme which has been used in the past on electrically
>         despun arrays is to have a discrete beam former with N beams
>         and then discretely switch to the best of the beams as the s/c
>         rotates. IMHO, this is a REALLY BAD  :-P  idea because there
>         will be abrupt phase and amplitude discontinuities when
>         switching from one beam to the next as the s/c spins.
>      2. A neat "zero click" adaptation of #1 can be done with a linear
>         or square array. For this geometry, the "optimum" combiner is
>         the Butler matrix which is the electrical realization of the
>         Cooley-Tukey FFT. Assume that tap X is the beam now, and that
>         Y is best for the next rotation step. If we use an in-phase
>         variable power splitter that can linearly interpolate between
>         the X & Y position, we can smoothly move the beam with no
>         discontinuities. The interpolation is done in POWER with
>         fractions [P] and [1-P] split between the X & Y taps. To build
>         this for an NxN array, we build the combiner that makes NxN
>         beams (of which [N-1]x[N-1] will be used -- we have no need to
>         make use of the beam on the array's "horizon"). I've tested
>         (in MATLAB) this idea for an 8x1 and 8x8 array. I haven't had
>         a magic idea on a Butler-like matrix for hexagonal geometry.
>      3. We could devise some continuous phase shifter to be applied to
>         each element. The required phase shift for any given pointing
>         direction is a linear phase gradient across the aperture --
>         i.e. when viewed from the target (earth), we need to
>         compensate for the geometrical phase offsets due to the plane
>         of the array. [ Note: If we can generate the phase gradient
>         easily, then we can free ourselves from any geometric
>         constraints -- the elements an be located anywhere on the
>         spacecraft.]
> Ideas are solicited!
> 73, Tom
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