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

John B. Stephensen kd6ozh at comcast.net
Fri Mar 23 13:05:20 PST 2007


Good point. However I and Q signals may not be needed for each antenna. The 
transmitter is BPSK so there can be one signal source at the final RF output 
frequency. Analog phase shifter ICs (see attachments) can be used to 
generate signals for individual power amplifiers at each antenna. The 
transmitter power amplifiers should be operated at saturation for best 
efficiency so the small amplitude variations in the phase shifters don't 
matter.

For the receiver, an LNA, mixer and LO phase shifter could be placed at each 
antenna. Amplitude variations in the LO have little effect as the LO input 
to the mixer is limiter. The mixers could be quadrature mixer ICs (such as 
an HMC597) if a low-IF receiver is used or single Gilbert-cell mixers (such 
as an LT5560) if the a high IF is to be used. A high IF probably uses less 
power and the conversion to I and Q baseband signals could occur after the 
outputs of all the mixers are summed. It might even be possible to sum the 
RF directly and use only an LNA, phase shifter and variable attenuator for 
each antenna element. The attenuators would compensate for phase shifter 
gain variation.

73,

John
KD6OZH

----- Original Message ----- 
From: "Alan Bloom" <n1al at cds1.net>
To: <K3IO at verizon.net>
Cc: "AMSAT Eagle" <Eagle at amsat.org>
Sent: Friday, March 23, 2007 19:42 UTC
Subject: [eagle] Re: Eagle Microwave Antenna Arrays -- mechanical concepts


> 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.
>
> 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
>>
>>
>> ______________________________________________________________________
>> _______________________________________________
>> Via the Eagle mailing list courtesy of AMSAT-NA
>> Eagle at amsat.org
>> http://amsat.org/mailman/listinfo/eagle
>
> _______________________________________________
> Via the Eagle mailing list courtesy of AMSAT-NA
> Eagle at amsat.org
> http://amsat.org/mailman/listinfo/eagle 
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