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

Robert McGwier rwmcgwier at gmail.com
Sat Mar 24 08:01:29 PST 2007

Alan Bloom wrote:
> On Fri, 2007-03-23 at 13:05, John B. Stephensen wrote: 
>> 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. 
> ...
> On Fri, 2007-03-23 at 18:39, Tom Clark, K3IO wrote:
> ...
>> 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.
> ...
> I've been assuming that the BPSK modulation must be filtered to reduce
> the bandwidth.  That means that the power amplifiers can't be run in
> class C since that would clip the rise/fall waveform of the modulation
> (unless we did some fancy pre-compensation).
> But maybe unfiltered is OK.  The satellite signal will be quite weak by
> the time it reaches earth, so maybe the unfiltered sin(x)/x sidebands
> would be acceptable.

It is okay and we are planning on hard limited amplifiers and BPSK.

> If so, that really simplifies things.  The modulator is simply a
> 180-degree phase shifter.  Something like a double-balanced mixer fed
> with a bipolar digital signal (on the DC-coupled "IF" port).   The PAs
> running in class C would have high efficiency and nice stable output
> power levels even if the input drive level is not very constant.
> Then the only problem left is to be able to vary the RF phase of each
> element.  Does anyone know a clever technique to control the phase of a
> microwave oscillator directly?  It's easy to do over a 90-degree or so
> range using a PLL, but we need to do it over several cycles of phase.
> Another method:
>> ... It wouldn't even be necessary to do the phasing in DSP --
>> programmable NCOs could be used.  ...
> The NCOs would have to run at a lower RF frequency and then be
> heterodyned up to C-band.  Still it wouldn't be too horrible a block
> diagram.  Let's see if I can draw it in ASCII:

This was all discussed in our San Diego meeting of last summer.  We 
traded off then and we are still trading off analog shifters versus 
tuning the phase of each element in the NCO's.

My plan is to have us start building actual components THIS SUMMER in a 
hard push to present project management with some real engineering trade 
offs so we can make hard decisions and build stuff.


> Alan


AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL,
"Taking fun as simply fun and earnestness in earnest shows
how thoroughly thou none of the two discernest." - Piet Hine

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