[amsat-bb] Circular or linear polarization for a cubesat - amateur preference?

[Peter Kazakoff]

Hi folks,

I'm currently working on finalizing the design of an undergraduate 3U cube
satellite (ECOSat-II at the University of Victoria in Canada) which will
retain some amateur satellite capabilities. The design won the Canadian
Satellite Design Challenge (CSDC) in 2013, and the prize is a launch into a
600 km sun synchronous polar orbit, so it in all likelihood will actually
fly.

Current specifications are:

   - RX: 70cm amateur space band, < 1 dB noise figure, omnidirectional
   pattern
   - TX: 2m amateur space band, 2 watts maximum transmit power,
   omnidirectional pattern

Supported modes:

   - Narrowband FM repeat
   - 40 kHz wide non-inverting linear transponder
   - 9600 baud DQPSK custom digital mode with forward error correction

The satellite uses an SDR board with 200 kHz I-Q bandwidth that we've
designed ourselves, so all the modes can run concurrently on the single
communications system.

The digital mode is something that we designed for our telemetry & control
link, but we'll publish a specification so amateurs with SDRs can still
play with anything that isn't critical for spacecraft operation (for
example: amateurs can poll the status of various subsystems and read the
telemetry files, but can't change the attitude control setpoint or update
firmware). If there's time before launch we'll probably also implement some
kind of message board service on this mode so amateurs can send
store-and-forward text messages.

Anyway, here's a SolidWorks render of the current design:
http://on.fb.me/1cRkB4J.

You can see that the satellite uses two dipole antennas, one for each band.
The problem that I see with this is that amateurs are going to need
circular polarized antennas for full-duplex transmission - as the antennas
are mounted 90 degrees apart, pointing a linear Yagi at the satellite means
that both linear polarizations can't be received at the same time. You can
do it with a crossed Yagi (circular polarized) but at the expense of 3 dB
of link margin in each direction. (This wasn't my design - it was designed
before I joined the team).

I'm considering trimming both dipoles down to half-wavelength for 70cm and
then using them as a single turnstile antenna. I can then feed the antenna
through a diplexer and 90 degree hybrid to give a circular pattern. I think
this was unattractive before because the design was hard to model, but
Keysight just gave us a copy of some pretty expensive high end RF design
software and field simulators so I should be able to get it all working in
software before building it.

The advantage to this approach is that an amateur station on the ground can
use a straightforward linear Yagi and only suffer 3dB of loss regardless of
spacecraft orientation about the zenith-nadir axis. Amateur stations with
crossed Yagis can get the full signal, provided they match the polarization
and the satellite isn't tumbling.

Thoughts?

-- 
*Peter Kazakoff*
ECOSat <http://csdc.uvic.ca/> Communications Lead
4th Year Electrical Engineering Student
University of Victoria
*(250) 920 - 6870*