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

Wed May 13 14:43:10 UTC 2015

Hi Peter,

If you would like to estimate the signals of both configurations, you can
also listen to existing satellites with certain antenna configurations that
are already in orbit.

Do you plan on stabilizing the satellite and if so, around which axis, or
do you plan to do pointing?
Its always difficult to select the correct antenna configuration based on
all variables. I do not think there is a certain preference, but you can
keep in mind that the majority of people entering the amateur satellite
hobby will first try linear antennas. The top stations may have
polarization switchable between LHCP, RHCP, and linear H/V or in case of
X-configuration, not exactly H/V.

If you want to get an idea of these signals, you can tune in to:
- Delfi-C3 (DO-64), 1200bd BPSK, canted turnstile (circular) on 3U body,
free tumbling in space (no fixed attitude)
- FUNcube-1 (AO-73), 1200bd BPSK with FEC, double dipole config, free
tumbling
- FUNcube-2 on UKube, 1200bd BPSK with FEC, monopole on 3U body, stabilized
- QB50p1 and p2 (EO-79 and EO-80), 1200bd BPSK AX.25, monopoles on 2U body,
stabilized
- Triton-1, 1200bd BPSK AX.25, monopole on 3U body, free tumbling

There are other satellites, but I am naming these because I built radio
hardware for all of them, and therefore know their configurations.

Notes:
* Delfi-C3 operates in sunlight only, and may not be on all the time
* Ukube had the FUNcube-2 payload that is not always on. You can also try
the main beacon 1200bps BPSK AX.25.
* The FUNcube telemetrey downlink is always ON and provides a nice way to
measure fading. The dashboard software also includes FEC error counts

If you need any help finding frequencies, let me know.

Wouter PA3WEG

On Wed, May 13, 2015 at 8:33 AM, Peter Kazakoff <peter at peterkazakoff.ca>
wrote:

> 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*
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