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

Wed May 13 13:09:15 UTC 2015

Regarding the antenna.  The only stations that will see pure circular
polarization are the ones directly below it, everyone else will see more
and more linear .  And stations are only directly below it (within 45
degrees) less than 5% of the total time in view.    AND they see signasl
that are 10 dB stronger, so circularity gains nothing.
See: http://aprs.org/LEO-tracking.html

The geopmetry of a LEO satellite pass is that the satellite is seen below
20 degrees to the horizon for more than 2/3rds of all pass times, so
everyone is seeing mostly linear signals anyway.  So it is not "circular"
polarization you want, but "cross polarization".  They are the same thing,
but the reality is that you just want to make sure that both polarizations
are covered since MOST of the time, MOST users are only going to be seeing
MOSTly just one or the other.

SO your antennas should not be two-monopoles out opposites sides of the
spacecraft, but two 90 degree fed monopoles on a corner.  This gives both
polarizations.

Besides two 1/4 wave monopoles fed on opposite edges of a 4" spacecraft
are not really a dipole since they are not fed at the center of a
half-wave dipole.  There will be additional nulls compared to a free space
dipole due to the 4" phasing separation of the monopoles.

Take these last comments with a grain of salt, but that's what I feel is
the reality of the situation.  Model and compare.  But the first part is
what most people overlook.... that is, almost all (95%) of satellite
operation is within 20 degrees of the horizon and most users then are
always off to the SIDE of the spacecraft.

Bob, WB4APR

-----Original Message-----
From: AMSAT-BB [mailto:amsat-bb-bounces at amsat.org] On Behalf Of Peter
Kazakoff
Sent: Wednesday, May 13, 2015 2:33 AM
To: amsat-bb at amsat.org
Subject: [amsat-bb] Circular or linear polarization for a cubesat -
amateur preference?

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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