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?
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@amsat.org] On Behalf Of Peter Kazakoff Sent: Wednesday, May 13, 2015 2:33 AM To: amsat-bb@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* _______________________________________________ Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
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@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* _______________________________________________ Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
participants (3)
-
Peter Kazakoff -
Robert Bruninga -
Wouter Weggelaar