Re: [amsat-bb] AO-91 and circular polarization?
Greetings Jerry,
thank you for this (yes I meant AO-92).
If I understand, you are saying the the satellite uses a conventional linear antenna.
However AO-92 is easy enough to access that a 3db loss from the polarity mismatch b/t linear and circular is not significant.
Correct? Todd
Sent from my iPhone
On Mar 11, 2018, at 6:46 PM, Jerry Buxton n0jy@amsat.org wrote:
Todd,
Correct is not really a consideration, the way I see it. Will a CP antenna work? Yes. Polarization should not matter for AO-92. (BTW I presume you mean AO-92, not AO-91 in which case L band just flat won't work no matter the antenna, period.) :-)
I use RHCP because I built my antenna for AO-51 and that is was in use at the time. I believe that when CP is used on a satellite, at least in the AMSAT history, it has been RHCP most of the time so building for RHCP may have advantages for future satellites.
Jerry Buxton, NØJY
Not sure where the 3db loss when on the opposite polarity came from. It is more on the order of 20db or more. I am a TV broadcast engineer and we take video and data feeds via various satellites. I can be locked into a bird in the vertical polarity and looking at the spectrum analyzer you can see all the carriers nice and tall. If I rotate the polarity all the signals go completely away and are replaced by all the carriers on the opposite pole. Broadcast satellites transmit two feeds on the same frequency only separated by polarity. This wouldn’t be possible at only a 3db loss. That’s why you must rotate your arrow antennas to follow the spin on the birds else the signal completely disappears.
Michael KC4ZVA EL98
On Mar 11, 2018, at 10:51 PM, Todd Deckard tdeckard@imris.com wrote:
Greetings Jerry,
thank you for this (yes I meant AO-92).
If I understand, you are saying the the satellite uses a conventional linear antenna.
However AO-92 is easy enough to access that a 3db loss from the polarity mismatch b/t linear and circular is not significant.
Correct? Todd
Sent from my iPhone
On Mar 11, 2018, at 6:46 PM, Jerry Buxton n0jy@amsat.org wrote:
Todd,
Correct is not really a consideration, the way I see it. Will a CP antenna work? Yes. Polarization should not matter for AO-92. (BTW I presume you mean AO-92, not AO-91 in which case L band just flat won't work no matter the antenna, period.) :-)
I use RHCP because I built my antenna for AO-51 and that is was in use at the time. I believe that when CP is used on a satellite, at least in the AMSAT history, it has been RHCP most of the time so building for RHCP may have advantages for future satellites.
Jerry Buxton, NØJY
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
The 3dB mentioned is a result of the polarity mismatch between CP and linear polarization. Mike is correct that there is 20dB or more when one CP is mismatched with another CP, or one linear is mismatched with another linear, but from linear to CP it's only ever about 3dB. IIRC, AO-92 uses a 1/4w whip for 70cm as the receive antenna for the L-band converter, so using a CP antenna on the ground will work just fine, and you will always have a 3dB loss, but it will be consistent. If I was building an antenna for that purpose, that I was going to operate beyond reach of the antenna, I would use CP as well.
On Mon, Mar 12, 2018 at 8:24 AM, Mike via AMSAT-BB amsat-bb@amsat.org wrote:
Not sure where the 3db loss when on the opposite polarity came from. It is more on the order of 20db or more. I am a TV broadcast engineer and we take video and data feeds via various satellites. I can be locked into a bird in the vertical polarity and looking at the spectrum analyzer you can see all the carriers nice and tall. If I rotate the polarity all the signals go completely away and are replaced by all the carriers on the opposite pole. Broadcast satellites transmit two feeds on the same frequency only separated by polarity. This wouldn’t be possible at only a 3db loss. That’s why you must rotate your arrow antennas to follow the spin on the birds else the signal completely disappears.
Michael KC4ZVA EL98
On Mar 11, 2018, at 10:51 PM, Todd Deckard tdeckard@imris.com wrote:
Greetings Jerry,
thank you for this (yes I meant AO-92).
If I understand, you are saying the the satellite uses a conventional
linear antenna.
However AO-92 is easy enough to access that a 3db loss from the polarity
mismatch b/t linear and circular is not significant.
Correct? Todd
Sent from my iPhone
On Mar 11, 2018, at 6:46 PM, Jerry Buxton n0jy@amsat.org wrote:
Todd,
Correct is not really a consideration, the way I see it. Will a CP antenna work? Yes. Polarization should not matter for AO-92. (BTW I presume you mean AO-92, not AO-91 in which case L band just flat won't work no matter the antenna, period.) :-)
I use RHCP because I built my antenna for AO-51 and that is was in use at the time. I believe that when CP is used on a satellite, at least in the AMSAT history, it has been RHCP most of the time so building for RHCP may have advantages for future satellites.
Jerry Buxton, NØJY
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
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
Ahh, between CP and linear. Sorry, I should have read more carefully.
Michael Vivona Sent from my iPad
On Mar 12, 2018, at 9:36 AM, Mike Thompson zryder94@gmail.com wrote:
The 3dB mentioned is a result of the polarity mismatch between CP and linear polarization. Mike is correct that there is 20dB or more when one CP is mismatched with another CP, or one linear is mismatched with another linear, but from linear to CP it's only ever about 3dB. IIRC, AO-92 uses a 1/4w whip for 70cm as the receive antenna for the L-band converter, so using a CP antenna on the ground will work just fine, and you will always have a 3dB loss, but it will be consistent. If I was building an antenna for that purpose, that I was going to operate beyond reach of the antenna, I would use CP as well.
On Mon, Mar 12, 2018 at 8:24 AM, Mike via AMSAT-BB amsat-bb@amsat.org wrote: Not sure where the 3db loss when on the opposite polarity came from. It is more on the order of 20db or more. I am a TV broadcast engineer and we take video and data feeds via various satellites. I can be locked into a bird in the vertical polarity and looking at the spectrum analyzer you can see all the carriers nice and tall. If I rotate the polarity all the signals go completely away and are replaced by all the carriers on the opposite pole. Broadcast satellites transmit two feeds on the same frequency only separated by polarity. This wouldn’t be possible at only a 3db loss. That’s why you must rotate your arrow antennas to follow the spin on the birds else the signal completely disappears.
Michael KC4ZVA EL98
On Mar 11, 2018, at 10:51 PM, Todd Deckard tdeckard@imris.com wrote:
Greetings Jerry,
thank you for this (yes I meant AO-92).
If I understand, you are saying the the satellite uses a conventional linear antenna.
However AO-92 is easy enough to access that a 3db loss from the polarity mismatch b/t linear and circular is not significant.
Correct? Todd
Sent from my iPhone
On Mar 11, 2018, at 6:46 PM, Jerry Buxton n0jy@amsat.org wrote:
Todd,
Correct is not really a consideration, the way I see it. Will a CP antenna work? Yes. Polarization should not matter for AO-92. (BTW I presume you mean AO-92, not AO-91 in which case L band just flat won't work no matter the antenna, period.) :-)
I use RHCP because I built my antenna for AO-51 and that is was in use at the time. I believe that when CP is used on a satellite, at least in the AMSAT history, it has been RHCP most of the time so building for RHCP may have advantages for future satellites.
Jerry Buxton, NØJY
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
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
Mike KC4ZVA,
All US Domestic Communication satellite in C-Band and Ku-band used orthogonal linear polarization. This is also known as "Frequency Reuse" first deployed by RCA in the 1970s. In a well designed commercial satellite system isolation between the Horizontal (0 deg) and the Vertical (90 deg) polarization should exceed 30 dB. Isolation is greatest at exactly a 90-degree offset between H-pol and V-pol. At a 45 deg offset, polarization isolation is only 3db. When adjusting a commercial satellite dish, it is better to "null out" the cross-pol that "peak" the co-pol. Circular polarization is still used on some Intelsat standard International satellite links.
I believe FO-29 is the only amateur satellite to use a circularly polarized antenna on the spacecraft. All of the CubeSats use small linear polarized antennas.
The primary reason to use a circularly polarized (CP) ground antenna with a Cubesat is to avoid the deep (-20 to -30 dB) signal null when the ground antenna is orthogonal (90 deg orientation offset) with the satellite antenna. There will still be times when the CubeSat antenna is pointed away from the earth and even a circular antenna will not help. Since most Cubesate tumble, these LP to LP nulls never last very long but they are an irritation during a QSO.
When using a CP ground antenna, you are effectively using a signal (wavefront) that contains both an H-pol and V-pol component that will always be at 45 deg offset to the linear polarized satellite antenna. When using a CP signal you are making a decision to "give up" 3 dB 100% of the time in signal to avoid the occasional -20 to -30 dB signal nulls possible in a linear pol to linear pol system.
If you watch a satellite operator using a hand help Arrow antenna - they are constantly peaking the antenna in direction and polarization. If the Arrow was a CP antenna there would be a lot less "arm twisting" but the Arrow would be 3 dB less efficient 100% of the time.
If your goal is CubeSat operation a CP antenna could be a good choice. It doesn't matter if the CP antenna is RHCP or LHCP because you are working a linear polarization satellite. If you are working FO-29 and your launch an LHCP signal and FO-29 is expecting an RHCP signal - you will experience a -20 to -30 dB loss because RHCP into LHCP will exhibit high isolation.
CubeSats have relatively strong signals and I highly recommend experimentation with small helical antennas and cross-pol yagis. You will have a lot of fun. If you want to get some arm and shoulder exercise use the Arrow antenna but the results will be very good.
73 - Paul - W2HRO
On Mon, Mar 12, 2018 at 9:24 AM, Mike via AMSAT-BB amsat-bb@amsat.org wrote:
Not sure where the 3db loss when on the opposite polarity came from. It is more on the order of 20db or more. I am a TV broadcast engineer and we take video and data feeds via various satellites. I can be locked into a bird in the vertical polarity and looking at the spectrum analyzer you can see all the carriers nice and tall. If I rotate the polarity all the signals go completely away and are replaced by all the carriers on the opposite pole. Broadcast satellites transmit two feeds on the same frequency only separated by polarity. This wouldn’t be possible at only a 3db loss. That’s why you must rotate your arrow antennas to follow the spin on the birds else the signal completely disappears.
Michael KC4ZVA EL98
On Mar 11, 2018, at 10:51 PM, Todd Deckard tdeckard@imris.com wrote:
Greetings Jerry,
thank you for this (yes I meant AO-92).
If I understand, you are saying the the satellite uses a conventional linear antenna.
However AO-92 is easy enough to access that a 3db loss from the polarity mismatch b/t linear and circular is not significant.
Correct? Todd
Sent from my iPhone
On Mar 11, 2018, at 6:46 PM, Jerry Buxton n0jy@amsat.org wrote:
Todd,
Correct is not really a consideration, the way I see it. Will a CP antenna work? Yes. Polarization should not matter for AO-92. (BTW I presume you mean AO-92, not AO-91 in which case L band just flat won't work no matter the antenna, period.) :-)
I use RHCP because I built my antenna for AO-51 and that is was in use at the time. I believe that when CP is used on a satellite, at least in the AMSAT history, it has been RHCP most of the time so building for RHCP may have advantages for future satellites.
Jerry Buxton, NØJY
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
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
Paul, thanks for your very nice write-up, and explains why my reception using my RHCP 440 ant(rotor-controlled) becomes quite unreadable when operating SO-50. And because I don't have a polarity switch for that ant, I can work SO-50 with better success using my Arrow/HT setup.
According to 2016 Getting Started with Amateur Radio, SO-50 also uses CP as downlink; "The -Z(bottom') of the satellite has four gold 400 MHz antennas in a turnstile array with the 436 MHz downlink antenna in the center. The satellite uses_left-hand circular polarization on the downlink."___ 73 Christy KB6LTY
--------------------------------------- Mike KC4ZVA,
All US Domestic Communication satellite in C-Band and Ku-band used orthogonal linear polarization. This is also known as "Frequency Reuse" first deployed by RCA in the 1970s. In a well designed commercial satellite system isolation between the Horizontal (0 deg) and the Vertical (90 deg) polarization should exceed 30 dB. Isolation is greatest at exactly a 90-degree offset between H-pol and V-pol. At a 45 deg offset, polarization isolation is only 3db. When adjusting a commercial satellite dish, it is better to "null out" the cross-pol that "peak" the co-pol. Circular polarization is still used on some Intelsat standard International satellite links.
I believe FO-29 is the only amateur satellite to use a circularly polarized antenna on the spacecraft. All of the CubeSats use small linear polarized antennas.
The primary reason to use a circularly polarized (CP) ground antenna with a Cubesat is to avoid the deep (-20 to -30 dB) signal null when the ground antenna is orthogonal (90 deg orientation offset) with the satellite antenna. There will still be times when the CubeSat antenna is pointed away from the earth and even a circular antenna will not help. Since most Cubesate tumble, these LP to LP nulls never last very long but they are an irritation during a QSO.
When using a CP ground antenna, you are effectively using a signal (wavefront) that contains both an H-pol and V-pol component that will always be at 45 deg offset to the linear polarized satellite antenna. When using a CP signal you are making a decision to "give up" 3 dB 100% of the time in signal to avoid the occasional -20 to -30 dB signal nulls possible in a linear pol to linear pol system.
If you watch a satellite operator using a hand help Arrow antenna - they are constantly peaking the antenna in direction and polarization. If the Arrow was a CP antenna there would be a lot less "arm twisting" but the Arrow would be 3 dB less efficient 100% of the time.
If your goal is CubeSat operation a CP antenna could be a good choice. It doesn't matter if the CP antenna is RHCP or LHCP because you are working a linear polarization satellite. If you are working FO-29 and your launch an LHCP signal and FO-29 is expecting an RHCP signal - you will experience a -20 to -30 dB loss because RHCP into LHCP will exhibit high isolation.
CubeSats have relatively strong signals and I highly recommend experimentation with small helical antennas and cross-pol yagis. You will have a lot of fun. If you want to get some arm and shoulder exercise use the Arrow antenna but the results will be very good.
73 - Paul - W2HRO
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FWIW, this page https://www.amsat.org/two-way-satellites/ao-7/ says that AO-7 has a CP antenna as well.
On Mon, Mar 12, 2018 at 12:58 PM, bruce hunter bhunter3@mindspring.com wrote:
Paul, thanks for your very nice write-up, and explains why my reception using my RHCP 440 ant(rotor-controlled) becomes quite unreadable when operating SO-50. And because I don't have a polarity switch for that ant, I can work SO-50 with better success using my Arrow/HT setup.
According to 2016 Getting Started with Amateur Radio, SO-50 also uses CP as downlink; "The -Z(bottom') of the satellite has four gold 400 MHz antennas in a turnstile array with the 436 MHz downlink antenna in the center. The satellite uses_left-hand circular polarization on the downlink."___ 73 Christy KB6LTY
Mike KC4ZVA,
All US Domestic Communication satellite in C-Band and Ku-band used orthogonal linear polarization. This is also known as "Frequency Reuse" first deployed by RCA in the 1970s. In a well designed commercial satellite system isolation between the Horizontal (0 deg) and the Vertical (90 deg) polarization should exceed 30 dB. Isolation is greatest at exactly a 90-degree offset between H-pol and V-pol. At a 45 deg offset, polarization isolation is only 3db. When adjusting a commercial satellite dish, it is better to "null out" the cross-pol that "peak" the co-pol. Circular polarization is still used on some Intelsat standard International satellite links.
I believe FO-29 is the only amateur satellite to use a circularly polarized antenna on the spacecraft. All of the CubeSats use small linear polarized antennas.
The primary reason to use a circularly polarized (CP) ground antenna with a Cubesat is to avoid the deep (-20 to -30 dB) signal null when the ground antenna is orthogonal (90 deg orientation offset) with the satellite antenna. There will still be times when the CubeSat antenna is pointed away from the earth and even a circular antenna will not help. Since most Cubesate tumble, these LP to LP nulls never last very long but they are an irritation during a QSO.
When using a CP ground antenna, you are effectively using a signal (wavefront) that contains both an H-pol and V-pol component that will always be at 45 deg offset to the linear polarized satellite antenna. When using a CP signal you are making a decision to "give up" 3 dB 100% of the time in signal to avoid the occasional -20 to -30 dB signal nulls possible in a linear pol to linear pol system.
If you watch a satellite operator using a hand help Arrow antenna - they are constantly peaking the antenna in direction and polarization. If the Arrow was a CP antenna there would be a lot less "arm twisting" but the Arrow would be 3 dB less efficient 100% of the time.
If your goal is CubeSat operation a CP antenna could be a good choice. It doesn't matter if the CP antenna is RHCP or LHCP because you are working a linear polarization satellite. If you are working FO-29 and your launch an LHCP signal and FO-29 is expecting an RHCP signal - you will experience a -20 to -30 dB loss because RHCP into LHCP will exhibit high isolation.
CubeSats have relatively strong signals and I highly recommend experimentation with small helical antennas and cross-pol yagis. You will have a lot of fun. If you want to get some arm and shoulder exercise use the Arrow antenna but the results will be very good.
73 - Paul - W2HRO
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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 (7)
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bruce hunter -
Burns Fisher -
Mike -
Mike Thompson -
Mvivona
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Paul Andrews -
Todd Deckard