Rotation of the polarization question
Hi I want to find a place where I can learn about the effects the atmosphere has on the polarization of signals from satellites. Does anyone know of any web sites where that might be discussed? Thanks Jerry |
Rotation of the polarization question
On 28 Mar, 15:25, "Jerry Martes" wrote:
Hi I want to find a place where I can learn about the effects the atmosphere has on the polarization of signals from satellites. Does anyone know of any web sites where that might be discussed? Thanks Jerry W4rnl talks about satellite antennas, might be worth a look |
Rotation of the polarization question
On Mar 28, 3:25 pm, "Jerry Martes" wrote:
Hi I want to find a place where I can learn about the effects the atmosphere has on the polarization of signals from satellites. Does anyone know of any web sites where that might be discussed? Thanks Jerry Referred to as Faraday Rotation.. ARRL handbook talks about it in the EME section. Rotation is less as frequency increases (so VHF is affected a lot more than UHF is affected more than microwaves) Amount of rotation is determined by the total electron content in the path, which, of course, varies with everything you can imagine, but primarily the amount of sunlight. It's related to the difference (dispersion) in propagation speed with frequency (or, index of refraction, if you want to work it that way).. More than 360 degrees of rotation isn't unusual at VHF. What's really interesting is that it rotates the same way no matter which direction you go, so if you had a planar reflector above the ionosphere, and you launched a wave that's say polarized North-South, and there's 45 degrees of rotation, when it came back down it would be rotated 90 and be polarized East West... This is one reason why GPS radiates on more than one frequency (you measure the phase difference between the two, and that allows you to estimate the TEC, which in turn allows you to estimate the change in propagation velocity, which improves the accuracy of your fix.) Most high accuracy orbiting radars that operate in P or L band do something similar, especially if they are making polarimetric measurements (handy if you want to do things like distinguish between trees and grass, for instance) How much detail do you need? I can probably give you URLs to cover everything from simple estimates of magnitude to mind bendingly detailed theoretical discussions. |
Rotation of the polarization question
wrote in message ps.com... On Mar 28, 3:25 pm, "Jerry Martes" wrote: Hi I want to find a place where I can learn about the effects the atmosphere has on the polarization of signals from satellites. Does anyone know of any web sites where that might be discussed? Thanks Jerry Referred to as Faraday Rotation.. ARRL handbook talks about it in the EME section. Rotation is less as frequency increases (so VHF is affected a lot more than UHF is affected more than microwaves) Amount of rotation is determined by the total electron content in the path, which, of course, varies with everything you can imagine, but primarily the amount of sunlight. It's related to the difference (dispersion) in propagation speed with frequency (or, index of refraction, if you want to work it that way).. More than 360 degrees of rotation isn't unusual at VHF. What's really interesting is that it rotates the same way no matter which direction you go, so if you had a planar reflector above the ionosphere, and you launched a wave that's say polarized North-South, and there's 45 degrees of rotation, when it came back down it would be rotated 90 and be polarized East West... This is one reason why GPS radiates on more than one frequency (you measure the phase difference between the two, and that allows you to estimate the TEC, which in turn allows you to estimate the change in propagation velocity, which improves the accuracy of your fix.) Most high accuracy orbiting radars that operate in P or L band do something similar, especially if they are making polarimetric measurements (handy if you want to do things like distinguish between trees and grass, for instance) How much detail do you need? I can probably give you URLs to cover everything from simple estimates of magnitude to mind bendingly detailed theoretical discussions. Hi Jim I am considering the design of a horn to illuminate an 8 foot diameter solid surface dish at 4 GHz, for reception of geosynchronous satellite signals that are linearly polarized. One rraa reader has informed me that the rotation is refered to as Faraday rotation and gave some links to it. I see that the amount of rotation may be small enough to be negligable for my application. I would like to know how rapid the rotation changes with time. Thanks Jerry |
Rotation of the polarization question
Jerry Martes wrote:
wrote in message ups.com... On Mar 28, 3:25 pm, "Jerry Martes" wrote: Hi I want to find a place where I can learn about the effects the atmosphere has on the polarization of signals from satellites. Does anyone know of any web sites where that might be discussed? Thanks Jerry Referred to as Faraday Rotation.. ARRL handbook talks about it in the EME section. Rotation is less as frequency increases (so VHF is affected a lot more than UHF is affected more than microwaves) Amount of rotation is determined by the total electron content in the path, which, of course, varies with everything you can imagine, but primarily the amount of sunlight. It's related to the difference (dispersion) in propagation speed with frequency (or, index of refraction, if you want to work it that way).. More than 360 degrees of rotation isn't unusual at VHF. What's really interesting is that it rotates the same way no matter which direction you go, so if you had a planar reflector above the ionosphere, and you launched a wave that's say polarized North-South, and there's 45 degrees of rotation, when it came back down it would be rotated 90 and be polarized East West... This is one reason why GPS radiates on more than one frequency (you measure the phase difference between the two, and that allows you to estimate the TEC, which in turn allows you to estimate the change in propagation velocity, which improves the accuracy of your fix.) Most high accuracy orbiting radars that operate in P or L band do something similar, especially if they are making polarimetric measurements (handy if you want to do things like distinguish between trees and grass, for instance) How much detail do you need? I can probably give you URLs to cover everything from simple estimates of magnitude to mind bendingly detailed theoretical discussions. Hi Jim I am considering the design of a horn to illuminate an 8 foot diameter solid surface dish at 4 GHz, for reception of geosynchronous satellite signals that are linearly polarized. One rraa reader has informed me that the rotation is refered to as Faraday rotation and gave some links to it. I see that the amount of rotation may be small enough to be negligable for my application. I would like to know how rapid the rotation changes with time. That depends on geomagnetic activity, because Faraday rotation is also a function of the earth's magnetic field. With a signal that started out linearly polarized, there are three sources of change: 1. Simple geometry - if a geostationary satellite orbiting above the 0deg meridian has an antenna that is horizontally polarized when viewed from a ground location on the same meridian, that same antenna will appear vertically polarized when viewed from a location 90deg west. This effect is easily calculated and compensated. 2. Faraday rotation, as above. 3. Dispersion ("smearing") of the linear polarization, at times of high geomagnetic activity. From experience with a 432MHz array that had fully rotatable polarization, polarization could range from accurately linear at geomagnetically quiet times (with a very deep null at 90deg offset), all the way to dispersion of the signal around the whole 360 degrees. A completely dispersed signal will have a 3dB loss compared with accurately aligned linear polarization, so you have to include that possibility in your path loss budget. (At lower frequencies, dispersion is also accompanied by ionospheric absorption, but at 4GHz you may not need to worry about that.) Bottom line: if you have enough signal/noise ratio to handle a permanent 3dB loss, then it will be much more convenient to ignore the fact that the signal started out linearly polarized. Instead, use a circularly-polarized feedhorn which will be insensitive to the polarization of the arriving signal. -- 73 from Ian GM3SEK |
Rotation of the polarization question
"Jerry Martes" wrote in
news:iyJOh.23960$FD1.9394@trnddc05: .... Hi Jim I am considering the design of a horn to illuminate an 8 foot diameter solid surface dish at 4 GHz, for reception of geosynchronous satellite signals that are linearly polarized. One rraa reader has informed me that the rotation is refered to as Faraday rotation and gave some links to it. I see that the amount of rotation may be small enough to be negligable for my application. I would like to know how rapid the rotation changes with time. Jerry, I am a little surprised that a geostationary satellite would use linear polarisation on a 4GHz feed, but that might just express a lack of experience. A long time ago, I worked with the Intelsat series, and they were circular polarisation. Earth stations had no means of adjusting the orientation of feeds, they were RH or LH circular, the uplink was opposite to the downlink IIRC. More recently, I worked on the design of a bird that used polarisation diversity. It used LH and RH circular, and reused the same frequency band on both polarisations. If your bird is truly linear, you could use a circular antenna with a slight reduction in G/T, but with the flexibility of eliminating the orientation variable and the mechanical aspects of an antenna with adjustable orientation (remembering that the feed orientation will vary with position of the earth station). Notwithstanding that transmission might be circular, the received signal might not be perfectly circular as a result of some of the effects you have described. Owen |
Rotation of the polarization question
"Jerry Martes" wrote in message news:iyJOh.23960$FD1.9394@trnddc05... wrote in message ps.com... On Mar 28, 3:25 pm, "Jerry Martes" wrote: Hi I want to find a place where I can learn about the effects the atmosphere has on the polarization of signals from satellites. Does anyone know of any web sites where that might be discussed? Thanks Jerry Referred to as Faraday Rotation.. ARRL handbook talks about it in the EME section. Rotation is less as frequency increases (so VHF is affected a lot more than UHF is affected more than microwaves) Amount of rotation is determined by the total electron content in the path, which, of course, varies with everything you can imagine, but primarily the amount of sunlight. It's related to the difference (dispersion) in propagation speed with frequency (or, index of refraction, if you want to work it that way).. More than 360 degrees of rotation isn't unusual at VHF. What's really interesting is that it rotates the same way no matter which direction you go, so if you had a planar reflector above the ionosphere, and you launched a wave that's say polarized North-South, and there's 45 degrees of rotation, when it came back down it would be rotated 90 and be polarized East West... This is one reason why GPS radiates on more than one frequency (you measure the phase difference between the two, and that allows you to estimate the TEC, which in turn allows you to estimate the change in propagation velocity, which improves the accuracy of your fix.) Most high accuracy orbiting radars that operate in P or L band do something similar, especially if they are making polarimetric measurements (handy if you want to do things like distinguish between trees and grass, for instance) How much detail do you need? I can probably give you URLs to cover everything from simple estimates of magnitude to mind bendingly detailed theoretical discussions. Hi Jim I am considering the design of a horn to illuminate an 8 foot diameter solid surface dish at 4 GHz, for reception of geosynchronous satellite signals that are linearly polarized. One rraa reader has informed me that the rotation is refered to as Faraday rotation and gave some links to it. I see that the amount of rotation may be small enough to be negligable for my application. I would like to know how rapid the rotation changes with time. Thanks Jerry Any particulat sat you are interested in. I you to have a downlink with GOES east and west and you wouldnt believe how simple of an antenna it took A pair of dipoles connected for circular polarity. LNA and downconverer were at he antenna.. Jimmie |
Rotation of the polarization question
Owen Duffy wrote in
: "Jerry Martes" wrote in news:iyJOh.23960$FD1.9394@trnddc05: . A long time ago, I worked with the Intelsat series, and they were circular polarisation. Earth stations had no means of adjusting the orientation of feeds, they were RH or LH circular, the uplink was opposite to the downlink IIRC. More recently, I worked on the design of a bird that used polarisation diversity. It used LH and RH circular, and reused the same frequency band on both polarisations. If your bird is truly linear, you could use a circular antenna with a slight reduction in G/T, but with the flexibility of eliminating the orientation variable and the mechanical aspects of an antenna with adjustable orientation (remembering that the feed orientation will vary with position of the earth station). Notwithstanding that transmission might be circular, the received signal might not be perfectly circular as a result of some of the effects you have described. Owen Hi: We just had a member that works in the satellite uplink/downlink business at Penn State University give a talk on the subject. The satellites use polarization as part of the frequency sharing system in geostationary satellites. When they buy time on a satellite they are given a frequency and a polarization to use. As there is a limited band of frequencies they use polarization to help share frequencies with some working horizontal and some vertical. The feeds on the dishes they use for uplinks and downlinks have motorized polarization feeds and they adjust them to the requested polarization. John Passaneau W3JXP |
Rotation of the polarization question
John Passaneau wrote in
: .... Hi: We just had a member that works in the satellite uplink/downlink business at Penn State University give a talk on the subject. The satellites use polarization as part of the frequency sharing system in geostationary satellites. When they buy time on a satellite they are given a frequency and a polarization to use. As there is a limited band of frequencies they use polarization to help share frequencies with some working horizontal and some vertical. The feeds on the dishes they use for uplinks and downlinks have motorized polarization feeds and they adjust them to the requested polarization. Yes John, I talked about that and I incorrectly used the term "diversity", but it is frequency reuse as I noted using polarisation. This can be done with circular (LH and RH) polarisation without a need to rotate the feed unit, which for many installations will mean another rotary waveguide joint which is to be avoided. Launching circular polarisation in a feed horn is not difficult, and developing autotrack error signals is relatively easy. An interesting feed for amateur applications is the septum feed which gives access to both circular polarisations from a fairly simple feed unit. Owen |
Rotation of the polarization question
Owen Duffy wrote in
: John Passaneau wrote in : ... Hi: We just had a member that works in the satellite uplink/downlink business at Penn State University give a talk on the subject. The satellites use polarization as part of the frequency sharing system in geostationary satellites. When they buy time on a satellite they are given a frequency and a polarization to use. As there is a limited band of frequencies they use polarization to help share frequencies with some working horizontal and some vertical. The feeds on the dishes they use for uplinks and downlinks have motorized polarization feeds and they adjust them to the requested polarization. Yes John, I talked about that and I incorrectly used the term "diversity", but it is frequency reuse as I noted using polarisation. This can be done with circular (LH and RH) polarisation without a need to rotate the feed unit, which for many installations will mean another rotary waveguide joint which is to be avoided. Launching circular polarisation in a feed horn is not difficult, and developing autotrack error signals is relatively easy. An interesting feed for amateur applications is the septum feed which gives access to both circular polarisations from a fairly simple feed unit. Owen Hi Owen: In this case the feed point has for lack of the right name a "pick up loop" that is motorized and that is what turns for the differnt polarisation. It is linear, not circular polarisation, at least on the geostationary satellites they use. I would guess that linear is easier for them to change. The way it works is they call a satatellite broker and buy time, he gives them a satellite, a frequency and a polarisation to use. Before the requested time the satellite transmits a test signal. They tune to that frequency and tune their antenna for max signal, then they transmit a test signal to the satellite, the satellite operator verfies that their signal meets spec.'s and then the channel is theirs for how every long they have paid for. It's more of a hands on operation than I would have though. Their main busness is sports as they up load all the sports events here on campus and for many of the non sports events like news events and some classes that are held via satellite. John Passaneau W3JXP |
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