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| SatSig Topics Index › LinkStar Forum: VSAT hubs and terminals › Satellite Freq allocation |
SatSig topic: Satellite Freq allocation(Read 6438 times) |
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Oct 29th, 2009 at 1:38pm
Can someone inform me if there is any world body responsible for satellite frequency allocation. since the C-Band freq range for 5.8GHz to 6.4GHz ,If two satellites have beams over the same spot, i dont think they will transmitt to earth at the same frequency else there will be interference on the receive frequency. if this is to be avoided, there must be a central body that does the freq allocation. My question is, is there a central body for freq allocation for satellite communication? or how is interference avoide or dealt with. Thanks Dapo |
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Oct 29th, 2009 at 4:23pm
In order to avoid unacceptable interference, your earth station antenna has a narrow directional beam, which you aim at just the wanted satellite. Your dish must meet certain specifications for high main beam gain, low off-axis sidelobe gain, and high polarisation discrimination to keep the interference to/from adjacent satellites and cross-pol interference to/from the same satellite within acceptable levels. Before a satellite is launched the owners must apply, via their government radio regulatory agency, to the ITU in Geneva, proposing a satellite design, its orbit location and a set of carriers, interference limitations and earth station specifications. Other nearby satellite owners then get a time to review and comment. Potential interference problems are resolved by a series of inter-system co-ordination agreements, whereby the network design is modified so that it neither causes or receives unacceptable interference. Once co-ordination is successfully achieved, the orbit registration is completed and the new satellite must be launched. Normally, carriers may be put at any spare frequency in a transponder but common sense indicates to avoid putting very high spectral density carriers, such as FM-TV, opposite to sensitive low spectral density carriers. Spot frequencies used for cross-pol CW testing must be chosen to match gaps in cross-pol spectrum. Best regards, Eric. |
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Oct 30th, 2009 at 2:11pm
Best regards Dapo |
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Oct 30th, 2009 at 6:05pm
Interference from terrestrial WiMax services is possible since many countries have licenced WiMax in the same C band as for satellite downlinks. Stupid, I know, but that is politics ! The lower end of the band 3.4-4.2 GHz is worst affected. WiMax causes severe interference into C band many African and Middle East cities. The WiMax interference may be on the same frequency as your wanted carrier. If the interference is mild you might reduce it successfully by hiding your dish at ground level behind walls or asking if your service provider can provide your service on a different downlink frequency whivh is presently clear of interference. If the interference is on a different frequency from your wanted carrier but is so extremely high that it overloads the LNB then the solution may be a front end microwave filter or different LNB. An extended C band version 3.4-4.2 GHz may suffer problems while an older standard band LNB 3.7-4.2 may be OK. The difference is similar to adding a blocking filter on the front against 3.4 - 3.7 GHz. Terrestrial interference may be investigated by removing the complete feed system and LNB etc from the antenna. Stand near the antenna and point the feed in all directions while watching the spectrum analyser. Interference from the opposite polarisation on the same satellite is a possibility, particularly if you are using linear polarisation. Simply rotate your feed to get the alignment correct. If you have circular polarisation then no adjustment is possible. A few, cheap, circular polarisation feeds have poor isolation (about 16 dB) while all normal ones have isolation around 27 - 30 dB. Interference from an adjacent satellite is possible if your dish is too small, distorted, mis-pointed or out of focus. Best regards, Eric. |
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Email me:eric@satsig.net |