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Satellite Freq allocation

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dapo
Ex Member


Oct 29th, 2009 at 1:38pm  
Hi,
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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Eric Johnston
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Reply #1 - Oct 29th, 2009 at 4:23pm  
A particular place on the ground may be within the coverage beams of perhaps as many as 40 satellites, all operating on the same frequency and on both polarisations.

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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dapo
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Reply #2 - Oct 30th, 2009 at 2:11pm  
Thanks for the explanation. My RX Freq is 1416621000 i could see a form of interference on my spectrum analyser. There is no microwave antenna around. Could the source of interference be from another satellite transmitting at my RX frequency. If so what can i do? I will appreciat your contribution.

Best regards

Dapo
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Eric Johnston
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Reply #3 - Oct 30th, 2009 at 6:05pm  
The frequency you mention 1416621000 (Hz) refers to your wanted receive frequency in your cable from the LNB towards the indoor modem.  With a typical C band LNB the satellite downlink frequency will be 5150 - 1416.6 = 3733.4 MHz.

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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« Last Edit: Oct 31st, 2009 at 10:55am by Admin1 »  
 
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