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Subject: Re: Astar 2a & 2b (North & South)


Author:
Bobben
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Date Posted: 06:19:26 11/22/03 Sat
In reply to: Charles 's message, "Re: Astar 2a & 2b (North & South)" on 23:29:26 11/20/03 Thu
 >Thank you for your reply Roger but can you assist
>further by commenting on the second part of my
>question namely: "Why can I get some North
>transmissions and not others" Thanks

five is on 2B North
Channel 4 is on 2A North

As Roger wrote the 2B & 2A does not have 100% identical beams. The footprints is a "design goal" and not something based on real measurements after the satellite
is placed in orbit.

five is also higher up in the band and will be more affected by attenuation in the coax cable.


Some Digibox'es could also have a problem with
selectivity, the receivers capability of removing unwanted signal.

It means that if you live far south (like Spain) and receive a north beam transponder any adjacent southbeam transponder could be 10dB or more stronger which will be seen as noise in the receiver because the bandwidth filter (36MHz) wont be able to filter/remove all unwanted signal from the adjacent transponders. It could also affect the receivers AGC and AFC.

five has got two adjacent southbeam transponders on the same polarisation. Its the same situation with Channel 4 but as Roger wrote the beams are not 100% identical.

In Norway I think I have measured the North beam to be up to 8-12dBW stronger than the southbeam.

Another effect is that if your LNB has a crosspol isolation of 20dB when receiving a northbeam there will probably be another southbeam on the other polarisation making the real crosspol isolation 10dB or even less if the difference between north/south beam is more than 10dBW.

So here in Norway with 20dB isolation the real isolation in some cases is only 8dB. The fact that the transponder on the other polarisation is offset in frequency helps to make the problem less because if the same center frequency was used for V/H then there would have been wery difficult to receive the southbeam here.

I would expect the difference between north/south beam is greater than the difference between north/southbeam in southern Spain because southern Spain is much far outside the northbeam footprint than southern Norway is with the southbeam. This will make the selectivity and crosspol isolation problem even worse in southern Spain compared to southern Norway.

If the isolation get too poor even a big antenna wont work. I have a LNB which should give 25dB isolation but it also depends on the antenna and if the offset antenna is pointed wery accurate in azimuth. Some systems could give 30dB isolation or even more but its usually 20dB with normal Universal LNB's.
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Re: Astar 2a & 2b (North & South)Brian00:20:00 11/27/03 Thu

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