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lnb summing

 
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Peter Mason
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Joined: 01 Oct 2015
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PostPosted: Thu Oct 01, 2015 10:17 am    Post subject: lnb summing Reply with quote

I would like to take an array of initially two (2) satellite lnb's and add them. Using the principal of keep it simple I would like to use a passive circuit but anything other than joining the coax together in a Y configuration simply antenuates the signals.

I have a boat and before I buy a full blown steerable dish that wiil flatten the battery at anchor I would like to use a set of horn arrays (selfsat ) and widen the field of view by pointing them appart then summing the lnb signals. The automatic static steering on a wider array can wait until this works.

How can I properly passively add the lnb signal?

Why can't we find extra terrestrial signals?

Peter

The wavelength that I estimate at 1.5 Mhz is 5mm so I presume layout is critical.
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hagster
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Joined: 09 May 2015
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PostPosted: Tue Oct 06, 2015 11:59 pm    Post subject: Reply with quote

A Wilkinson Divider will add your 2 signals. But I doubt the result will be what you are expecting.

Remember that you are adding complex vectors that will sometimes add constructively and sometimes destructively. The end result will be an interference pattern of the 2 not a simple wider beam. The actual resultant pattern will depend upon the exact physical construction.

If you did achieve the wider field of view, this would also come with a reduction in gain.
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Peter Mason
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PostPosted: Wed Oct 07, 2015 3:21 pm    Post subject: Reply with quote

Thank you for the reply.
I did not think of the complex nature of the signal but would you expect variable phase signals from two signals received some cm apart from the same point source?
The wilkinson divider looks like the sort of thing I was seeking.
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hagster
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PostPosted: Wed Oct 07, 2015 5:32 pm    Post subject: Reply with quote

Peter Mason wrote:
Thank you for the reply.
I did not think of the complex nature of the signal but would you expect variable phase signals from two signals received some cm apart from the same point source?
The wilkinson divider looks like the sort of thing I was seeking.


Peter.

Thinking about this a bit more I think the answer misht be more complex than I first suggested. I would have to spend some more time to understand the problem fully. Maybe someone else on this forum could give a better answer from their own experience.

Basically an LNB is a low noise amplifier followed by a down convertor that mixes the gigahertz signal down to a much lower IF so it can be fed down long lengths of cheap nasty coax without too much loss. Hence when you mix the signals with your coupler/divider(you will need to know this IF frequency so you can choose an appropriate divider/coupler) you are mixing two signals that have been downconveted from seperate LO's and are hence not synchronised. This will generate all sorts of artifacts in the signal. I really don't know what the effect of this will be on your signal, but I doubt it will enhance your setup.
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helix
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PostPosted: Wed Oct 14, 2015 2:50 pm    Post subject: Reply with quote

I think you will have a lot of problems with grating lobes. You are making an array of two dishes. But the phase centers of the two dishes are quite far apart in terms of wavelengths. So because there will be so many, narrow-angle grating lobes, there's really no guarantee that "pointing" your array at the satellite will produce any improvement whatsoever. In fact, I would expect terrible performance as the boat moves/rocks and the satellite moves through the sky.
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Peter Mason
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PostPosted: Wed Oct 14, 2015 5:12 pm    Post subject: Reply with quote

I am getting the impression it will not work and using a tracking antenna is an approach that would be more sucsessful. Thank you to all contributors.
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