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VSAT technology and installation >> Dish pointing and alignment >> Using 1 Antenna with two Modems

Message started by Caiphus on Apr 21st, 2008 at 6:16pm

Title: Using 1 Antenna with two Modems
Post by Caiphus on Apr 21st, 2008 at 6:16pm
Hi Guys...

Is it possible to install a VSAT using 1 x Antenna, 1 x BUC, 1 x LNB and then put a Splitter to connect the signal to two Modems? I have two iDirect 5100 will be using iSCPC to connect to another remote site and one will be for internet access.

Title: Re: Using 1 Antenna with two Modems
Post by USN - Retired on Apr 21st, 2008 at 8:26pm
I can't speak for iDirect - and I've made statements like this that have later come back to bite me. But typically the answer would be no. Multiple connections with a single reflector are not unusual. But in most cases, each modem is associated with an independent carrier. That is; it's own uplink equipment, downlink equipment, and associated splitters and combiners. I suspect the iDirect hardware selection may not be sophisticated enough to fulfill such a requirement.


Title: Re: Using 1 Antenna with two Modems
Post by Eric Johnston on Apr 21st, 2008 at 10:36pm
On the receive side, the signals from the one LNB will split successfully into the two modems, with about 3.7 dB loss assuming a 2 way splitter.   You will need a splitter that feeds the DC power supply to the LNB via one side only and blocks the DC path on the other.  You may well need a PLL type LNB for the SCPC mode.  If the LNB needs an external 10 MHz reference, you need to make sure that both modems don't send a 10 MHz reference simultaneously, and that your chosen splitter can actually pass the 10 MHz reference.

On the transmit site there are big problems with power supply, 10 MHz reference and intermodulation.  Both carriers will be on simultaneously so the BUC rating needs to be about 5 times the composite power to keep intermodulation interference under control and you will need considerable skill to set this up.   You may well need a 40 watt BUC and associated separate power supply.  It will probably be cheaper to use two antennas and two lower power BUCs.

I do not recommend trying to use two modems, both with transmit on, unless you really know what you are doing.

I would investigate further.  I think I have heard mentioned in this forum that iDirect can have dual receivers in one modem for mesh operation.  Maybe one transmit modem can communicate occasional bursts direct to the large hub (for internet access) and also transmit a regular series of interleaved bursts on another freqeuncy to another site (iSCPC) for mesh communication to another remote site.  Note that the link budget for the remote to remote link will require that the dishes / BUCs at both ends are much larger.  I hope TDMAMike can help here.

Can the second modem operate in receive only mode and can the transmit modem frequency hop between sending bursts to the hub and to the other mesh site?.  Obviously this is not true continuous SCPC on the mesh link but a steady stream of bursts at a high bit rate can have equal throughput.

Best regards, Eric.

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