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Equipment sizing for VSAT HUB

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Posts: 26
Mar 28th, 2013 at 5:12pm  
Considering if we are designing a VSAT  network and selecting HUB and remote antenna what is the relation between HUB and remote antenna sizes. I was told we always keep large antenna sizes on HUB to have better RX SNR at remote. Can anyone explain what is the correct method for selecting antenna size although we need to consider link budgets.

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Eric Johnston
Senior Member

Posts: 2109
Reply #1 - Mar 29th, 2013 at 7:56pm  
The way I do it is to make a spreadsheet link budget and then duplicate the figures column so that you have two link budgets side by side. One for the hub to remote carrier and one for all the return link carriers merged together in an imaginary larger bandwidth carrier. e.g.  4 Mbit/s outlink and 1 Mbit/s return link. The imaginary 1 Mbit/s return link might represent 4 x 256k actual return link TDMA carriers.

The satellite operator will tell you the total uplink pfd that you are allowed and the corresponding total downlink eirp power for your leased bandwidth.  You can decide to distribute this power between your outlink carrier and your imaginary composite return link carrier.

Write this into the spreadsheet so that the total satellite uplink pfd and total satellite downlink eirp are displayed relative to the values you have been told you can have.

Start with earth station sizes likely to work, like a 4.5m or 6m hub and 1.2m or 1.8m remotes and then adjust all the variables till you have good working links in both directions. You can make adjustments for different dish sizes and BUC powers, modulation and bit rates, as per your personal preferences and inputs from your colleagues. If marketing insist on extremely small remotes then the implications in terms of capacity can be demonstrated.  If finance insist on extremely low operating cost the effect in terms of larger dish sizes, high quality linear BUCs etc can be demonstrated. If customers want high reliability, evaluate the effect of larger rain margins and uplink power control on dish size and peak BUC power.

Expect to make at least 10 printouts of alternative good system designs, each meeting different assumptions. The 'best' solution will almost certainly be a compromise that will hopefully be able to meet changes in customer expectaions over the next 10 years or however long you intend to operate this network.

Do all the spreadsheet and formulas yourself so that you understand what you are doing.

Although link budgets will give dish sizes for receiving, the dish sizes you need for transmit may be larger. There are two reasons. First you may decide that it is cheaper to use a larger dish and lower power BUC. Don't forget electricity and air-conditioning (cooling) costs.
Second, and this an over-riding constraint, you must not transmit unacceptable side-lobe interference into nearby satellites. The interference limit will be specified by your satellite operator in terms of an off-axis eirp power flux density formula involving the log of the off-axis beam angle.

Best regards, Eric.
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« Last Edit: Mar 30th, 2013 at 8:47am by Admin1 »  
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