Satellite Internet

Satellite Internet Forum.

Welcome, Guest.
Welcome to this satellite broadband discussion forum. Wherever you are and whatever your problem we are here to help each other. Connecting to the internet via satellite is not always easy but is critically important to those in remote places or with poor terrestrial infrastructure. Both service providers and customers are encouraged to contribute. Register at the bottom of the forum home page if you wish to contribute or ask question. May 2018: GDPR: Updates to Privacy and Cookies policies: As you may know, a new EU data protection law called GDPR will apply from Friday 25th May 2018. As part of satsig's commitment to protecting the privacy of site visitors and forum members, I have therefore updated the Privacy and Cookie policies. There are now links leading to these policies: Disclaimer, Terms of Use and Privacy, Forum User Agreement, Forum Rules and Cookies at the bottom of the home page and all forum pages. Read the Forum rules.
      Satellite internet forum          
Pages: 1

Equipment sizing for VSAT HUB

(Read 2643 times)
Ex Member
Ex Member


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.

Martin
Back to top
 
 
IP Logged
 
Eric Johnston
Senior Member
***
Offline


Personal text from Profile,
Options, Top line

Posts: 2108
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.
Back to top
« Last Edit: Mar 30th, 2013 at 8:47am by Admin1 »  
 
IP Logged
 
Pages: 1