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VSAT technology and installation >> iDirect Forum: hubs and terminals >> BUC for small antenna

Message started by vydrik on Apr 28th, 2011 at 12:51am

Title: BUC for small antenna
Post by vydrik on Apr 28th, 2011 at 12:51am
Hi, I know that there is a power limitation for sub-meter antennas, to avoid interference.

Is it a BUC limitation? What is the power limits for 60cm and 75cm antennas?


Title: Re: BUC for small antenna
Post by Eric Johnston on Apr 28th, 2011 at 2:33pm
For 1m antennas or larger I believe the limit is -14 dBW per 4kHz into the VSAT antenna regardless of size.

So if you wanted to transmit a carrier bandwidth of 200 kHz the maximum power is 10log(250/4)-14 dBW = 3 dBW = 2 watts

For smaller dishes (at Ku band) the main beam is becoming so broad that it starts to directly include the adjacent satellites. You need to talk to the satellite operator about power limits and carrier bandwidth.  It may be necessary to use spread spectrum techniques to keep the interference down.

If you need very small antennas why not consider Ka band operation such as Hylas ?  The higher frequency of Ka band means that a smaller transmit dish is feasible.

Best regards, Eric.

Title: Re: BUC for small antenna
Post by vydrik on Apr 28th, 2011 at 6:12pm
Eric, many thanks.

Yes, I'm interested in the power limits to prevent an interference with adjacent satellites, is it possible to estimate it before I go to the satellite operator? Is there a rule of thumb for this?

Title: Re: BUC for small antenna
Post by Eric Johnston on Apr 28th, 2011 at 7:06pm
Actual limits are negotiated between satellite operators and vary according to satellite spacing, beam sensitivites etc.  The process is called "intersystem coordination" and may take many weeks prior to an orbit slot and thus satellite being approved. Different figures will apply according to what is agreed. The figures may differ from one transponder to another as a transponder operated in high gain mode for VSAT return links will have quite different signal levels compared with a transponder used for one TV broadcast or big VSAT outlink carrier.

The -14 dBW/4 kHz figure above I believe applies to an agreement in-between a number of US satellite operators over America.

If you are contemplating using a small antenna make sure you have measured transmit radiation patterns for your own antenna.  What is its gain on axis ?  What is its gain at 1, 2, 2.2, 2.3, 2.5, 2.7, 3, 3.1, 3.2, 3.3, 3.4, 3.5, 4, 4.5 5, 5.5, 6 deg off axis etc. ?

Your interference into the adjacent satellite depends on your transmit sidelobe gain and your transmit power per 4 kHz into your feed.  The effect of the interference on the other satellite service depends on their wanted signal level.  If they have spot beams with high sensitivity on their uplink then their wanted signal will be very low power spectral density so your uplink must be correspondingly restricted. Alternatively if they have a broad uplink beam with low sensitivity then their uplink signal will have a high power spectral density and to avoid interference into your satellite they may be using a large 6m - 10m dish for their uplink.

Best regards, Eric.

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