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Message started by haylingchrist on Jun 26th, 2008 at 7:47pm

Title: Dish size and BUC power
Post by haylingchrist on Jun 26th, 2008 at 7:47pm

I'd like to know if (all else being equal) it's generally necessary to increase BUC power as the dish size goes up?

I've currently got a 1-way setup but I'm thinking about going 2-way and would like to understand the theory a bit better.

If I double my dish area to improve performance in a marginal location, I'd expect the received signal strength to roughly double. However, I don't think this is going to make much difference on the transmit side. I'm still transmitting with the same power and the larger dish might help efficiency a bit but not much? Should I expect to have to double BUC power to match the increased dish size?


Title: Re: Dish size and BUC power
Post by Eric Johnston on Jun 26th, 2008 at 9:12pm
If you double the dish area you need to reduce the transmit BUC power by half in order to continue transmitting the same uplink eirp power towards the satellite.

If you double your dish area and retain the same BUC you will reduce your normal transmit power by half, i.e. -3 dB, by adjusting down the output level from your modem.   During rain your modem may then be able to automatically increase the power by up to 3 dB, but just during rain, in order to provider higher %age time availability.

Assuming you have a small dish...    On the receive side, the doubling of the dish area will improve the overall C/N or Eb/No by nearly 3 dB (i.e. nearly double the quality).   You don't get quite 3 dB improvement as the signal has already suffered some degradation due to noise on the uplink and from intermodulation noise added in the satellite transponder.  Various interferences also add noise.   In particular, with a larger receive dish, you will find adjacent satellite interference on the downlink is reduced.

Note that larger dishes need more skilled assembly and alignment.  There is no point buying a 2.4m dish and only getting the performance of a 1.8m dish due to poor panel alignment / distortion.

Best regards, Eric.

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