Link equation

[geoffrey]

Could anyone who understands the link equation please comment on the following regarding dish size for Astra 28.2E:

In Israel on a ChannelMaster 1.2m offset dish with an Invacom C120 mount LNB fitted to a ChannelMaster Offset feed I have measured (using a Satlook Digital NIT) a carrier to noise ratio (C/N) of 4.6dB on 11.973V

What should I get using the same LNB on a Prime focus feed on a 3.6m prime focus dish?

The reason I ask is that the figure I have measured on the 3.6m dish is a C/N ratio of 9.0dB.

The 3.6m dish was an old DH 2 piece dish that was missing its correct joining strip and I suspect that its performance is compromised.

Should I be getting something nearer to a C/N of 14dB?

 

[bignick]

Could anyone who understands the link equation please comment on the following regarding dish size for Astra 28.2E:

In Israel on a ChannelMaster 1.2m offset dish with an Invacom C120 mount LNB fitted to a ChannelMaster Offset feed I have measured (using a Satlook Digital NIT) a carrier to noise ratio (C/N) of 4.6dB on 11.973V

What should I get using the same LNB on a Prime focus feed on a 3.6m prime focus dish?

The reason I ask is that the figure I have measured on the 3.6m dish is a C/N ratio of 9.0dB.

The 3.6m dish was an old DH 2 piece dish that was missing its correct joining strip and I suspect that its performance is compromised.

Should I be getting something nearer to a C/N of 14dB?

I don't think you will get anywhere near 14dB. The dB scale is logerithmic (spelling?) Which means that a rise of just over 1.2dB I'm told is about a doubling of strength. I think 9dB is therefore more than adequate.

Anyone that knows better please feel free to put me right

Cheers

Nick

PS The NIT gives Signal to Noise (SNR) readings not CNR - Just being pedantic, sorry

 

[geoffrey]

Not pedantic at all !

Its possible to measure carrier/ noise ratio C/N manually with the Satlook by measuring the db range between the noise floor and a specific peak. Unless I am missing something I think that this is the C/N measurement used in the Link Equation. The signal to noise measurement is given directly by the Satlook when its managed to lock a signal but seems to be a different thing altogether.

 

[geoffrey]

One thing I missed out that seems to be important in the calculations are the elevations angles for the dishes. The elevation angle for the offset 1.2 ChannelMaster was 30.4 degrees and for the prime focus 3.6m DH dish 53 degrees.

 

[geoffrey]

Someone may be interested in my progress with these calculations:
According to http://www.satellite-calculations.com/ I think that the measurements I have given for C/N are really C+N/N and need to be corrected. This means that my measurement for the ChannelMaster 1.2m C/N is actually 2.75dB and the DH 3.6m C/N 8.42dB. The efficiency of the ChannelMaster dish is probably 66% and the DH prime focus 44% (unless anyone knows better). The antenna noise for the ChanelMaster is 34K and the DH 28K for similar true elevations. Using these numbers in the SMW calculator at www.smw.se it seems that the DH dish should be giving me a 10.6 C/N and is therefore behaving as if its a 3.0m dish when it gives me 8.42dB.