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Noise temperature and G/T calculations
5th Oct, 2017 at 12:29pm
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For many years I have had a noise temp calculator page at

http://www.satsig.net/noise.htm

The way the noise temperature contribution of the waveguide and the  noise temperature contribution of the cable have been the subject of  a number of emails providing alternative methods of doing the calculation. Quite which is right or wrong is unclear to me.

If you are an expert or know someone who is, please ask them to comment.

Code
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var antcontribution = 1.0 * form.anttemp.value * Math.pow(10,- form.wgloss.value / 10.0);

var wgcontribution = 1.0 * form.wgtemp.value * ( Math.pow(10,form.wgloss.value / 10.0) -1.0);

var lnbcontribution = 1.0 * lnbtemp;

var cablecontribution = (1.0 * form.cabletemp.value * (Math.pow(10,form.cableloss.value / 10.0) - 1.0))/( Math.pow(10,form.lnbgain.value / 10.0)) ;

systemp = antcontribution + wgcontribution + lnbcontribution + cablecontribution + modemcontribution;


form.gain.value = round(form.antgain.value - form.wgloss.value,3);

form.gont.value = round(form.gain.value - 10*Math.log(systemp)/Math.log(10.0),3);

 



The above are extracts from the javascript code embedded in the html page.  To see all the code, go to the page and do View Source (Right Click in Chrome)

To test the equations I have made two examples below. I would have though that they should both give same result, with cable instead of waveguide in the second example.  The calculation results are different. Why ?



The noise from a 1 dB attenuator is 75K at the output but what at the input ?

Best regards, Eric



« Last Edit: 5th Oct, 2017 at 5:03pm by Admin1 »  

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Welcome to the new forum

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Re: Noise temperature and G/T calculations
Reply #1 - 5th Oct, 2017 at 6:52pm
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I have spent all day trying to figure out what is wrong.
The latest modification is :

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 var cablecontribution = (1.0 * Math.pow(10,form.cableloss.value / 10.0) * form.cabletemp.value * (Math.pow(10,form.cableloss.value / 10.0) - 1.0)) /  ( Math.pow(10,form.lnbgain.value / 10.0)) ;
 



If you now set the noise temp and attenuation of everything to zero.

Then set either the waveguide  to 1 dB and 290K
or

cable to 1 dB and 290K you get the same answer for G/T !!



Feed back welcome.
« Last Edit: 6th Oct, 2017 at 1:21pm by Admin1 »  

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