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*Subject*: Re: [amsat-bb] Sun noise*From*: "John Santillo" <u1004467@xxxxxxxxxxx>*Date*: Sat, 21 Dec 2002 14:52:02 -0500

Hello all, can we take this a little bit further. From this information is there a way to calculate the G/T of this system? I've seen this figure of merit many times before but have never fully understood how to calculate it. Thanks, John N2HMM ----- Original Message ----- From: "Tom Clark, W3IWI" <w3iwi@toad.net> To: "'Eric Van Offelen'" <eric@vanoffelen.com>; "'Amsat-bb'" <amsat-bb@AMSAT.Org> Sent: Saturday, December 21, 2002 14:15 PM Subject: RE: [amsat-bb] Sun noise > > > > How much sun noise should i expect on 2.4 GHz with the > > following set-up: > > - 90 cm offset dish > > - +/- 0,2 dB estimated loss in adapters > > - 0,7 dB NF preamp > > Merry Christmas and 73's > > Eric > > EA5GIY > > Eric -- let's calculate it from first principles: > > Your dish is 90 cm and the wavelength is 13 cm. Therefore your dish has a > beamwidth of a bit more than 13/90 radians = 8.3 degrees. For a circular > aperture, Airy's criterion would predict the diffraction limited beam to be > 1.22 times this large, or about 10.1 degrees in diameter. To be simple, I'll > call this 10 degrees. > > The radio sun at 13 cm, depending on sunspot count, is about 40,000 Kelvin > and is about 1 degree in diameter. Therefore the sun fills about (1/10)^2 or > ~1% of the beam. Therefore, the sun should contribute about (1/100)*40000 = > 400 degrees Kelvin if you dish has perfect (100%) aperture efficiency. > > Since I know nothing about the quality of the feed and dish, I will guess > that you actually have about 50% aperture efficiency (meaning that half of > the power is in the main beam and half is scattered into sidelobes). > Therefore the sun probably contributes ~400/2 or about 200 Kelvin. > > You say the preamp has 0.7 db NF and you have 0.2 dB connector loss. You do > not say how long the short piece of coax between the feed and the preamp is, > so I will guess that it adds another 0.1 dB of loss. Therefore your real > Noise Figure is 0.7+0.2+0.1 = 1 dB. > > Many graphical and slide rule devices make the next calculation be easier, > but we go back to first principles to find that the correspondence between > Noise Figure and Temperature is > T = 290 * [10^(Noise Figure/10)-1] Kelvin > Which, for NF=1, is 75 Kelvin as an estimate of your system temperature. > > So this says that the sun should have S/N = 200/75 = 2.66 times the noise in > the receiver. Therefore if you have a true power detector on the output of > the receiver and set it to 1 volt = receiver noise off the sun, it should go > up from 1 volt to (1+2.66) =3.66 volts when pointing at the sun, or a > Yfactor of 3.66:1. > > You can check this by making the dish look at large trees with many leaves. > These trees will contribute about 200-250 K of noise, but since they fill > the entire ~10 degree beam, they will also contribute about the same noise > as we predict for the sun. > > Another idea is to have a 90 cm wide person stand in front of the dish > completely covering the dish. The person will contribute about 300-320 > Kelvin of noise. If you happen to transmit, this makes the person bring a > new meaning to the words "dummy load" ;<} > > Hope this helped -- seasons greetings de Tom W3IWI > > ---- > Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA. > To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org > > ---- Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA. To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

**References**:**RE: Sun noise***From:*Tom Clark, W3IWI

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