Re: [amsat-bb] The effect on MDS by illumination

[Anthony Monteiro <aa2tx@xxxxxxxxxxxx>]

Hi Ed,

Since you have accounted for fractions of a dB, you might
as well include the loss due to gaseous absorption in the
atmosphere at 2.4GHz in calculating MDS.

A good approximation is

                 Loss = .35/sin(x) dB

where x is the elevation angle between 5 and 90 degrees.

This means you should really add an extra 4 dB of signal
loss at low elevation angles, somewhere around .5 dB at 45 degrees
and 0 dB at really high angles.

Tony AA2TX

---

At 11:02 PM 8/20/01 -0800, Edward R. Cole wrote:
>This has been a topic of interest for some time, so I decided to finally
>address it:
>
>I added the calculations of Trx and NF for general info:
>----------------------------------------------------------------------------
>--------
>Let's analyze a theoretical AO-40 2.4G receiving system:
>l1 = coax loss = 0.1 dB L1 = 1.0
>N1 = Preamp NF = 1 dB   T1 = 75.1
>g1 = Preamp gain = 30 dB        G1 = 1000
>l2 = coax loss = 3 dB           L2 = 2.0
>N2 = converter NF = 6 dB        T2 = 864.5
>g2 = converter gain = 20 dB     G2 = 100
>l3 = coax loss = 3 dB           L3 = 2.0
>N3 = IF receiver NF = 8 dB      T3 = 1539.8
>To = 290 K {ambient temp}
>
>First we need to convert all the loss, gain, and noise figures from dB to
>ratios, using:
>Tn=To[10^(Nn/10)-1]
>Ln = 10^(ln/10)
>Gn = 10^(gn/10)
>for n = 1, 2, 3 above
>
>And solve for Trx:
>Trx = To*[L1-1] + T1*L1 + To*L1/G1*[L2-1] + T2*[L1*L2]/G1 +
>To*L1*[L2/G1*G2]*[L3-1] + T3[L1*L2*L3]/G1*G2
>
>Spreadsheets make quick work of lengthy calculations:
>http://www.setileague.org/software/spreadsh.htm (cascade.xls)
>Trx = 85.7 K
>NF = 10*Log(1 + Trx/290) = 1.12 dB
>----------------------------------------------------------------------------
>---------------
>Now the main topic:
>
>MDS stands for minimum detectable signal.  You could say that MDS is the
>signal
>level when receiver S/N = 0 dB.  It is the level where noise power equals
>signal power [Pn = Ps].  System noise power can be characterized by system
>noise temperature through Plank's Law:
>
>Pn = k*T*B
>Pn = system noise power in mw
>k = Boltzmann's constant = 1.38*10^(-20) mw/Hz-K
>T = is our system noise temp in K
>B = system bandwidth in Hz = 2500 Hz for SSB
>
>Well, T consists of receiver noise temp [Trx], plus sky noise [Tsky], and
>ambient noise [Tgnd]:
>T = Trx + Tsky + Tgnd
>
>Tsky at 2400 MHz is mostly celestial noise...about 10K
>Tgnd is thermal noise from the environment; mostly the thermally hot earth.
>To = 290 K is the accepted value for the temp of the earth.
>Tgnd ~ To/Gsidelobe
>
>OK lets say that our sidelobes are all below 10 dB.
>Gsidelobe = 10
>Tgnd = 290/10 = 29 K
>
>so T = 85.7 + 10 + 29 = 124.7 K
>
>We can express Pn in terms of dBm by:
>Pn = 10*Log [kTB] = -198.6 + 10*Log(124.7)+ 10*Log(2500)
>Pn = -198.6 + 21 + 34
>Pn = -143.6 dBm
>
>Hey that's pretty good sensitivity.
>
>Now lets change our illumination so that Gsidelobe is -15 dB:
>Gsidelobe as a ratio is 31.6
>
>Tgnd = 290/31.6 = 9 K
>T = 85.7 + 10 + 9 = 104.7 K
>
>Pn = -198.6 + 10*Log(104.7) + 34
>Pn = -198.6 + 20 + 34
>Pn = -144.6 dBm
>
>So we just lowered our MDS from -143.6 to -144.6 dBm, that's 1 dB
>
>It turns out that lowering our sidelobes to -15 dB reduces dish efficiency
>from
>55% to 50%.
>That mean our gain has dropped by 10*Log(50/55) = 0.41 dB
>
>What does this mean?
>Pn dropped 1 dB and Gant dropped 0.41 dB, so MDS improved a net .59 dB
>
>Not very much, but any improvement helps.
>
>Try this for a helix with -6 dB sidelobes to a feed with -15 dB
>sidelobes...{left as an exercise for the student...doesn't this just remind
>you
>of school days?}
>
>Ed
>Check out similar analysis by Dr. Paul Shuch, Exec. Director of the
>SETI-League
>[three paragraphs under Fig.2]:
>http://www.setileague.org/hardware/feedchok.htm
>
>
>
>
>----
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