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R: Downlink budget calculation




----- Original Message ----- 
From: Joel R. Stanley <j_r_stanley@wyan.org>
To: <amsat-bb@AMSAT.Org>
Sent: Wednesday, June 30, 2004 8:57 PM
Subject: [amsat-bb] Downlink budget calculation


> Hello,
> 
> I'm trying to calculate how much gain (from the antenna and preamp) I
> need for satellite reception.
> 
> We have the link budget equation Prx = (Ptx + Gtx) + Grx - Afs - Am,
> where Prx is the received power (dBW), Ptx + Gtx is the EIRP (dBW), Grx
> is the receive gain (dBi), Afs is free-space attenuation loss (dB), and
> Am is miscellaneous loss (dB).  My receiver specifications state that
> its sensitivity is "better than -15dBu for 12dB S+N:N".
> 
> Okay, a dBu is dB relative to one microvolt.  How does this relate to
> Prx in dBW?  If I take that -15 dBu, and convert to volts, then
> calculate power assuming 50 ohm impedance (V^2/50) and convert that to
> dBW, is that the correct value to use to find Grx?
> 
> Thanks and 73 de Joel KC8DQH.
> 

Hi Joel, KC8DQH

One microvolt attenuated by 15 dB or -15 dBuV is corresponding to
 
    (-15/20)                                           -6
10               = 0,177 uV  = 0,177 x 10     = 0,000000177 volt

Assuming 50 ohm the input impedance of your RX and assuming that
the above voltage is the applied voltage needed by your RX to receive 
a signal with a ratio (S+N)/N of 12 dB than the power applied to the
input of your receiver must be
                              2                             -16
P= 0,000000177     / 50 = 6,26 x 10        watt

and hence
                                   -16
10 log       6,26 x 10         =  -152 dBW  or  -122 dBm
            10


Best 73" de

i8CVS Domenico

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