You find the
( here used ) formulas in THE SATELLITE handbook, ARRL edition (
ISBN 0-87259-658-3 8-12 & 8-13 ). These calculations deliver
good results.

First we calculate
the path loss ( dB ).

Path loss is
: 32.4+20logf+20logrho.

Note : in this
site you can find a small script ( JAVA ) which calculates this
formula above. To see : click here

f
is the used ( up or down ) frequency ( MHz ) and rho
is the slant range ( km ).

Second the power
sum,
ps ( dBmW ).

Power sum is
: actual transponder power ( dBmW ) plus satellite antenna gain ( dBi ) plus
ground station antenna gain ( dBi ) minus
path loss ( dB ).

Remark : use
dBi if comparing iso, for instance
a 0.5 wave dipole in free space delivers ca 2.15 dBi. An antenna
with a gain of 6 dBd has 8.15 dBi. Iso and the 0.5 wave dipol are
standards.

Third the
total received noise power,
trnp
( dBmW ) and
( at the end ) the Signal Noise Ratio,
SNR (
dB ), see THE SATELLITE handbook or below.

Signal
Noise Ratio
is : ps minus trnp.

If you want
to analyse the own situation : below you find the required formulas.

Calculation
total received noise : Ptrn
(
trnp above ).

Ptrn(mW)=k•Trs•B=>Ptrn(dBmW)=10log(k•Trs•B)=10logk+10logTrs+10logB.

k is the Boltzmann
factor 1.38•10to(-20) in mW/Hz•K (K=273+C), Trs is the resulting
system temperature in K, B is the bandwidth
in Hz. Remark : 10logk delivers a -198.6 value.

Calculation
resulting system temperature : Trs.

The receive
system
noise ( a hardware result ), cosmic noise
( received by the antenna ) and earth radiated noise going into the lobs of the
antenna. ( for instance earth radiated noise at a temperature -5C=268K
). Attention, Trs expressed in temperature, see below.

Trs=Tr+Tsky.

Tr
is the receive system temperature in K, Tsky is the temperature of the sky in
K. Tsky is 'seen' by the antenna.

Calculation
receive system temperature : Tr.

Tr=Ta[10to(Ft/10)-1].

Ta
is the ambient
temperature ( hardware ) in K, Ft is
the system noise figure in dB, concerning
the coax, preamp(s) and input(s) in the circuit. Figure 11.3 ( 11-2
), in the satellite handbook, explains more. Very clear is : every
module must be on the right place !
The part ( in the formula above ) :
10to(Ft/10) is the system
noise factor fT.

Calculation
Signal Noise Ratio : SNR.

SNR(dB)=Prs(dBmW)-Ptrn(dBmW).

Prs
is the received signal power,
the power sum,
ps ( see
above ).

You find some
calculations ( examples ) in : the VUSat transponder ( fourth submenu
button ). And a script ( JAVA ) which calculates all the formulas, described.
Also reachable via the fifth submenu button.