Free Space Pathloss Calculation and EME link budget
Part 1 - JAVA application
Please let me know, if you like this calculator. The JAVA source code of EME System Calculator is also availbale for you here. This ZIP File contains the CLASS Files AND the JAVA File.
The latest version however is integrated into the VMT software package.
Part 2 - EXCEL Spreadsheet
How do I calculate the effective range of my station in free space environment ? The solution is quite simple: Add all known parameters of your station, take the formula for free space path loss and don't forget the Boltzmann konstant.
I have added a Microsoft EXCEL (Version 5 or 7) spreadsheet EME System Sheet, where you can enter or modify all parameters and see immediately the effect on the result. This spreadsheet works correct and takes care of all noise contibutions to your system. Moon noise cannot be calculated.
Part 3 - Fundamentals
Propagation on the earth's surface is a different problems and this adds a number of restrictions.
Moonbounce propagation adds other difficulties:
The virtual diameter of the moon is ca. 0.5° it is a certain fraction of the whole sphere. Therefore an additional loss of around 50 dB has to be introduced. Doing so you will come to the general RADAR equation:
a * Pt * Gr * Gt * La^2 Pr=------------------------------ ( 4 * PI )^2 * d^2 a: cross section of target Pr: receive power Tr: transmit power Gr: gain of receive antenna Gt: gain of transmit antenna La: Lambda = wavelength d: distance to target
Here is the "dB" version:
Pr [dB] = Pt + Gr + Gt + 10 * log (a) + 20 * log (f) + 40 log (d) - 103.4 with d in km a in m^2 f in MHz
The reflectivity of the moon is only 7%
Here is a calculation example for standard free space propagation:
Noise power calculation Definition: Noise Power NP = 4 * KTB / (4*R) = KTB where: K = Boltzmann konstant = 1.38E-23 J/K B = Bandwidth in Hertz T = Ambient temperature = use 290 K (Kelvin!) /* no Celsius or Fahrenheit ...*/ NF = Noise Figure in dB /* ideally should be transformed to Noise temperature !!! */ SNR= Signal to Noise Ratio for detection Definition: Pathloss Pl = 32.45 + 20*log(f) + 20*log(d) where f = Frequency in MHz d = distance in kilometers Antenna gains should be given in dBi, that means dB over an isotropic radiator. IF your gain is in dBd /* db over halfwave dipole */ then add 2.14 dB to your value.
Here is an example from the EXCEL speadsheet:
Free space path loss calculation (April 1997) --------------------------------------------- Christoph Petermann 09.04.1997 Pommernweg 11 D24229 Schwedeneck www.df9cy.de --------------------------------------------- The framed fields may be changed only ! Signal to Noise Ratio / Sensitivity Limit Method Noise Power P = 4 k T B / ( 4 R ) = k T B Bandwidth B [Hz] = 1,0E+02 Hz Boltzmann Konst. [ J/ K]= 1,38E-23 J /K Temperature [K] = 290,00 K Noise Power [dBm]@290K -153,98 dBm My systems Signal to Noise Ratio: SNR [dB] 5,00 dB Receiver Noise figure 0,30 dB Receiver Noise temperature 20,74 K Losses prior to LNA 0,20 dB Losses in Noise Temperature 13,67 K Antenna temperature (Sky) 20,00 K All system Noise Temperature 40,94 K Noise Power [dBm]@T_sys -162,48 dBm Sensitivity [dBm]@T_sys -157,18 dBm Calculation of maximum possible Free Space Range Transmit antenna gain 35,00 dBi Gain over isotropic Receive antenna gain 35,00 dBi Transmit power 60,00 dBm equal to 30,00 dBW equal to 1000,00 Watt Receive Sensitivity -157,18 dBm Frequency 1296,00 MHz Maximum path loss incl. antenna gains Pl= 287,18 dB Range out of Pl=32.45+20log(f)+20log(d) R= 4206 Mio.km EME Pathloss for a given frequency Frequency 1296,00 MHz Moon distance 386000,00 km Moon diameter 3400,00 km RADAR Equation 53,14 dB reflectivity of Moon surface 7,00 % Pathloss 277,15 dB Expected Signal to noise ratio 10,03 dB Method PL=32.45 + 20*log(Moondistance * 2) + 20*log(f) + Spherical loss from RADAR equ. + reflectivity loss reflectivity loss : the moons' surface reflects only 7% to the earth RADAR Equation: the virtual moon represents only a fraction of the total sphere. Therefore an additional loss must be introduced. This is called in general the RADAR EQUATION © C.Petermann DF9CY June 1997 "References: VHF/UHF Manual; ARRL Publications et al." The calculations are valid for free space only
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