VSWR to S11 Calculator

This tool calculates the S11 from the Voltage Standing Wave Ratio (VSWR).

Enter VSWR (a value greater than or equal to 1) 


	

			

		

						

				

				

                

			

			
						

		

		






  S11 to VSWR 





Formula





S11 = (VSWR-1)/(VSWR+1)





S11 (dB) = 20*Log10(S11)





The input reflection coefficient, S11 < 1





Example Calculation





For a VSWR of 1.5, the corresponding value of S11 is 0.2. 





As the value of VSWR increases, S11 increases as well and the quality of the match decreases.





Background





Many RF components such as filters and amplifiers can be modeled as 2-port networks with their scattering or S-parameters. 





In a 2 port network, S11 is the forward reflection coefficient (at port 1) when port (2) terminated in 50 ohm. 





For the 2 port network shown below, S11 = b1/a1






Two-port network




VSWR is a measure of the match between the characteristic impedance of the source and the attached load. The load can be an antenna or any RF module such as a filter or amplifier. The greater the degree of mismatch, the greater the amount of power reflected back. 





This calculator provides the VSWR when the source and load impedances are specified. It can for instance be used to calculate the VSWR when the source and load are 50 Ω and 75 Ω, respectively.  





VSWR is often expressed as a ratio to 1. For example, if the VSWR is 2.5 then it is written as 2.5:1. 





What’s a good value of VSWR?





A good value of VSWR is 1.5 or lower. In an ideal situation, the VSWR=1. In this case, the load is perfectly matched to the feedline characteristic impedance and S11 (linear) = 0 and S11 (dB) = -∞. In this situation, no signal energy is reflected and all of it is transmitted forward.





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