Microstrip Transmission Line Characteristic Impedance Calculator Using a Formula by Brian C. Wadell

  The characteristic impedance (Zo) of a microstrip transmission line can be calculated using the active calculator or the formulas at the bottom of the page.

[diagram of PCB layers]          Where   εr= Relative Dielectric Constant
                      W = Width of track
                        t = Thickness of track (1 oz/ft2 = 0.035 mm)
                        h = Thickness of dielectric

Enter the εr of the PCB:  
Enter the width of the track:    mm
Enter the thickness of the track:    mm Effective Dielectric Constant (εeff): 
Enter the thickness of the dielectric:    mm Characteristic Impedance (Zo):    Ohms 
 

This calculator uses Visual Basic and will work in Internet Explorer and other browsers with Visual Basic plug-ins. For more information see About our calculators

The following formulas were derived from formulas used in an article by Rick Hartley http://www.jlab.org/accel/eecad/pdf/050rfdesign.pdf.  It is not too clear from the article by Rick Hartley where to use εr and where to use εeff (although there is a note to use eeff instead of er). The substitutions selected give results that are almost identical to the "benchmarks" in http://www.polarinstruments.com/support/cits/IPC1999.pdf, and these benchmarks were obtained by numerical methods. The accuracy of results obtained from this calculator can be checked by making a sample PCB and measuring the resulting impedance. Guidance for this can be found at Measuring Characteristic Impedance of PCB tracks using a Vector Network Analyser.

[calculation formula]

[Microstrip formula]

 The above formulas are in Transmission Line Design Handbook by Brian C Wadell, Artech House 1991. The main formula is attributable to Harold A. Wheeler and was published in, "Transmission-line properties of a strip on a dielectric sheet on a plane", IEEE Tran. Microwave Theory Tech., vol. MTT-25, pp. 631-647, Aug. 1977. The effective dielectric constant formula is from: M. V. Schneider, "Microstrip lines for microwave integrated circuits," Bell Syst Tech. J., vol. 48, pp. 1422-1444, 1969.

This calculator is provided free by Chemandy Electronics in order to promote the FLEXI-BOX

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