I'm trying design a video transmitter around an 5.8GHz Airwave module. The documentation says the trace to the antenna must have a characteristic impedance of 50 ohms. How do I calculate the characteristic impedance of my trace?

  • \$\begingroup\$ See the section titled "Sizing the feedline" in this article. \$\endgroup\$ – Toby Jaffey Oct 28 '10 at 19:13

As a quick rule of thumb for 50 ohms, you want the ratio of the trace width to the height above the ground plane (probably your board thickness) to be around 1.8 with 2-oz copper. With a standard 0.063" thick PCB, that means a trace width around 120 mil. (Fat!)

Formulas for calculating this can be found in various places. If you access to an engineering library, try chapter 4 of High Speed Digital Design by Johnson and Graham. (Equation 4.90 on p. 187 is the key one.)

Here's an online calculator: http://referencedesigner.com/tutorials/si/si_06.php

  • \$\begingroup\$ Thanks! What happens if the trace doesn't meet this requirement? (e.g. extra noise, reduced transmission distance?) Also, is there any problem with trace length? \$\endgroup\$ – Thomas O Oct 28 '10 at 17:23
  • \$\begingroup\$ Re width, is this radius or diameter? \$\endgroup\$ – Thomas O Oct 28 '10 at 17:25
  • \$\begingroup\$ You don't care about length-- characteristic impedance is a measure of the ratio of inductance to capacitance at any point in the trace. Let me think about the effects of missing the 50 ohm target for a minute. \$\endgroup\$ – pingswept Oct 28 '10 at 17:41
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    \$\begingroup\$ Width means neither radius not diameter. I'm not sure what you mean with your second comment. \$\endgroup\$ – pingswept Oct 28 '10 at 17:42
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    \$\begingroup\$ OK, here's my understanding of mismatched characteristic impedance. As a signal passes from source, through the trace, and into a load, the signal is attenuated in the trace, and some signal is reflected at each intersection. With matched impedance, there are no reflections. If the trace is too wide (too much capacitance) or too narrow (too much inductance), reflections and attenuation increase. \$\endgroup\$ – pingswept Oct 28 '10 at 18:09

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