[Blabber] Help with microstrip traces

Ian Harris imharris at gmail.com
Tue May 14 12:22:13 UTC 2013


Hi guys,

So I'm stuck.  I'm building a wifi module and I need to create a 50 ohm
trace from the module to the antenna.

Looking on the internet, I can find impedance calculators, and after poking
around it seems that I need to use a four layer board (since in two layer
boards the dielectric is just too thick).

So in a four layer board, I can find examples where the
dielectric<http://www.expresspcb.com/expresspcbhtm/specs4layerstandard.htm>is
0.017" (17mil).  Copper is, say 1oz (which the internet
tells me <http://www.pcbuniverse.com/articles.php?a=4> is 1.37mil "high").
 The dielectric constant is 4.6 (same source).

Now according to a calculator <http://emclab.mst.edu/pcbtlc2/microstrip/>,
the required strip width (trial and error) for 50R is around 30mil.

However, on the demo board for this module, the antenna trace, while a
reasonably solid trace is *almost* the SMD pad width - but not quite - and
the SMD pad width is 27mil. If I had to guess, I'd say it's around 20mil.

So how do they get the impedance down to 50R? The calculator says that a
20mil trace / 1.37mil copper / dielectric (17mil) / 4.6 constant = 88R.
 I'm presuming that's not close enough.

Change copper from 1oz to 0.5oz makes almost no difference.

Any ideas?

kind regards
Ian.
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