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Smith Chart Readings

Question asked by Dirty on Nov 12, 2007
Latest reply on Nov 20, 2007 by dgun

I am measuring inductors that are mounted as shunts and am curious on the smith chart readings I get. 

Some readings start as a short and move to the 50Ohm center, but other times the chart shows a negative impedance on the outside of the short that moves into the 50 ohm center.  S21 shows a gain where the impedance is negative, but how can this be?

It all depends on whether I am using a co-planar type ground plane or if the coil is actually wrapped to the backside ground plane.  Am I really doing something wrong by using a coplanar ground if I see a gain in S21?