See...
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>Newsgroups: alt.binaries.schematics.electronic
>Subject: Smith Chart Amusements - SmithChartDoodles.JPG
>Message-ID:
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>I'm trying to remember from 40+ years back, when I last used a Smith
>Chart... what do the circular patterns mean?
>
>I suspect that they are perhaps due to a distributed R-L-C network?
Loops like that indicate that the device input (or output) phase has gone through 360 degrees. Such loops on a Smith Chart are present if there are unwanted resonances and/or positive feedback in the circuit or if the device is NOT unconditionally stable at all input impedances.
You didn't specify what device you were testing or how the Smith Chart was produced. My guess(tm) is that it's some kind of active device doing a fine job trying to oscillate by ringing merrily every time the frequency sweep crosses one of the resonances or instability points. Wave your hand over the circuit and see if the loops move around. If they do, you've got problems.
Gripe: I don't like guessing the missing details (because I'm lazy):
- Whatcha trying to accomplish?
- Whatcha got to work with? (Circuit, hardware, test equip, etc).
- What did you do to make that mess happen?