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I find that plotting unwrapped phase (not restricted to +/- 180 degrees) can provide insight that conventional phase plots cannot. For one thing, it lets you judge group delay at a glance without generating a separate curve. It also makes high-order resonances stand out. I've attached two examples from a NanoVNA-H4 ground probe measurement. I don't know of any VNA software that unwraps phase, but it's not hard to implement.

Brian

More examples.

Brian

MATLAB unwraps phase with a single command line.

NanoVNA App has graph selections for S11 and S21 phase unwrap. This program only works on Windows and I prefer it to NanoVNA Saver for several reasons (easy graph selection, better scaling options, robust calibration, better measurement comparison)

Roger

Thanks, Roger. I've seen plenty of NanoVNA App plots here, but none with phase unwrapped.

Brian

On 8/13/23 2:41 PM, Brian Beezley wrote:

Thanks, Roger. I've seen plenty of NanoVNA App plots here, but none with phase unwrapped.
Brian

Maybe because nobody bothered to try? <grin>

Sometimes, I find the derivative of unwrapped phase (group delay) more useful, too.

On Tue, Aug 15, 2023 at 09:25 PM, Jim Lux wrote:

Sometimes, I find the derivative of unwrapped phase (group delay) more useful,
too.

Jim, your comment inspired me to add group delay to my program. I don't see its relevance for ground probe data, but it provides a powerful diagnostic tool that can reveal anomalies otherwise invisible. The first two plots show an ordinary-looking phase plot and the corresponding group delay plot. The big group delay spike corresponds to a barely perceptible flat spot in the phase curve. It's easier to see in the third plot with a narrower frequency span. I think it's due to a VNA frequency stepping issue.

Brian

Make sure you are taking frequency steps that are small wrt to the DUT bandwidth. GD on a xtal filter is one example.

On 8/18/23 8:15 AM, Brian Beezley wrote:

On Tue, Aug 15, 2023 at 09:25 PM, Jim Lux wrote:

Sometimes, I find the derivative of unwrapped phase (group delay) more useful,
too.

Jim, your comment inspired me to add group delay to my program. I don't see its relevance for ground probe data, but it provides a powerful diagnostic tool that can reveal anomalies otherwise invisible. The first two plots show an ordinary-looking phase plot and the corresponding group delay plot. The big group delay spike corresponds to a barely perceptible flat spot in the phase curve. It's easier to see in the third plot with a narrower frequency span. I think it's due to a VNA frequency stepping issue.

Yeah, derivatives do accentuate noise - but sometimes that's actually useful to spot a discontinuity in a system that should otherwise be "smooth" And when you have multiple resonances, is makes them obvious.