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I found the first citation particularly interesting. I tend to push microstrip lines a bit higher in frequency but the comments here are very useful. Much like coax, operating frequency can be pushed higher with smaller cross-sections. Unfortunately, insertion loss increases and power handling decreases.
With microstrip, I have often been curious about the best impedance for lowest loss. My suspicion is that it is probably in the vicinity of 70 ohms but I have never looked too deeply to make that case.