100 ohms differential trace dimensions?

Can someone give me a good guess of the width and gap between two traces to make a 100 ohm differential impedance line. I have analyzed it various ways, and looked at numerous online "calculators", and I am not converging on an answer. Every program gives me wildly different results. NOTE: there is NO ground plane. Air above and below the board.

My best guess so far is W=100 mils, and S= 4 mils. Traces are on flex board (5 mil thick polyimide er=3.0).

Thanks, Rich

There is no unique answer. Z0=SQRT(L/C). Somewhat oversimplified: You can change the C by decreasing the gap width, or change the L by changing the line width.

I have modelled the line in Sonnet which I think you are using. The model file is attached.
Below is the plot of the line impedance vs. line width for two different slot widths (2.5mils and 5mils). As you can see, you have 100 ohm differential impedance for w=40mils@s=2.5mils and also for w=90mils@s=5mils.

Thanks volker. That is very helpful. It is for an RFID antenna at 915 MHz printed on a flex pcb.

What is that file format ".zon" that you attached? Can that be imported into sonnet lite plus?

This is the "packed project" with results. Import it into the Sonnet editor with File > Import > Packed Project.

Thanks. I wish I had a few days to sit back and just play with Sonnet and learn all these tricks. I am learning along the way (i.e. the "hard" way).