[help] I measure a simple structure, but...

Hello, Folks,

We use a PA module and can't reach the performance in its specification with our experiment board. I suspect something not good in our fixture, so I did a simple measurement:

I made a 50 Ohm microstrip with 18.3mil width and 1 inch length on Ro4003 board with 8mil thickness. Put the PCB on our fixture and measure the insertion loss. It's about -4dB at 15GHz! And also poor return (I can't remember the exact value). There are two coax-microstrip adapter in our fixture, which is with 1.3 mm diameter inner conductor and 4.1 mm diameter outer conductor and teflon as insulator. I suspect the abrupt of the coax-microstrip interface causing the mismatching. Then I changed to another adapter with 0.8mm inner conductor, 2.7 mm outer conductor and teflon insulator, I expect better insertion loss, but finnally nothing improved obviously! Who can explain what cause the mismatching of the structure?

I also notice some bigger companies use a precise 3.5 mm coax-microstrip adapter with very thin inner conductor for this band. Do you know where I can buy this kind of adapter?

Thank you very much!

Any reply?