DC to 40 GHz coaxial-to-microstrip transition for 100-μm-thick GaAs substrates

The design, design approach, and test results are presented for a simple coaxial-to-microstrip transition. The approach provides improved performance over the basic coaxial-to-microstrip transition by causing a TEM-to-quasi-TEM transformation through a tapering of the coaxial line for 432±51 μm (≪6% of λ₀ at 40 GHz) prior to impinging a portion of the resultant quasi-TEM field directly across the microstrip´s dielectric at the coaxial-microstrip interface. Tests show that the return loss for the transition into a 50-Ω microstrip line on a 100-μm-thick GaAs substrate is better than 16.9 dB per transition from 200 MHz to 40 GHz. A cover having a height of 1.9 mm and a width of 2.6 mm had little or no influence on test results