Analytical expression for the input impedance of a microstrip probe in waveguide

We present a closed-form expression for the the input impedance of a microstrip probe in a rectangular waveguide. The probe extends only part way across the waveguide and is therefore compatible with RF components that require an open circuit at low frequencies. Our analysis is based on the spectral-domain method and is able to take into account the orientation of the antenna with respect to the direction of propagation. We have examined the validity of our model by carrying out extensive impedance measurements at 5GHz. In those cases where the probe did not extend more than half way across the waveguide, excellent agreement was obtained. We show that the bandwidth of a probe that stretches only part way cross the waveguide is very much greater than the bandwidth of a probe that stretches all of the way across the waveguide and that is earthed at both ends. Moreover, the input resistance is lower and more suited to submillimetre-wave detectors such as SIS tunnel junctions. Our expression suggests that it should be possible to develop low-impedance, wideband probes for nearlydouble-height waveguide, and this implies that the upper frequency limit to which probes and waveguides can be manufactured can be extended well into the THz frequency range. A related, and often neglected consideration, is that the ohmic loss associated with an oversized waveguide is very much smaller than the ohmic loss associated with a reduced-height waveguide.