Exciting traveling waves in high Q structures using microstrip

Exciting traveling waves in high-Q resonant structures can simplify low noise oscillator designs (Tobar et al., 1999), (Tobar et al., 2000), (D.P. Tsarapkin, N.A. Shtin, 2002), and also has applications for newly proposed Lorentz invariance tests (Tobar et al., 2005). In this work, we use microstrip probes to excite traveling waves in a sapphire dielectric resonator. Previous work has indicated that matching microstrip probes to a sapphire resonator can be a difficult requirement (D.P. Tsarapkin, N.A. Shtin, 2004). A model has been developed, which takes into account leakage of the microstrip line in the reverse direction to which we excite the traveling wave. From such a model we can define the standing wave ratio (SWR) from the amount of forward and reverse traveling wave in the resonator. Ideally for a perfect traveling wave the SWR = 0, and for a standing wave the SWR = 1. By comparing the model with experiment we find that we can excite a traveling wave with an SWR of 0.35 and an unloaded Q-factor of 150,000 in a WGH12,0,2 mode at 9,997 GHz, in a sapphire of 5 cm diameter and 3 cm height. From the model, we also propose new oscillator designs for low noise applications