Effects of Insulating Dielectric Layers in Millimeter-Wave Planar Transmission Lines

Abstract

Insulating dielectric layers added to waveguiding structures to reduce the conductor losses at high frequencies complicate the structure and usually introduce dominant mode crossing and leakage. A qualitative study is used to derive a design constraint that ensures that insulated structures fully utilize the conductor loss reduction available from the insulating dielectric layers. A numerical example is provided in which it is shown that in certain cases arbitrary dielectrics cannot be used for insulating layers. INTRQDUCTIONIn an effort to reduce the conductor losses in millimeter-wave waveguides, insulating dielectric layers are being introduced to planar dielectric waveguides [1,2] and to microstrip-type waveguides [3]. The insulating layers consist of low dielectric constant materials and are placed between the higher dielectric constant guiding layer and structure metallization to induce decaying fields in the insulator. The reduced field strength at tie conductors induce smaller currents, and therefore, smaller conductor losses. A recent example of such a modificaton is the microslabTm waveguide, where insulating dielectric layers were added to the microstrip geometry [3] (see Fig. 1).The introduction of dielectric layers complicates the structure and makes the operating characteristics more complex. The modified structures can exhibit leakage [4] and dominant mode crossing (see [5,6] for example). Oliner has shown in [4] that insulated planar structures can leak and that the leakage can be avoided with proper design. Therefore, the design procedure for minimizing the conductor loss must also incorporate the requirements for a non-leaky structure. In this paper, we neglect dielectric loss and investigate the conductor loss and leakage in a qualitative manner in order to provide a general design criterion for planar insulated waveguides that fully utilizes the loss reduction available from the insulating layers.Planar waveguides can be approximately analysed using an "inside/outside" technique [4]. Referencing the general waveguide of Fig.2, the inside (outside) region has an effective dielectric constant 8effil (eeffo). The step in e,ff provides the lateral confinement. Confmemeni is maintained in the y-direction with either a dielectric discontinuity or metallization. CONDUCTOR LOSS STUDYTo determine in a qualitative manner the conductor loss and leakage properties of insulated strip waveguides, we will analyse the inside and outside regions seperately. For the planar waveguides being considered for millimeter-wave circuits and integrated optics, the inside/outside-regions are layered parallel-plate waveguides. It is sufficient to analyse the three-layer case shown in Fig.3. Waveguides with more than three layers behave similarly as long as one layer has a higher 8eff than the others..