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Suspended striplines are widely used for several reasons. They provide high Q-factor, wide bandwidth and good temperature stability. This tutorial reviews some of the basic principles and new concepts for suspended striplines.
There are several reasons for the wide use of printed transmission lines. They are simple to produce, they operate over a wide bandwidth, and have small weight and dimensions. The earliest form of printed transmission lines was the stripline (Figure 1a).1,2 The printed stripline consists of a strip conductor centered between two parallel ground planes with two identical dielectric substrates. Small air gaps can exist between the two substrates because of fabrication faults. A dominant leak, mode that can exist in such a structure results in undesirable crosstalk and spurious performance.3 The detrimental effect of such gaps can be eliminated by using mirror image conductors (Figure lb) on top of the bottom substrate and on the bottom of the top substrate. Due to multiple points of contact along the two conductors, the electric field in the gap is cancelled. In practice, the widths of two center conductors are purposely made slightly different (W^sub 1^ # W^sub 2^). The wider conductor is given the width required for the desired characteristic impedance and the other conductor is made slightly smaller. In this way, even if the conductors are somewhat misaligned, the effective width appears constant with the proper value.
Many RF and microwave components are manufactured using suspended striplines (SS), which yield a higher Q-factor than printed stripline and microstrip lines (ML).4-7 The SS is a modified version of the stripline. In the SS (Figure lc to e) the strip conductors are placed on the surfaces of a thin dielectric substrate, and the substrate is then suspended in a metal enclosure. The major portion of the electromagnetic field is confined to the air gaps between dielectric...