The Radioman's Manual of RF Devices, Principles and Practices

Chapter 8: Coaxial Cavity Filters

RF filters make many things possible in RF communications that would otherwise be impossible or impractical. This chapter discusses coaxial cavities and the various filter arrangements. In order to properly illustrate the various filter responses and characteristics, a tracking generator and spectrum analyzer are employed for many of the test setups. In the field, simpler equipment setups can be used to make minor adjustments to a filter.

8.1 The Band-Pass Cavity

A cutaway view of a typical quarter-wave coaxial band-pass cavity is shown in Figure 8 1. The inner portion of the cavity consists of a fixed conductor and a moveable conductor. The moveable conductor is connected to a tuning rod that can be used to lengthen or shorten the effective length of the inner conductor, which is what tunes the cavity to its resonant frequency. The tuning rod must be resistant to expansion and contraction with widely changing temperatures, since this would change the resonant frequency of the cavity. An alloy material called invar is used for the tuning rod because of its nearly zero temperature coefficient. It is relatively invariant with wide temperature changes.

Figure 8 1: This is a cutaway view of the inside of a band-pass cavity. The tuning rod is usually made of an alloy called Invar which is short for invariant. This alloy has a near-zero temperature coefficient, resisting any expansion or contraction with widely varying temperatures. Expansion or contraction of the tuning rod would cause significant changes in the resonant frequency of the cavity.