Astonishingly, 200 years ago in France in 1826, Felix Savary observed the oscillatory nature of Leyden jar discharges. This oscillatory behavior is an essential requirement for resonance.1 The theory of resonance would take another 50 years to mature. In 1887, Heinrich Hertz in Germany devoted one section of his papers to describing his experimental observations of what he called “Resonance Phenomena”.1 That is nearly 150 years ago, which is still amazing.

Cavity filters are ubiquitous today, and they are not new to the world of RF and microwave engineering. Cavity filters have been around for almost 80 years, since Fano and Lawson completed their initial work in 1948.2,3 They are widely used in RF and microwave systems where high selectivity, low insertion loss, and excellent power handling are required. They are commonly found in SATCOM, radar, EW, ECM, and even test and measurement systems. One critical aspect of cavity filter design is how energy is coupled into and out of the resonant structure. Among the various techniques available, coupling loops and coupling probes (henceforth, more commonly “loops” and probes”) are the two most common methods. Each method relies on a different electromagnetic coupling mechanism and offers distinct advantages depending on frequency, power level, bandwidth, and mechanical constraints.

This article furnishes the reader with a basic understanding of the operating principles, design considerations, and practical tradeoffs of loop and probe coupling methods for cavity filters.