VHF / UHF Filters and Multicouplers: Application of Air Resonators

Chapter 9

Directive Couplers

9.1. Introduction

Figure 9.1.

We use the adjectives “directional” and “directive” indiscriminately to qualify the couplers that present this property. A directive coupler is formed by two parallel lines, one said to be the main and the other the coupled, the second being close to the first and consequently being influenced by its radiative electromagnetic field. The ensemble is therefore an octopole since it comprises four accesses. The lines can be of any type, microstrip on a single circuit, microstrips in opposition, wired, hybrids, or even wave-guides. In the last case, it will simply be necessary to create one or more openings in the common wall to guarantee the coupling. When the parallelism of the lines and the matching of the four accesses are done, the coupler presents a directional function which makes the coupled line deliver a signal that is proportional, either only to the direct power crossing the main line (exit 3), or only to the reflected power (exit 4). The theoretical study is based on the principle that the main signal induces in the coupled line two signals propagating in opposite directions, called even mode (for that propagating in the same direction as the main signal), and odd mode, for the second. Each one gives rise to a different impedance: Z0e for even mode and Z0o for odd mode, these two quantities depending on the type ...