VHF / UHF Filters and Multicouplers: Application of Air Resonators

Chapter 8

Auxiliary Devices

8.1. Introduction

HF multicoupling and filtering rely to a great extent on theoretical and technological knowledge of cavities. However, we are obliged in practice to use supplementary devices to achieve the integrality of the specifications of a system. For example, it is impossible to assure a channel-to-channel isolation, which is measured in tenths of dB, with the cavities alone: it is necessary to add circulators. If we have difficulties with the adaptation, it will be necessary to add impedance adaptors. If we need to cancel an off-band increase of the response curve, it will be necessary to use a secondary filtering, and if there is no space to do so by means of cavities, we will have to find another technique, etc.

It is thus indispensable that technicians dispose of a complete arsenal that allows them to solve all kinds of situations, and we will call these objects auxiliary.

8.2. Circulators

8.2.1. Operating principle

These are passive components, and thus linear, at least in a certain dynamic, but non-reciprocal. Their specific properties are based on those of ferrites subject to a strong magnetostatic field. In informal language the ferrite can designate both the material, and the product, or object. Ferrites are sintered ceramics (pulverized mixture, compression, cooking, fabrication) based on iron oxide. From the chemist’s point of view, we can refer to as ferrites all the components of the form Fe2O3, XO, where X can be either Fe (this ...