HF guide structures - Introduction

Electromagnetic waves can be emitted into free space by means of an antenna, for example. This is known as free wave propagation, and is widely used for radiotelephone systems, radar or radio communications. However, there are many situations in which it is necessary or more appropriate to transmit this electromagnetic energy in structures generally referred to as "guides". As the name suggests, the electromagnetic wave is transmitted by guided propagation through a mechanism of successive reflections off walls and/or interfaces. At low frequencies, we more commonly speak of wires or cables transmitting electrical signals. This is a simplifying view that allows simple calculation using line theory. However, in all rigor, the dynamic phenomenon is always fundamentally the guiding of an electromagnetic wave.

The guide is the structure linking the generator or its equivalent (which injects the power) and the load at the end of the guide. The latter may be the input to an amplifier, mixer, filter or antenna. It is therefore the key element in the interconnection of devices. Knowing its impedance enables you to adapt the elements connected to it.