Types of waveguides. Wave propagation in waveguides.

A waveguide is a hollow or dielectric-filled metallic tube in which directed propagation of an electromagnetic field occurs. In a waveguide, radiation losses are practically absent. Conductive losses in the metal are smaller than in a coaxial line because there is no inner conductor as in coaxial cables; an air-filled waveguide has very low dielectric losses.

Waveguides are most commonly used in the frequency range of SHF, as at these frequencies coaxial cables exhibit significant attenuation of electromagnetic waves (e.g., for transmitting satellite antenna signals). At centimeter wavelengths, the line is often replaced by a waveguide, which is a metal tube with circular or rectangular cross-section in which the electromagnetic wave propagates. The waveguide walls act as a shield, preventing the electromagnetic waves from spreading in other directions and forcing them to propagate only along the waveguide.

Waveguides are also used to transmit power over long distances (for example, to feed high-power TV antennas installed on transmission towers).

Stripline Waveguide

A stripline waveguide consists of flat transmission lines made of two or three very thin conductors separated by a dielectric. Stripline waveguides are widely used in printed circuit technology. They are typically used for the following purposes:
1. As transmission lines for high-frequency power in ranges from hundreds of MHz to tens of thousands of MHz;
2. For creating simple SHF low-pass and high-pass filters;
3. For power splitters;
4. As a transition to coaxial lines or waveguide paths. Stripline waveguides use high-quality dielectrics such as fluoroplastics and polystyrene, and conductors are made of copper, and in some cases, silver.

Advantages and Disadvantages of Waveguides

Compared to coaxial lines, waveguides have lower energy losses because they lack an inner conductor and insulators. The highest voltage in a waveguide occurs between diametrically opposite points on its inner surface (if circular) or between opposite walls (if rectangular). The distance between these points is greater than the distance between conductors in a coaxial line, reducing the risk of breakdown at high voltages.

However, waveguides have a limitation that restricts their application. In coaxial or balanced lines, waves of any frequency can propagate, while in a waveguide only waves with a frequency above a certain critical value, known as the cutoff frequency f_cr, can propagate. In other words, only waves with a wavelength shorter than a certain critical wavelength x_cr can propagate. The critical wavelength is approximately twice the transverse dimension of the waveguide. For example, if a waveguide has a diameter of 3 cm, the critical wavelength is about 6 cm. Longer waves cannot propagate through such a waveguide.

Along the waveguide there is always some attenuation of the wave, meaning its energy gradually decreases. This is due to currents induced on the inner surface of the waveguide walls, which dissipate part of their energy as heat. Energy losses would be absent only if the walls were made of a perfect conductor.

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