At certain frequencies and power levels, the electrical and material characteristics of conductors and dielectrics can change and produce unexpected effects. When the microwave frequency is high, there will be a phenomenon of electron migration to the surface of the conductor, known as the skin effect.
In addition, if the power level is too high, the dielectric material between two conductors will ionize into a conductive material, causing microwave dielectric breakdown. When the skin effect occurs, as the frequency of electromagnetic waves inside the waveguide device increases, more and more electrons migrate to the surface of the waveguide device.
Because the high-frequency conductivity inside the conductor is lower than that near the surface, the inner wall of the RF waveguide device is more important for wave propagation than the outer wall. Therefore, the skin effect is an important reason why the loss of conductors is much greater at high frequencies than at low frequencies.
For non-ferromagnetic conductors, their DC resistance is much higher than their resistance at RF and microwave frequencies. In addition, for ferromagnetic conductors such as nickel, iron, and steel, the higher the permeability, the greater the increase in AC resistance.
For these reasons, high-conductivity metals such as silver and gold are often used to electroplate the inner walls of waveguide devices - especially in high-frequency microwave, millimeter-wave, and high-power applications. At a frequency of only 1 GHz, 98% of the electrons inside a copper plate will migrate to an area within a few microns near its surface.
At high frequencies such as microwave and millimeter-wave, most electrons will migrate to smaller areas, making the surface condition of conductors extremely important. This means that the plating material finally formed on the inner wall is the only conducting material inside the RF waveguide device that affects electromagnetic propagation. Therefore, surface consistency and smoothness are crucial in reducing the overall RF loss of waveguide devices.
Another phenomenon that affects high-frequency and high-power systems is the breakdown of the dielectric between the conductive walls of waveguide devices induced by electromagnetic energy, which can lead to high-energy arcs.
The high-energy arc generated inside the waveguide device will cause high-power standing waves and produce heat that may cause damage or even strip off the metal plating on the surface of the waveguide device material.
The power threshold that causes microwave breakdown depends on the characteristics of the dielectric, temperature, distance between conductors, frequency, and pressure of the gaseous dielectric inside the waveguide device. For high-power waveguide filters, the spacing between the filter cavities covered with metal is also crucial for preventing secondary electron avalanche breakdown. The waveguide system can inject gases with higher breakdown characteristics to increase the power threshold that causes microwave breakdown.
In addition, after the waveguide system is assembled, sometimes dry gas is used to remove moisture in order to prevent condensation and high humidity that can reduce the microwave breakdown threshold. To achieve this, a special flange with inlet and outlet valves can be used.
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