Microwave cavity

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A microwave cavity is a closed metal structure that confines electromagnetic fields in the microwave region of the spectrum. Such cavities act as resonant circuits with extremely low loss at their frequency of operation. Their Q factor may reach several hundred thousand compared to a few hundred for resonant circuits made with inductors and capacitors at the same frequency.

For frequencies over a few hundred megahertz in the VHF range, conventional inductors and capacitors present difficult problems. The losses of both increase with frequency.

This type of inductor is usually wound from wire in the shape of a helix with no core. Skin effect causes the high frequency resistance of inductors to be many times their direct current resistance. In addition, capacitance between turns causes dielectric losses in the insulation which coats the wires. These effects make the high frequency resistance greater and decrease the "Q".

This type of capacitor will use air, mica, ceramic or perhaps teflon for a dielectric. Even with a low loss dielectric, capacitors are also subject to skin effect losses in their leads and plates. Both effects increase their equivalent series resistance and reduce their Q.

Even if the Q of VHF inductors and capacitors is high enough to be useful, each suffers from the problem of being composed of some of the other. The shunt capacitance of an inductor may be more significant than its desirable series inductance. The series inductance of a capacitor may be more significant than its desirable shunt capacitance. As a result, in the VHF or microwave regions, a capacititor may appear to be an inductor and an inductor may appear to be a capacitor.

The energy of an air core inductor should be almost totally in its magnetic field. Some energy is stored in the electric field due to the capacitance between its turns. The latter energy is an unwanted feature. The energy of a capacitor should be almost totally in the electric field of its dielectric. Some is stored in the magnetic field from the current in its leads. This is unwanted as well.

Air is almost loss free for high frequency electric or magnetic fields. Microwave cavities confine electric and magnetic fields almost exclusively to the air spaces between their walls. The currents in the cavity walls are small because they are at a high impedance point. While losses are small from these currents, cavities are frequently plated with silver to increase their electrical conductivity and reduce the losses even further. Copper cavities frequently oxidize, which increases their loss. Silver or gold plating will prevent this. Even though gold is not quite as good a conductor as copper, it prevents oxidation and the resulting deterioration of Q with aging. Because of its much higher cost, it is used only in the most demanding applications.

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