The visible portion of the electromagnetic spectrum gives us light, but it is the microwave portion – between 300 MHz and 300 GHz — that enables us to communicate, wirelessly and speedily. Within the microwave region, the narrow band of frequencies clustered around 2.4 GHz has been the focus of recent interest; that’s where Bluetooth, ZigBee and Wi-Fi live.
But things get really interesting at higher frequencies – when the wavelengths of signals are roughly the same as the dimensions of the equipment. At those frequencies, lumped-element circuit theory — represented by discrete resistors, capacitors and inductors — no longer holds. Instead, distributed circuit elements and transmission-line theory apply; waveguides replace coaxial transmission lines and cavity resonators replace L-C circuits.
Cavities are resonant metal structures within which microwaves form standing waves. They are used in oscillators and transmitters and as filters with very high Q factors. They enable point-to-point communication systems with very large information capability – used on the surface of the Earth, in satellite communications and in deep space radio communications. Point-to-point licensed microwave links are true fiber replacements with full duplex communications.
The cavity is a critical part of any microwave system design, affecting frequency of operation, stability, output power, temperature, and allowable bias voltage. But, construction of cavities is a fine art that depends on the application, the construction material and the properties of other components. All these elements are tightly coupled so each must be selected with care.