page_title: Microwave cavity
section_title: Losses in LC resonant circuits
hierarchical_section_title: Microwave cavity / Comparison to LC circuits / Losses in LC resonant circuits
caption_attribution_description: Español: Ondámetro de banda Ku English: Ku band (12-18 GHz) absorption wavemeter. This device was used to measure the frequency of microwaves before frequency counters for those frequencies were available. It consists of an adjustable microwave cavity calibrated in frequency.
context_page_description: A microwave cavity or radio frequency cavity is a special type of resonator, consisting of a closed metal structure that confines electromagnetic fields in the microwave region of the spectrum. The structure is either hollow or filled with dielectric material. The microwaves bounce back and forth between the walls of the cavity. At the cavity's resonant frequencies they reinforce to form standing waves in the cavity. Therefore, the cavity functions similarly to an organ pipe or sound box in a musical instrument, oscillating preferentially at a series of frequencies, its resonant frequencies. Thus it can act as a bandpass filter, allowing microwaves of a particular frequency to pass while blocking microwaves at nearby frequencies.
  A microwave cavity acts similarly to a resonant circuit with extremely low loss at its frequency of operation, resulting in quality factors up to the order of 10⁶, compared to 10² for circuits made with separate inductors and capacitors at the same frequency.
context_section_description: Conventional inductors are 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 factor.
  Conventional capacitors 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 factor of VHF inductors and capacitors is high enough to be useful, their parasitic properties can significantly affect their performance in this frequency range.  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 capacitor may appear to be an inductor and an inductor may appear to be a capacitor. These phenomena are better known as parasitic inductance and parasitic capacitance.
caption: An absorption wavemeter.  It consists of an adjustable cavity calibrated in frequency.  When the resonant frequency of the cavity reaches the frequency of the applied microwaves it absorbs energy, causing a dip in the output power.  Then the frequency can be read off the scale.
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