wavemeter basics and wavemeter types | transmission wavemeter, absorption wavemeter, cavity wavemeter
This page covers wavemeter basics and wavemeter types. It compares transmission wavemeter vs absorption wavemeter vs cavity wavemeter and mentions difference between transmission wavemeter, absorption wavemeter and cavity wavemeter types.
What is Wavemeter
As we know cavities are used in microwave circuits as circuit elements and as measuring instruments such as frequency meters. Fixed cavities are used in filters (as elements) and in oscillators (as frequency controls). Tunable cavities are used in wavemeters and receiver pre-selectors.
The cavity is constructed by varying its dimensions usually lenfth and calibrating the variation. As shown in the figure-1; wavemeter is located between generator and load. The resonant frequency of LC circuit is defined by following equation.
Following are the wavemeter types.
• Transmission wavemeter
• Absorption wavemeter
• Cavity wavemeter
Transmission Wavemeter type
The figure-2 depicts transmission wavemeter type. As shown, maximum energy reaches the load only when cavity is tuned to resonant frequency. The transmission wavemeter type is coupled via E-plane Tee or probe to main line or guide. It has coupling loop at the output with detector. Usually indicator is attached with the detector probe which indicates maximum output at resonance frequency. This frequency of oscillation is read from wavemeter.
Absorption Wavemeter | Reaction Wavemeter
The figure-3 depicts absorption wavemeter type. It is also known as reaction wavemeter. This type of wavemeter is most often used. It is loosely coupled to RF source which absorbs energy from it. When this wavemeter is adjusted to resonance frequency, the resonant circuit absorbs energy. This is indicated or displayed by dip on meter. The frequency measurement can be done based on the reading from the dial.
Here "d" is either zero or approx. equal to λg/2. This results into short at waveguide wall when cavity is far from resonant condition. In this absorption wavemeter type, cavity impedance is in series with load. This results into dip of load power when wavemeter is tuned to resonant frequency. The magnitude of dip depends on cavity impedance magnitude. This in turn depends on Q (Quality factor) and distance (d).
Cavity Wavemeter type
The figure-4 depicts Cavity wavemeter type. In this type of wavemeter, axis of cavity is made perpendicular to main waveguide. The coupling from cavity to waveguide is obtained using small circular hole or Iris.
As shown in the figure, block of polyron (i.e. absorbing material) is used on back cavity. These blocks prevent false resonance by damping out any oscillations which might exist in this section.
The device indicates wavelength directly on micrometer head attached to plunger with proper design. Accuracy of cavity wavemeter depends on various factors viz. initial calibration, Q, temperature stability and skills of the operator. In general higher Q yields high accuracy.
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