Wavemeter Types: Transmission, Absorption, and Cavity Wavemeters Explained

This page explores the fundamentals of wavemeters and dives into the different types available. We’ll compare transmission, absorption, and cavity wavemeters, highlighting the key differences between them.

What is a Wavemeter?

Wavemeters are essential tools for measuring the frequency of electromagnetic waves, particularly in microwave circuits. 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, as depicted in Figure 1, are crucial components in wavemeters and receiver pre-selectors. These cavities are constructed by adjusting their dimensions, typically length, and calibrating the resulting variation in frequency. As shown in the figure, the wavemeter is placed between the generator and the load to measure the frequency of the signal.

The resonant frequency of an LC circuit is defined by the following equation:

Wavemeter Types

There are primarily three main types of wavemeters:

• Transmission Wavemeter
• Absorption Wavemeter
• Cavity Wavemeter

Let’s delve into each of these types.

Transmission Wavemeter

Figure 2 illustrates a transmission wavemeter.

In this configuration, maximum energy is transferred to the load only when the cavity is tuned to its resonant frequency. The transmission wavemeter is coupled to the main line or waveguide using an E-plane Tee or a probe. It includes a coupling loop at the output connected to a detector. Typically, an indicator is linked to the detector probe, signaling maximum output at the resonant frequency. This frequency of oscillation is then read directly from the wavemeter.

Absorption Wavemeter | Reaction Wavemeter

Figure 3 shows an absorption wavemeter, also known as a reaction wavemeter.

This type is frequently used. It’s loosely coupled to the RF source, absorbing energy from it. When the wavemeter is tuned to the resonant frequency, the resonant circuit absorbs energy. This is indicated by a dip on the meter. The frequency measurement is then determined from the dial reading.

Here, “d” is either zero or approximately equal to λg/2. This results in a short at the waveguide wall when the cavity is far from the resonant condition. In this type, the cavity impedance is in series with the load. This leads to a dip in load power when the wavemeter is tuned to the resonant frequency. The magnitude of the dip depends on the cavity impedance magnitude, which in turn depends on the Q factor (Quality factor) and the distance (d).

Cavity Wavemeter

Figure 4 depicts a cavity wavemeter.

In this design, the axis of the cavity is positioned perpendicular to the main waveguide. Coupling from the cavity to the waveguide is achieved using a small circular hole or Iris. As shown, a block of polyron (an absorbing material) is placed on the back cavity. These blocks prevent false resonance by damping out any oscillations within this section.

The device displays the wavelength directly on a micrometer head attached to a plunger with proper design. The accuracy of the cavity wavemeter is affected by several factors, including initial calibration, Q factor, temperature stability, and the operator’s skill. Generally, a higher Q factor leads to greater accuracy.