Measurement methodologies using microwave frequencies and RF resonators are considered the most accurate way to obtain measurement data to calculate the weight, humidity, or content of water in a variety of materials.
Advantages of Microwave Frequencies
Today, microwave frequencies up to 100 GHz are increasingly being used in industrial applications. When utilized with a special resonator that adapts to the material, one can reliably obtain electrical data like complex permittivity and complex permeability and, in turn, accurately calculate the weight, humidity, or content of water. Because of the wide range of available microwave frequencies, the measurement method can be customized based on the material. Also, microwave sensors provide an overall ease of use, especially compared with nuclear sensors.
Resonators are high quality factor (Q factor) structures that resonate at certain frequencies. A sample material inside of the cavity region affects the center frequency and Q factor of the cavity. Various resonant techniques are available, but the perturbation method is the most commonly used technique. The electrical data like permittivity and permeability of a material can be determined using the frequency shift between f Res unloaded and f Res loaded. The Q factor is calculated based on the frequencies 3dB down to the magnitude at resonance, in the plot marked as flu/flo and fuu/fuo. When both the shift in resonance frequency and Q factor are measured, one can determine two corresponding physical data, for example, the weight and humidity. In many cases only one physical data is required; therefore, it’s only necessary to measure the shift in resonance frequency or the Q factor, rather than both. A typical measurement system using the resonant cavity method consists of a resonator cavity, a signal processing part, and software control by a computer.