Abstract
In this study, an ultra-compact humidity sensor based on a double-folded substrate integrated waveguide (SIW) re-entrant cavity was proposed and analyzed. By folding a circular re-entrant cavity twice along its two orthogonally symmetric planes, the designed structure achieved a remarkable size reduction (up to 85.9%) in comparison with a conventional TM010-mode circular SIW cavity. The operating principle of the humidity sensor is based on the resonant method, in other words, it utilizes the resonant properties of the sensor as signatures to detect the humidity condition of the ambient environment. To this end, a mathematical model quantitatively relating the resonant frequency of the sensor and the relative humidity (RH) level was established according to the cavity perturbation theory. The sensing performance of the sensor was experimentally validated in a RH range of 30%–80% by using a humidity chamber. The measured absolute sensitivity of the sensor was calculated to be 135.6 kHz/%RH, and the corresponding normalized sensitivity was 0.00627%/%RH. It was demonstrated that our proposed sensor not only has the merits of compact size and high sensitivity, but also benefits from a high Q-factor and ease of fabrication and integration. These advantages make it an excellent candidate for humidity sensing applications in various fields such as the agricultural, pharmaceutical, and food industries.
Highlights
Humidity is one of the most important indexes for evaluating environmental quality, and it is crucial to monitor and control humidity accurately in many scenarios
A novel double-folded SIW (DFSIW) re-entrant cavity dedicated to humidity detection is presented in this paper
Compared to the conventional circular substrate integrated waveguide (SIW) cavity, the proposed structure has achieved a size miniaturization up to 85.9%
Summary
Humidity is one of the most important indexes for evaluating environmental quality, and it is crucial to monitor and control humidity accurately in many scenarios. This design suffers from a poor sensitivity (only 30.73 kHz/%RH at 6.9 GHz), and its loaded Q-factor is relatively low due to the open boundaries inherent with quarter-mode structures From these reported works, one can see that the realization of SIW humidity sensors simultaneously possessing high sensing performance and miniaturized size still remains a challenge. When moist air enters the sensitive regions, it causes a strong perturbation to the electric field and results in a large resonant frequency shift of the cavity This structural design offers an effective solution for balancing the sensing performance and physical dimensions of a passive microwave humidity sensor, which is of great significance for promoting the development and application of microwave humidity sensing devices
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