Abstract-In this letter, the design of a microstrip bi-Yagi and microstrip quad-Yagi array antenna is presented. These designs are a derivative of the original microstrip Yagi antenna array and can achieve a high gain and a high front-to-back (F/B) ratio in comparison to the conventional microstrip Yagi structure first proposed by Huang in 1989. The proposed bi-Yagi and quad-Yagi antenna arrays can also achieve a higher gain (3-6 dB) than the conventional microstrip Yagi array. Simple fabrication techniques can be used with these designs due to the placement of the feeding network on the same layer with the antenna elements. Furthermore, simulations and measurements demonstrate with very good agreement that the proposed arrays can achieve a gain as high as 15.6 dBi (compared to a gain of 10.7 dBi that is achieved by the microstrip Yagi antenna array) while maintaining an F/B ratio that is relatively high.Index Terms-Bi-/quad-Yagi array, capacitive coupling, frontto-back ratio, microstrip Yagi antenna array, quasi-endfire.

Abstract-A novel design of a highly sensitive wireless passive RF strain transducer is presented based on a patch antenna loaded with an open loop that is capable of sensing strain independently in two directions. An original idea of utilizing a cantilever at the gap of the open loop significantly improves the sensitivity of resonant frequency shifts. The frequency shifts in two distinct resonant modes are detected based on two dominant orthogonal modes of the patch resonators. In measurements, the prototypes achieved a sensitivity of 2.35% frequency shift per 1% strain, more than twice that of existing strain transducers of the same class. In simulations, the new design achieved a theoretical sensitivity up to four times as high as existing designs of RF passive wireless strain transducers. The ground plane allows for the sensitivity of the sensor to be independent from the applied surface. An implementation example of the passive remote sensing system based on the proposed strain transducer is also discussed as a proof-of-concept case. Based on calculations, the interrogation method in the example shows a radar cross section fluctuation of 3.8 dB corresponding to the strain induced at the sensor.

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