This article presents an overview of 3-D-printed microwave and millimeter-wave components integrating gallium-based (Ga-based) liquid metal (LM). The LM is considered here as a fourth degree of freedom for the design of 3-D-printed antennas, filters, and wireless sensors. The 3-D printing combined with LM technology can be considered as a 4-D printing technology. It can benefit from the advantages offered by 3-D printing technologies (low cost and fast manufacturing) and LM at room temperature (flexibility and reconfigurability). To date, in microwave and millimeter-wave applications, 4-D printing technology is mainly used to achieve the metallization and/or reconfigurability and/or mechanical flexibility of high-frequency 3-D-printed components.

This article reports the yield estimation of vineyards from using a mobile millimeter-wave radar system. The proximal radar sensor is a 122-GHz FM-CW radar mounted on a rover that moves along the furrows between vine plants. The proposed yield estimation is based on the classification of radar echoes measured for various grape varieties, for different meteorological conditions, and at different stages of grape development. As yield estimation errors are found to be lower than 10%, the proposed radar technique is suitable for precision viticulture applications.

In this paper, we report the accurate estimation of vine grape yield from a 3-D radar imagery technique. Three ground-based frequency-modulated continuous-wave radars operating, respectively, at 24, 77, and 122 GHz are used for the contact-less estimation of grape mass in vineyards. The 3-D radar images are built from the beam scanning of the vine plants and allow estimating the mass of grapes from the computation of appropriate statistical estimators. These estimators are derived from the measured polarization and magnitude of radar echoes. It is shown that the estimation of grape mass from the proposed ground-based radar imagery technique at millimeter-wave frequency range may be accurate within 1%.