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Abstract-The wireless communications in a tree canopy is essential for pre-harvesting control of fruit productions.To efficiently communicate between a sensor node and a sink node, channel characteristics in a tree canopy must be well-established. In this paper, propagation channel characteristics at the frequencies of 2.45 and 5.2 GHz have been estimated for designing a wireless communication system in a tree canopy. The proposed solution is based on measured path loss, time-varying signal strength and Angle of Arrival (AoA) for various paths in a tree canopy to estimate the channel. Since the waves reflect, refract, diffract and scatter from the foliage, it is complicated to find the true travelling path between a transmitter and a receiver at the nodes. The AoA estimator is used for physical interpretation of the channel. The experimental results demonstrate the channels in a tree canopy are mostly matched with the General Extreme Value model. The measured path gains illustrate that the appropriate antenna patterns must be selected to enhance the reliability of the system.

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This paper presents a microwave reflectometer using self-mixing oscillator antennas. A unique characteristic of the proposed reflectometer is its simple structure without a circulator and a directional coupler. Each self-mixing oscillator antenna utilizes a Gunn diode as an active device and a rectangular waveguide as a resonator and a radiator. The determination of dielectric properties of materials was carried out at 9.79 GHz and the measured results using the proposed reflectometer were in good agreement with those by a dielectric probe. In addition, the reflectometer has several industrial applications, e.g. fruit classification, moisture content sensing, etc.