## 1 Introduction
A radio channel model for near ground agriculture applications is rarely considered among the latest literature. Then, it is evident the need for reliable near-ground narrowband radio channel modelling which allows understanding of the undergoing propagation behavior. The path loss model becomes crucial for the design and the evaluation of robust wireless systems for smart agriculture applications. Moreover, it is to evaluate the maximum effective distance between adjacent terminals, hence to estimate the number of sensors needed to cover a certain area. When placing nodes close to the ground in agriculture fields, the transmitted signal experiences a high-level of attenuation due to the components of the field like grass and soil, as well as terrain roughness. As we mentioned above, path loss caused by near-ground placement of antennas is mainly explained in terms of Fresnel zones obstruction [15; 16; 23; 24; 25]. This signal strength loss plays a key role on the quality of service (QoS) causing unreliable communication between nodes leading to an unsuccessful WSN application that will increase both the number of data packet retransmissions and the power consumption in the nodes, causing radio link failure in last term. In this paper a narrowband radio channel model is proposed to predict path loss, in order to fit the signal strength decay caused by agriculture fields found in the propagation link for a near-ground radio scenario at microwave frequency bands. An experimental measurement campaign was carried out using identical transmitter and receiver directional antennas working at 868 MHz, 2.4 GHz and 5.8 GHz, and general purpose lab instrumentation, in agriculture fields with three types of ground: soil, short and tall grass. Firstly, the received power is measured at two different heights of 20 cmand 40 cm above ground. The path loss was then estimated as dependent of the radio link range and antenna height, introducing modifications according to the observations and results achieved. Following, in a different scenario setup, analyzed was the communication quality between commercial sensor nodes dedicated to smart agriculture by measuring the Radio Signal Strength Indicator (RSSI) value as a quality indicator, when placing the nodes at the same two different antenna heights, 20 and 40 cm, only for the case of a short grass field. These nodes were working at 2.4 GHz under ZigBee protocol. From the RSSI measured values, the value of the bit error rate (BER) was derived for different transmitted signal modulation schemes (BPSK, 8-PSK, 16-QAM) in order to compare the theoretical performance of those modulations in a near-ground scenario. In Section 2 are described the measurement systems used in the two experiments and the planned campaigns. Section 3 introduces the experimental results achieved for the three types of grounds and antenna heights. Section 3 also contains the analysis of the experimental results. The proposed path loss model, based on a log-distance fitting, is briefly described in Section 3. Section 4 elaborates a discussion for comparison between the achieved path loss model and results and other existing works. Finally, conclusions close this paper.