## 2 Radio Wave Propagation Measurements in a Corn Field
### Measurement Equipment and Setup  
Figure 2 shows the configuration of the measurement setup consisting of a portable RF signal generator (AS108, Pico Technology, Tyler, TX, USA, frequency range 300 kHz-8 GHz) directly connected to a reference dipole. The RF generator was configured to generate a continuous wave (CW) signal with a frequency of 0.9 GHz or 2.4 GHz and a power level of 0 dBm. The generator and the antenna were positioned in the corn field and relocated, as illustrated in Figure 3a,d. The transmitted CW signal was captured by a real-time spectrum analyzer (R5550-427, thinkRF, Ottawa, ON, Canada, frequency range 9 kHz-27 GHz) using a reference dipole antenna and stored on a laptop connected to the analyzer through an Ethernet interface for post-processing. The spectral analyzer and antenna were located outside the cornfield, as shown in Figure 3b. The coordinates of the receiving antenna are 43\({}^{\circ}\)05\({}^{\prime}\)19\({}^{\prime\prime}\) N 24\({}^{\circ}\)02\({}^{\prime}\)17\({}^{\prime\prime}\) E.  
Figure 1: Measurements in a corn field in the agricultural area near Dabravata Village, Yablanitsa Municipality, Bulgaria: (**a**) Google Earth image; (**b**) photo from the corn field. Four reference dipoles with \(\lambda/4\) parallel conductor balun using a 3.6 mm diameter semi-rigid cable were constructed according to IEC/IEEE 62209-1528:2020 Std. [40] to carry out the measurements. Two dipoles operated at 0.9 GHz and the other two at 2.4 GHz. The mechanical dimensions and \(\mid\)S\({}_{11}\)\(\mid\) of the dipoles are presented in Table 1. A vector network analyzer (Tektronix TTR503A, Tektronix, Inc., Beaverton, OR, USA) was used to measure S parameters. Before measurements, an Open-Short-Load calibration was performed. To study the near- and above-ground radio wave propagation in real-world smart agriculture scenarios, the transmitting (Tx) dipole antenna (more specifically its feed point) was positioned in the corn field at a distance from the ground equal to one-third of the wavelength (0.11 m and 0.04 m at 0.9 GHz and 2.4 GHz, respectively) and 0.5 m above the ground. We experimentally determined the minimum height for installing the Tx dipole above the soil by measuring the S\({}_{11}\) parameters at different heights. It is well known that the proximity of the dipole to the ground can significantly impact the antenna's parameters and characteristics. Figure 4 displays the measured \(\mid\)S\({}_{11}\)\(\mid\) at a distance from the ground equal to one-third of the wavelength. The results show that placing the dipole at a distance less than 0.3\(\lambda\) m from the ground significantly affects antenna characteristics, including resonant frequency and input impedance, due to proximity effects. The receiving (Rx) dipole antenna positioned outside the cornfield was at a height of 2.0 m and 3.4 m above the ground, as shown in Figure 3b,c. We selected 29 transmitter-receiver locations in obstructed line-of-sight (O-LoS) conditions to perform measurements for vertically and horizontally polarized antennas. Received signal levels (Rx Levels) from the spectrum analyzer were recorded at 1 m, 5 m, and every 5 m after that as the transmitter moved away from the receiver, resulting in 29 locations total for co-polarized vertical-to-vertical (V-V) antennas at both 0.9 GHz and 2.4 GHz. The procedure was repeated using co-polarized horizontal-to-horizontal (H-H) antennas. In addition, Tx and Rx antennas were located so that the line connecting the antennas was perpendicular to the corn rows, as depicted in Figure 3d. For each of the presented scenarios, ten consecutive values of the received signal level were measured for each location of the transmitting antenna. The graphs show the average values of the ten measurements.