## 4 Experimental Setup  
The experiment was conducted to examine the effect of fog on electromagnetic signal propagation in the Ka band, in a frequency region of 26.5-40 GHz. An Agilent (USA) Vector Network Analyzer model N5230C covering frequencies up to 40 GHz is used. The experiment was conducted in a closed tunnel, shown in Figure 4, where artificial fog was introduced under controlled conditions. The Ka radiation was transmitted through the fog, while measuring attenuation and phase shifts. Independent optical measurements were performed simultaneously, to characterize fog visibility and water vapor concentration inside the tunnel. The tunnel dimensions were 18 m length with a cross-section of 2 m \(\times\) 2 m (width \(\times\) height), allowing free passage across the mmW radiation beam, see Figure 4. The transmitter/receiver system was placed on one side of the tunnel and a 2 m \(\times\) 2 m metallic board with Lambertian surface was positioned perpendicularly on the other side, in order to enhance the back-scattering signal to the radar's receiver. As a result, the mmW radiation traveled through a total distance of 36 m (forth and back). The tunnel was built to a minimum of clapboard construction to reduce a parasitic back-reflections of stray radiation toward the receiver. The entire length of the tunnel was covered with a nylon sheet to allow a stabilizing of a water droplets cloud to settle. This experimental setup was built within a closed structure to minimize the impact of the outdoor weather on the meteorological conditions, cloud concentration, and droplets size distribution inside the tunnel. Through four openings along the tunnel, a thick artificial fog was introduced with a stable size distribution whose mass median diameter (MMD) is approximately 8 \(\upmu\)m. This droplet distribution represents a compromise between the distribution of cumulus clouds and radiative fogs [29; 30], which  
Figure 4: Schematic illustration of experimental setup [16]. Figure 3: Graphs of the 28 GHz link budget power ratio Pr/Pt as a function of distance d.  
mmW radiation may propagate in a natural atmosphere. The cloud was generated using four ultrasonic foggers based on piezoelectric transducers whose operation is controlled automatically during the experiment to achieve the required visibility, shown in Figure 5. A fast homogenization of the cloud inside the tunnel was achieved, by small fans that were distributed along the tunnel's corners to reduce effects of stray radiation to the receiver, shown in Figure 6. The fans were turned off during the measurement itself. The mmW propagation measurements were carried out sequentially from high to low cloud concentrations. The decrease in fog concentration, while maintaining homogeneity and other cloud  
Figure 5: The fog-streaming machine. Figure 6: The fog chamber with the small fans used to distribute the fog uniformly. parameters stable within the tunnel, is achieved spontaneously during the experiment. The highlighted gray area in Figure 7 shows a typical homogeneity while gradual reduction in visibility (panel b) and corresponding cloud concentration (panel a) is achieved within the tunnel, as indicated by the similarity of the black and the red curves. A course of \(\sim\)12 successive propagation measurements during this time period were completed within 20 min to ensure maximal reliability when comparing results. The classification of droplets size distributions was done using a SprayTech Malvern Instrument equipped with a 300 mm lens, allowing for real-time measurement of droplet diameters in the range of 0.1-900 \(\upmu\)m. The technique is based on a diffraction of light in 632.8 nm that is measured by a spaced array of 36 detectors.