## 5 Experimental Setup  
Now we demonstrate the effect described in the preceding sections using a high resolution FMCW radar operating at the higher band of the millimeter waves, at 330 GHz [27]. A set of experiments have been conducted for studying the effect of fog on the radar performances even at very low visibility conditions, down to 0.5 m. The results were used to demonstrate the effects and validate the model for such extreme conditions. The experimental setup was based on a radar system placed in a confined indoor space filled with artificial fog. The radar system and a metal target were placed in the two far ends of the closed corridor. The fog was created by a thermal fog machine. The chamber has a dimension of 20 m \(\times\) 3.5 m \(\times\) 4 m, where the distance between the radar and the target was \(d=18.8\) m.  
Schematics for the FMCW radar used in the experiment is given in Figure 4a. The HP-8350B frequency-sweeping synthesizer is employed as a primary driver of the Local Oscillator (LO). Its frequency was tuned to sweep from 10 to 10.31 GHz and multiplied by a factor 32 providing linear FM signal starting from \(f_{0}=320\) GHz with a sweep of \(\text{BW}=10\) GHz. The transmission power was 10 dBm. Both the transmitting and receiving antennas were ELVA-1 custom design Gaussian horn-lens antenna with a gain of 40 dBi and linear horizontal polarization. A photograph of the 320-330 GHz FMCW radar is given in Figure 4b. The operational parameters of radar are summarized in Table 2. The detected signal obtained at the output of the harmonic mixer at the receiver chain was analyzed using a spectrum analyzer, model R&S FSV40. The spectrum of the detected signal was measured to find strength of the IF signal and its spectral components. Figure 4: The 320-330 GHz Frequency-Modulated Continuous-Wave (FMCW) radar used in the experimental setup: (**a**) Block diagram and (**b**) Photo including transmitting and receiving antennas. A convenient way to create stable, sustainable, homogeneous, low visibility fog in a relatively large indoor space is by using artificial fog. The artificial fog was generated using a fog machine type MAGNUM 850 with maximum fog output of 200 m\({}^{3}\) per minute. The water based aerosol fog was created by using vaporizing proprietary water and glycol-based (glycols, poly-glycols) fog juice [28]. According to the manufacturer, the particle size produced by this thermal fog machine, was within the wide range of 0.25-60 \(\upmu\)m [29]. Although the artificial fog used in the current experiment was somewhat different in its characteristics from 'natural' water fog and clouds, its small particle sizes fit to our experimental requirements. The typical droplet size in natural fog is about 10 \(\upmu\)m. The artificial fog droplet diameter was measured to be around \(R_{p}=4\)\(\upmu\)m [28]. This leads to a size parameter \(2\pi R_{p}/\lambda=0.1\) at 330 GHz. This is well below the Mie scattering regime and within the Rayleigh approximation. The spatial fog spread in space was homogenized using a series of small fans along the corridor (see Figure 5). The composition of the artificial fog was based on aqueous glycol solution. The solution components and their respective refractive index were triethylene glycol (\(n=1.4531\)), 1,3-butylene glycol (1.4401), propylene glycol (1.4324), and deionized water (1.330). Since the weight percentage of each component was not provided by the manufacturer, an estimation was made for the refractive index as 1.439 [29]. The initial thermal conditions along the corridor were 16 \({}^{\circ}\)C and 70% relative humidity. The process of filling the space with the artificial fog lasted several minutes to reach a visibility less than one meter. The filling process can be seen in Figure 6. The droplet number concentration \(n_{d}\) and liquid water content \(W_{0}\) can be estimated using expression (17). For a visibility of \(Vis=0.5\) m we found that in our case, \(n_{d}=4000\) cm\({}^{-3}\) and \(W_{0}=30\) g/m\({}^{3}\).