# Scaled Modeling and Measurement for Studying Radio Wave Propagation in Tunnels
## 4 Experiments in a Rectangular Pedestrian Tunnel  
The experimental system is shown in Figure 4. It consists of a transmission system \(T_{x}\) and a receiver \(R_{x}\). The transmitter was based on a 9 kHz-20 GHz analog signal generator (KEYSIGHT EXG N5173B), set for a continuous wave transmission with a Continuous Waveform (CW) power of 19 dBm, fed to a 1-18 GHz Broadband Horn Antenna (A-INFO JXTXLB-10180), with a gain of 4.97 dBi (at 1 GHz), 13.2 dBi (at 2.4 GHz) and 12.69 dBi (at 10 GHz). The A-INFO antenna cross-section is 244 \(\times\) 164 mm\({}^{2}\). This type of antenna was also used at the receiver site, which was connected to a spectrum analyzer for measuring the received signal power and frequency. Its high dynamic range and sensitivity allow measuring even very low level of received power, well below the minimum measured signal power, which was in the experiment \(-\)60 dBm. The \(R_{x}\) antenna and the spectrum analyzer were placed on a mobile cart. The \(T_{x}\) antenna was mounted on a tripod, as shown in Figure 4a.  
Figure 3: Simulations of the link budget along a pedestrian concrete tunnel considering different permittivity values for frequencies of (**a**) UHF 1 GHz, (**b**) UHF 2.4 GHz, and (**c**) SHF 10 GHz.  
Both the transmitting and receiving antennas were placed at the center of a tunnel cross-section, with smooth concrete walls and with a rectangular cross-section as shown in Figure 4b. The measurements were taken for the UHF and SHF bands, from 4 m between the antennas in order to fulfill far-field conditions and avoiding near-field measurements, ending at 45 m, with the receiver being moved in steps of 25 cm. The position of the receiver antenna relative to the center of the tunnel was verified at every step to ensure the measurements were consistent.  
A comparison between the simulation and the experimental results of wave propagation along the tunnel for the three frequencies is shown in Figure 5. The rest of the physical parameters, including the height, width, antenna location, and frequency, were identical to those of the experimental scenario. In the experiment, the Minimum Detectible Signal (MDS) power was set to \(-80\) dBm. It was well above the minimum measured signal power which was \(-60\) dBm. The graphs show that the simulations based on ray tracing had correlated well with the measured results for all three frequencies, which confirms that ray tracing is suitable for modeling radio wave propagation in tunnels in the UHF and SHF regimes. The measured signal power is a result of coherent summation of the ensemble of rays propagating from the transmitting horn and arriving at the receiving horn aperture. The spectrum analyzer shows the total resulted power collected by the receiving horn.  
Figure 4: (**a**) Experimental setup in the tunnel, and (**b**) block diagram of experimental setup.  
Figure 5: \(Cont\).  
Inspection of the experimental measurements in Figure 5 reveals a good fit with the simulation results. The main discrepancy is due to the limited dynamic range of the receiver that cannot measure low power dips emerging when distractive interferences occur. Taking into account this limitation, the experimental results show that the link between the budget estimation with the ray-tracing model is also reliable for wireless communication links operating in narrow tunnels or corridors.