# Highway Tunnel Defect Detection Based on Mobile GPR Scanning
## 2 Materials
### The GPR Antenna  
The 400 MHz source pulse oscillogram had a considerable detection depth, but it could not distinguish the shallow anomalies. While the other one from 900 MHz had a shallower detection depth and higher detection precision. So in this paper, a bi-frequency GPR (see in Figure 3) that can receive 400 MHz and 900 MHz signals simultaneously is developed and manufactured to obtain the precise characteristics of the tunnel structure. The main control unit of GPR is mainly composed of field-programmable gate array(FPGA), power management(PM), data acquisition and processing(AC&P), and radar  
Figure 3: The GPR antenna is designed with two frequency-independent signal channels of 400 MHz and 900 MHz. Figure 2: To ensure that the electromagnetic wave signal is incident vertically into the surface, an antenna deflection mechanism is provided on the top of the manipulator. timebase control(RTBC) unit. Upon these, the control of transmitting and receiving signals and the conversion of echo modulus is realized. The FPGA unit adopts the XILINX SPARTAN-6 series, and the structure is based on the Virtex, a mature architecture, to realize the low risk, cost, and power consumption design. The radar data is connected with the display and control through the network port to realize the system, real-time data transmission, and display function. The PM and AC&P unit is assembled on a circuit board and placed above the substrate, responsible for the power system and processing unit. As shown in Figure 4, the antenna is placed in the metal back cavity to enhance the antenna's penetrating ability and avoid the noise and environmental scattering signals above the ground influencing the reflected signal of the underground target, making the measured data challenging to extract. The receiving and transmitting antennas are also separated by a metal plate to improve the isolation between antennas. The top of the back cavity has a noticeable mirror effect to enhance the radiation ability of the antenna. In a specific range, the radiation pattern of the back cavity antenna with different lengths and widths is consistent. Therefore, designing the cavity size is the critical factor affecting the performance of the butterfly antenna. The size of the shielding shell should be reduced as far as possible to ensure the compactness of the radar system in the premise of enhancing the penetrating ability of the antenna. The design of the cavity height for the mirror effect should be determined by the center frequency of the electromagnetic wave, considering that any size of the cavity is a narrowband system relative to the wide-band pulse signal. The mirror effect of the cavity top to antenna forms a two-unit antenna array, and the total intensity of downward radiation is:  
\[ \begin{array}{l}E=E_{1}\left(1-e^{i2\lambda h}\right)\\ k=\frac{2\pi}{\lambda_{0}}\end{array}  \tag{1}\]  
where \(E_{1}\) is the radiated electric field of the antenna itself; \(k\) is the wavenumber; \(\lambda_{0}\) is the wavelength of the center frequency of the electromagnetic wave. According to the formula, we can know that when \(h\) is 1/4 \(\lambda_{0}\), the radiated energy is the largest, two times as much as \(E_{1}\). The stability of the GPR antenna time and amplitude stability, and time-window linearity were tested before packaging. Which are defined as below:  
\[ \begin{array}{l}J_{time}=t_{\max}-t_{\min}\\ J_{amp}=\frac{A_{\max}-A_{\min}}{A_{\mathrm{rec}}}\times 100\%\\ C=\frac{2\left|\frac{b_{\max}-b_{\min}}{b_{\max}-1}\frac{b_{\max}-b_{\max}}{b_{ \max}-2}\right|}{\left(\frac{b_{\max}-b_{\min}}{b_{\max}-1}+\frac{b_{\max}-b_{ \max}}{b_{\max}}\right)}\end{array}  \tag{2}\]  
Figure 4: The antenna is placed in a metal box. The signal energy is enhanced by adjusting the height of the box. where \(I_{time}\), \(I_{amp}\), and \(C\) are the antenna's time stability, amplitude stability, and time-window linearity index, respectively.