# A Wideband Dielectric Waveguide-Based 160-GHz Radar Target Generator
## 3 Components
### Targets  
The radar targets for the RTG should simulate realistic applications for a tank level probing radar. Two scenarios with a high and a low reflective medium are considered, whereby the target distance should be adjustable arbitrarily. The reflectivity \(\Gamma\) of the medium is determined by its permittivity \(\varepsilon_{r,m}\) with  
\[\Gamma=\frac{1-\sqrt{\varepsilon_{r,m}}}{1+\sqrt{\varepsilon_{r,m}}} \tag{2}\]  
due to the impedance discontinuity between medium and air. With a permittivity \(\varepsilon_{r,\mathrm{H_{2}O}}=5.73\)[17] at 160 GHz water is a high reflective medium and the reflectivity is \(|\Gamma|=-7.73\) dB. Palm oil with a permittivity of 1.8 should be simulated as a weakly reflecting medium. The reflectivity amounts to \(|\Gamma|=-16.72\) dB.  
A reflection in a waveguide can be achieved by a specific perturbation of the field. Since the dielectric waveguide has field components both inside and outside the medium, a reflection can be excited by manipulating the field distribution at the surface. This can be achieved by means of attachable discontinuities and has the advantage that the waveguide geometry does not have to be modified, and the targets can be placed at arbitrary distances. The strength of the reflection depends on the dimensions and the geometry of the discontinuity.  
The two targets are realized with rods made of conductive material. Two rods are attached at the top and bottom in the H-plane (cf. Figure 2) of the waveguide at the same position in the z-direction as shown in Figure 6. The advantage of the chosen realization is, that, compared to ring-shaped structures, the rods can be easily attached and removed.  
With the cuboids, the high reflective medium can be simulated. The cuboid height \(h\) is selected such that the evanescent field outside the waveguide has almost completely decayed. This is achieved with a height of 2 mm [12]. The length \(l\) of the cuboid determines the frequency response. In order to obtain a reflection as uniform as possible in the frequency range from 150 GHz to 170 GHz, the length is 2 mm. The dimension of the width \(b\) should be considerably larger than the waveguide dimension and is set to \(b=20\) mm. The simulated and measured reflection coefficients of the target are shown in Figure 7a. The simulated mean of the reflection is \(-5.77\) dB which is approximately equal to the reflection of water. The flatness in the used frequency range is 1.1 dB. The measured reflection coefficient is shifted by 5 GHz to higher frequencies due to a tilting of the cuboids. This results in a flatness of 3.1 dB in the considered frequency range. The increased flatness has only negligible effects on the radar response.  
Figure 6: Positioning of the different targets at the dielectric waveguide.  
Due to the tapered approximation of the metal to the dielectric waveguide, the reflection coefficient of the metal cylinders is lower. By increasing the cylinder diameter \(d\) the reflection coefficient decreases since the tapered length is longer. For a diameter of \(1\,\mathrm{mm}\) the reflection coefficient is \(-15.82\,\mathrm{dB}\) as shown in Figure 7b. This is comparable to the reflection of palm oil. The width \(b\) is again considerably larger than the waveguide dimension with \(b=20\,\mathrm{mm}\). The measured reflection coefficient agrees very well to the simulations with a mean value of \(-16.17\,\mathrm{dB}\). The measured flatness amounts to \(1.56\,\mathrm{dB}\).  
A tolerance analysis of the positioning shows that a shift or tilting of the targets has significant effects on the reflection coefficient. For this reason, a precisely fitting bracket was designed, which can be mounted around the dielectric waveguide as shown in Figure 8.