## 4 Experimental Results  
The measured and EM simulation frequency responses of the three manufactured GGW bandpass filter prototypes shown in Figures 7 and 8b,c are plotted in Figures 11-13, respectively. The prototypes were measured from a vector network analyzer (R&S ZVA67). A WR-75 TRL (through, reflect, line) calibration KIT and a TRL calibration procedure were used for de-embedding the effects of the cables and SMA-rectangular waveguide transitions from the measurements of the prototypes between 8 GHz and 15 GHz. In this way, the \(S\)-parameters measurements of the prototypes were obtained between their input and output ports. This frequency range (8 GHz-15 GHz) allows single-mode operation (TE\({}_{10}\)) in the WR-75 rectangular waveguide and GGW (Figure 2). Below 8 GHz and above 15 GHz, accurate measurements of the prototypes cannot be carried out, since calibration errors and power fluctuations occur due to the cut-off frequency of the WR-75 rectangular waveguide TE\({}_{10}\) mode and the superposition of the first- and higher-order modes in their structures (Figure 2), respectively. The conductivity (\(\sigma\)) of the conductor and the roughness (\(R_{a}\)) of the surfaces were considered in the EM simulations (HFSS) of these three prototypes. Figure 11: EM simulated and measured \(S\)-parameters for the aluminum fifth-order wideband GGW Chebyshev filter (Figure 7). Figure 12: EM simulated and measured \(S\)-parameters for the silver metallized resin-based fifth-order wideband GGW Chebyshev filter (Figure 8b). The measured 5-parameters (Figure 11) of the fifth-order GGW Chebyshev bandpass filter manufactured in aluminum using a CNC milling machine show a good agreement with the simulated data. Regarding the EM simulations, the measurements show a slight shift of 50 MHz towards higher frequencies due to the manufacturing tolerance, which is \(\pm\)100 \(\upmu\)m. Between 10 GHz and 12 GHz, corresponding to the lower frequency of the recommended band of the WR-75 standard and the higher cut-off frequency\(f_{c,H}\) of the filter, the return and insertion losses are, respectively, \(RL=20\) dB and \(IL=0.3\) dB. These losses deteriorate between 8.3 GHz and 10 GHz, due to the high dispersion of the quasi-TE\({}_{10}\) mode (Figure 2) and to the frequency dependence of the impedance inverters (metallic pins) in this frequency range [19; 47]. The out-of-band region has a spurious free range up to 15 GHz with a maximum rejection level \(RLSB=70\) dB. The resin-based prototypes (Figures 12 and 13) exhibit a bandwidth \(BW=3.7\) GHz and a stopband with a spurious free range up to 15 GHz like the previous aluminum-based GGW bandpass filter. The measured results of both resin-based prototypes agree well with the simulated data, although some differences in return loss and number of reflection zeros can be seen. The measured return and insertion losses in the frequency range 10 GHz-12 GHz for the resin-based filter (Figure 12) metallized with a silver layer are \(RL=20\) dB and \(IL=1\) dB, respectively. Its maximum rejection level in the stopband is \(RLSB=65\) dB. Except for \(RL=15\) dB, the resin-based filter (Figure 13) metallized with a conductive paint spray has similar results: \(IL=1.2\) dB and \(RLSB=71\) dB. A comparison between machined (Figure 11) and 3D printed (Figures 12 and 13) prototypes shows both resin-based filters present worse insertion loss than the aluminum-based one. This is because the surface roughness of the 3D printed prototypes has a negative effect on the conductor loss of the silver and conductive paint layers. The roughness of the metallized surfaces is larger than the one obtained with the CNC technique, and consequently the attenuation increases [51]. In addition, the conductive paint has a poor conductivity (\(\sigma=10^{4}\) S/m), which is approximately 6300 and 2800 times less than the value of the silver and 6082 aluminum, respectively.