## 4 Experimental Results
A vertically polarized radiation is achieved for both sum (\(\Sigma\)) and difference (\(\Delta\)) patterns, coherent with the E-field polarization of the \(TE_{11y}^{\mathit{cir}}\) mode in Figure 3, and the \(TE_{01}^{\mathit{cir}}\) mode in Figure 5. A very good agreement is observed between the experimental and the simulated performance. Nevertheless, there is a remarkable difference between the two normalized radiation patterns in the main beam broadside direction (\(\theta=0^{\lx@math@degree}\), null depth of the monopulse pattern), which is below \(-35\) dB, \(-42\) dB and \(-35\) dB, respectively, and an almost symmetrical response around the broadside direction. These are very important parameters for adequate monopulse operation. Figure 12: The measurement setup of the Dual-Mode Conical Horn Antenna (DM-CHA) in the anechoic chamber. It is also noteworthy that the angular coverage range of the monopulse pattern is approximately \(\pm 11^{\circ}\) (the gray box in Figure 13). This is an important parameter for 5G high data rate and radiolocation satellite communications in the millimeter-wave band. This coverage is directly related to the direction of arrival of the received user signal from Earth, which is calculated by comparing the received signal levels from the difference and sum antenna patterns (monopulse function). The limit of the monopulse coverage is defined in the angular direction in which the sum and difference pattern levels are equal. The system is not operative in angular directions where the difference pattern level is higher than the sum pattern [2]. Wider angular coverage of the monopulse system would require multibeam or scanning (mechanical or electrical) beam capability. In addition, the simulated and measured results of the normalized radiation pattern in polar plot in azimuth (\(\varnothing=0^{\circ}\)) and elevation (\(\varnothing=90^{\circ}\)) planes have been presented in Figure 14.  
Once the monopulse performance of the antenna system has been demonstrated, the aperture efficiency is studied. This parameter is the ratio of the effective radiating area to the physical area of the aperture. As it is explained in [62] (Section 13.5), the theoretical maximum achievable aperture efficiency in a conical horn antenna is 84%, under \(TE_{11y}{}^{cir}\) excitation with ideal uniform phase at the antenna aperture. This is impractical since it would require an infinite horn length \(L_{horn}\). In Figure 15, the simulated and measured antenna directivities of the sum (\(\Sigma\)) radiation pattern are shown, as well as the related aperture efficiency based on the horn antenna aperture area size. The simulated directivity is higher than 22 dBi throughout the frequency band, with a peak value of 22.4 dBi at 38.5 GHz. In terms of aperture efficiency, the antenna presents more than 70% in simulation. This value perfectly matches with the expected value for a conical horn antenna with the horn diameter-to-length ratio actually implemented (\(D_{horn}/L_{horn}=0.6\)) [62] (Section 13.5). These simulation values indicate that the \(TE_{11y}{}^{cir}\) mode of the sum (\(\Sigma\)) pattern has been properly generated. The corresponding measured values are directivity over 21.6 dBi, with a maximum of 21.9 dBi also at 38.5 GHz, and antenna aperture efficiency higher than 65%. The measured 0.5 dB directivity and 5% aperture efficiency reductions compared with simulations can be translated in a maximum E-field phase deviation of \(15^{\circ}\) at the entire antenna aperture area, perfectly explained by fabrication tolerances. Figure 13: Simulation and measurement results of the normalized radiation pattern in azimuth plane (\(\varnothing=0^{\circ}\)) at different frequencies: (**a**) 37.5 GHz, (**b**) 38 GHz and (**c**) 38.5 GHz. The simulated and measured antenna realized gain are also included in Figure 9:0 The conductivity considered in the simulations for the alloy material coming from the DMLS manufacturing of the CHA was 25,000 S/m.