A dual-band antenna at terahertz frequencies could be useful for communication and radar applications, but is often difficult to fabricate using traditional techniques. Here a design discussed in  is evaluated with XFdtd EM Simulation Software which uses a conical horn to radiate a lower frequency band at 94 GHz and a tapered dielectric strip to carry the higher band of 340 GHz. The antenna produces equal beamwidth patterns in the E and H planes with gain around 18 dBi for both frequencies.
Device Design and Simulation
The feed end of the antenna consists of two rectangular waveguide ports that are orthogonally-aligned with the lower frequency port on the side of the structure and the higher frequency port in line with the center axis of the horn. A tapered dielectric strip of quartz runs the length of the device and partially fills the waveguide feed at 340 GHz. The fields at the higher frequency are then guided by the strip while the lower frequency port at 94 GHz feeds into a rectangular to circular waveguide transition region and then into the conical horn. The device is shown in Figure 1 in three dimensions with the tapered quartz strip shown in a red color and the 94 GHz port visible on the +X side of the horn. Views of the horn from the top (+Y) direction and front (+Z) direction are shown in Figure 2 and Figure 3, where the tapered strip can be seen to partially fill the waveguide in the X direction.
The two ports are excited by broadband modal waveguide sources to generate the S-parameter results. In Figure 4, the return loss for the lower band port is shown and can be seen to be below -15 dB at 94 GHz. Figure 5 shows the return loss for the upper band port which is nearly -20 dB at 340 GHz. The coupling between the ports is extremely low and generally below -70 dB.
The propagation of the 94 GHz fields as a function of time is shown in Figure 6, where the transition from rectangular to circular waveguide may be seen followed by the radiation from the horn. The steady-state field distribution in Figure 7 shows smooth field transitions across the horn. The higher frequency excitation through the orthogonally-connected second port shows in Figure 8 how the dielectric strip guides the time varying waves out of the antenna. The steady-state field distribution at 340 GHz shows tight containment of the fields around the strip, as seen in Figure 9.
The radiation patterns of the two bands are shown in Figure 10 and Figure 11, where both frequencies have similar beam patterns and gain with low side lobes. In Figure 12, the gain patterns in the E and H planes are shown for the 94 GHz radiation and the cross-polarization levels are very low while the main beam is symmetrical. At 340 GHz, the patterns in the E and H planes have higher cross-polarization but still nearly symmetrical main beams and low side lobes as shown in Figure 13. The radiation and system efficiencies for both frequencies are over 95% as there is little loss in this antenna design.
The results of the simulations show how the lower frequency band is radiated by the horn structure while the higher band is carried by the tapered dielectric strip. The dual-band horn design shows good performance at both frequencies with high gain, symmetrical beams and low sidelobes. The antenna has high efficiency and good return loss characteristics at both frequencies of interest.
 X. Wang, C. Deng, W. Hu, Y. Liu and X. Lv, "Design of a 94/340GHz horn antenna loaded with dielectric for dual-band operation," 2017 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, San Diego, CA, USA, 2017, pp. 561-562, doi: 10.1109/APUSNCURSINRSM.2017.8072323.