# Low-Profile Dual-Band Reflector Antenna for High-Frequency Applications
## 2 Antenna Configuration and Design
### Composite Feed  
A composite feed operating in MMW/THz bands is proposed. As shown in Figure 3, the composite feed consists of a higher-band feed and a lower-band feed. The higher-band feed is a circular waveguide, while the lower-band feed is composed of a dielectric, a conical coaxial horn, and a waveguide connector. The dielectric serves as a supporting structure that maintains the relative position of the conical coaxial horn and the circular waveguide stable. Due to its small size, the dielectric has minimal influence on the lower-band feeding. Figure 2: Sketch of ray paths in the lower and higher bands. The diameters of circular waveguides are important for the working band. Therefore, it is crucial to determine the dimensions of the waveguides first. Firstly, the diameter of the circular waveguide for the higher-band feed is chosen to be \(r_{3}=0.68\lambda_{h}\), ensuring that the TE\({}_{11}\)-mode electromagnetic waves propagate within the frequency range of 0.90-1.10 \(f_{h}\), as shown in Figure 4b. Secondly, the diameter of the coaxial waveguide for the lower-band feed is chosen to be \(r_{4}=3.22\lambda_{h}\), which can sustain the propagation of the first higher-order TE\({}_{11}\) mode within the frequency range of 0.91-1.10 \(f_{l}\). Figure 4a shows the transmission coefficients of the first and second higher-order modes of electromagnetic waves in the lower band, propagating in the coaxial waveguide. As a result, the desired first higher-order TE\({}_{11}\) mode of electromagnetic waves can be propagated in the coaxial waveguide. Thirdly, the waveguide connector is chosen as the mode converter. The first higher-order TE\({}_{11}\) mode of the coaxial waveguide can be excited by the TE\({}_{11}\)-mode transmission mode of the waveguide connector, as the electric field distribution of the TE\({}_{11}\)-mode is similar between rectangular waveguides and coaxial waveguides. Finally, port 1 of the coaxial waveguide is connected to the rectangular waveguide, as shown in Figure 4b. The figure displays the TE\({}_{11}\)-mode transmission coefficients of the circular waveguide and the coaxial waveguide within the higher and lower bands separately. Therefore, the TE\({}_{11}\) mode of a coaxial waveguide can be excited by a rectangular waveguide with low loss in the lower band. The TE\({}_{11}\)-mode transmission coefficients of the coaxial waveguide in the lower band can be optimized by the parameters of the rectangular waveguide connector, as shown in Figure 3a. The parameters of the dielectric can be adjusted properly to achieve an acceptable matching impedance in the lower band, and the diameters of the stepped cylinders located at the bottom of the dielectric can be adjusted properly to reduce the feed mismatch loss in the higher band. The simulated reflection coefficients of the composite feed are shown in Figure 5, suggesting that the simulated reflection coefficients are below \(-15\) dB in both bands. Figure 4: Simulated transmission coefficients of waveguide: (**a**) Coaxial waveguide within lower band. (**b**) Coaxial waveguide with waveguide connector and circular waveguide. The simulated gain and aperture efficiency in the higher band are shown in Figure 6, based on the full-wave simulations. As a result, the reflector antenna achieves a gain higher than 43 dBi and an aperture efficiency higher than 39% over the entire higher band. Meanwhile, a relative 2 dB gain bandwidth of the higher operating band is approximately 20%, around \(f_{h}\). Furthermore, the simulated normalized co-polarization (Co-pol) and cross-polarization (Cr-pol) radiation patterns at \(f_{h}\) are shown in Figure 6b. The simulated Cr-pol patterns are \(-50\) dB lower than the maximum direction in both E- and H-planes. Table 1 summarizes the estimated aperture efficiency and gain loss reduction attributed to \(f_{h}\). The calculated total losses at \(f_{h}\) are 3.7 dB, with taper loss, spillover loss, and dielectric loss being the main contributing factors.