# Design and Implementation of a Multi-Band Active Radar Calibrator for SAR
## 3 Results  
### Design of the ARC  
#### 3.1.1 Antenna Design  
The form and performance of the antenna determine the whole structure and performance of the ARC to some extent. Considering the operation at a multi-band frequency of one ARC, the antennas must be able to work at a frequency range from the L-band to Ka-band. It is almost impossible to design an ultra-wideband antenna that works from the L-band to the Ka-band with the specifications of gain, a 3 dB beam width, an antenna pattern, and isolation between receiving and transmitting antenna to satisfy the requirements for the ARC at all four bands. So, a multi-band antenna needed to be designed for the ARC. We designed a combined antenna consisting of an L/C-band common aperture horn antenna [17; 18], an X-band dual-polarimetric corrugated horn antenna, and a Ka-band corrugated horn antenna for the multi-band ARC. A schematic illustration of the combined antenna is shown in Figure 2. The types of polarization used are H-pol for the L-band, H- and V-pol for the C- and X-bands, and V-pol for the Ka-band.  
There are many key technologies and difficulties in the design of the multi-band antenna such as L/C-band common aperture design, C/X-band dual-polarimetric and high polarization isolation design, L-band miniaturization design, and so on. The dual-polarimetric and high polarization isolation design technology has been introduced in existing literature [6; 19]. We emphasize the key technology of L/C-band common aperture design. A schematic illustration of the L/C-band common aperture antenna is shown in Figure 3.  
Figure 2: Schematic illustration of the combined antenna.  
The L/C-band common aperture antenna consists of a common radiation section, an L-band feed waveguide, a C-band transmission section, and an ortho-mode transducer (OMT). The amplitude and phase distribution of the aperture field will affect the antenna's radiation performance, so the structure design of the common radiation section is very important. Generally speaking, the feed port of an antenna is located at the transmission section. However, it can be seen from Figure 3 that the L-band feed waveguide is placed at the common radiation section, which reduces the length of the antenna significantly.  
In addition, the demand for isolation between the receiving antenna and transmitting antenna is very high in order to ensure precision in the RCS of the ARC. In general, the isolation should be at least 25 dB better than the gain of the radio frequency circuit [14]. In order to satisfy the demand, on the one hand, the level at 90 degree from the boresight should be designed to be as low as possible, and on the other hand, choke slots were placed on the sidewall of the radome. Based on the measures above, the isolation reaches 79 dB for the L-band, 80 dB for the C-band, 82 dB for the X-band, and 84 dB for the Ka-band, based on measurement in an open space. The isolation is very high and it is hard to measure. First we chose an open space and the Tx and Rx antennas pointed up to the sky in the measurement, and then the IF band of vector network analyzer was set to 100 Hz.  
The gains designed on respective center frequencies are 12 dB for the L-band, 22 dB for the C-band, 23 dB for the X-band and 27 dB for the Ka-band, with a typical sidelobe level of \(-\)17 dB for the four bands, which minimizes multipath effects and background noise.