## 2 Instrument Design
### Radiometer Assembly
#### 2.1.1 Antenna Set  
The antenna set of MERITXELL is formed by eight dual linear polarization antennas that collect the electromagnetic radiation in different passive Earth observation bands, commonly used in microwave radiometry. These eight bands are located into seven bands named: L-band (1.400-1.427 GHz), S-band (2.69-2.70 GHz), C-band (7.14-7.23 GHz), X-band (10.6-10.7 GHz), K-band (18.6-18.8 GHz, and 23.6-24.0 GHz), Ka-band (36-37 GHz), and W-band (86-92 GHz). For the sake of clarity, each of the bands at K-band is coined with a different name in this work. The 18.6-18.8 GHz band is named K', whereas the 23.6-24.0 GHz band is named K\". Figure 2 shows a front view of the eight antennas of instrument MERITXELL without their radome. In the case of the L-, S-, and C-band, the antennas for each band are 4 \(\times\) 4 patch arrays with a 25\({}^{\circ}\) beamwidth at \(-\)3 dB, a Main Beam Efficiency (MBE) of 95%, a Cross-Polarization Isolation (CPI) better than 35 dB in the main beam (i.e., \(-\)3 dB beamwidth), and a radiation efficiency \(\eta_{A}\) of 95%. Parameters obtained for the antennas have been measured at UPC anechoic chamber as mounted in the whole system. Each one of the patches that conform these array antennas is a dual-polarization coaxial-fed microstrip antenna printed in a 0.6 mm fiberglass circuit board with dielectric air. The signals collected by the 16 patches are combined with different weights depending on the position in the array by means of a microstrip power combiner circuit for each polarization, vertical (V) and horizontal (H). Barlett (triangular) tapering is applied in both directions to reduce the side lobes and improve the main beam efficiency, a critical parameter for microwave radiometer antennas. Eventually, the signal output ofeach power combiner is guided through a coaxial cable to the front-end stage. These antennas have been developed following the novel architecture implemented in the L-band AUtomatic RAdiometer (LAURA), described in [25], because of their excellent performance. In fact, the one used for the L-band measurements is an improved replica of the LAURA antenna, whereas the S-, and C-band antennas are scaled versions of the previous one. Figure 1: MERITXELL overall block diagram. The design has been divided in five sub-systems: **(1)** antenna set; **(2)** front-end; **(3)** back-end; **(4)** additional sensors; and **(5)** monitoring & control. For the X-, K-, Ka-, and W-band, the chosen antenna for each one is a corrugated horn combined with a Fresnel lens that produces a Gaussian beam. The corrugated horns contribute to increase the bandwidth, as well as the MBE and CPI figures as compared to other kinds of horn antennas. The resulting antennas have a beamwidth at \(-3\) dB with a maximum of \(6^{\circ}\) at X-band, and a minimum of \(3.2^{\circ}\) at W-band, their MBE is about 98%, their CPI is higher than 37 dB at the main beam (i.e., \(-3\) dB beamwidth), and the radiation efficiency \(\eta_{A}\) is about 99%. The signal collected by each horn is separated into V and H polarization signals using an Orthomode Transducer (OMT). Eventually, each one of the OMT outputs feeds its corresponding waveguide to the front-end stage. Figure 3 shows the antenna used for the Ka-band with the corresponding output waveguides for V and H polarizations. Eventually, Table 2 summarizes the features of the antenna set built in MERITXELL.