A three-frequency millimeter-wave feed horn was developed as part of an advanced component technology task that provides components necessary for higher-frequency radiometers to meet the needs of the Surface Water and Ocean Topography (SWOT) mission. The primary objectives of SWOT are to characterize ocean sub-mesoscale processes on 10-km and larger scales in the global oceans, and to measure the global water storage in inland surface water bodies, including rivers, lakes, reservoirs, and wetlands.
In this innovation, the feed provides three separate output ports in the 87-to- 97-GHz, 125-to-135-GHz, and 161-to- 183-GHz bands; WR10 for the 90-GHz channel, WR8 for the 130-GHz channel, and WR5 for the 170-GHz channel. These ports are in turn connected to individual radiometer channels that will also demonstrate component technology including new PIN-diode switches and noise diodes for internal calibration integrated into each radiometer front end. For this application, a prime focus feed is required with an edge taper of approximately 20 dB at an illumination angle of ±40°. A single polarization is provided in each band. Preliminary requirements called for a return loss of better than 15 dB, which is achieved across all three bands. Good pattern symmetry is also obtained throughout all three-frequency bands. This three-frequency broadband millimeter- wave feed also minimizes mass and provides a common focal point for all three millimeter-wave bands.
In order to achieve similar E and H plane beam widths over the combined 87-to-183-GHz band ring, loaded slots are employed in the corrugated portion of the feed. The feed operates in a flareangle limited condition, which gives approximately constant beam width across the entire band, and provides a common phase center located near its apex. The half-flare angle for the feed is approximately 30°. Analysis and optimization of the overall feed design employed a combination of finite element and mode-matching tools.
The illumination requirements and relative frequency spacing for this application are similar to those required for the Scanning Multichannel Microwave Radiometer (SMMR) on Seasat, the (TOPEX)/Poseidon, and the Jason missions. However, in this particular application the required fractional bandwidth is larger. Thus, while the three-frequency feed horn described here shares many features in common with the feed previously developed for the above missions, enhancements are necessary in order to achieve broad band performance and manufacturability in the millimeter- wave bands.