Tech Briefs

Compact Ka-Band Antenna Feed with Double Circularly Polarized Capability

This design could be used for the feed component in highgain antennas.

NASA has an interest in utilizing the Ka-band frequency allocation. One of the main reasons for migrating to Kaband is the need for higher frequency bandwidth to enable higher data rates. A dual circular polarized wideband antenna for Ka-band communications applications was designed leveraging a novel Ka-band polarizer design used in Lunar Reconnaissance Orbiter/Solar Dynamics Observatory (LRO/SDO). The proposed design merges two components (polarizer and antenna) into one unit, reducing its overall size. Its simulated bandwidth extends beyond the allocated bandwidth for NASA at Ka-band. This novel design could be used as well for the feed component in high-gain antennas (HGAs).

Satellite communications at the Kaband frequencies has been a topic of interest for the last decade. NASA has had a Ka-band frequency allocation underused for many years due to various reasons. When the transmitter’s output data rate after encoding is 450 Msps, the spectrum needs to be severely filtered to avoid interference with NASA’s DSN frequency allocation, which promotes large amounts of inter-symbol interference at the receiver. Transitioning to Ka-band will mitigate this problem since the allocated bandwidth is larger, allowing much higher data rates to be sustained without such signal distortions.

The Earth-shaped reflector antenna at Ka-band consists of a single reflector configuration where the feed is located at the focal point of the system using supporting struts, and the reflector is shaped to produce an Earth-shaped radiation pattern. The waveguide used to carry energy to the antenna feed can be located along one of, or be part of, the struts supporting the feed above the reflector.

The antenna design consists of merging a corrugated feed antenna with a polarizer to produce a satisfactory solution of blockage reduction (size reduction) of the reflector aperture. What is usually a two-component system has been reduced to a single part, reducing mass, size, and cost. This antenna could be used in Kaband HGA or Earth-shaped antenna reflector combinations. The polarizer/ antenna combination also allows it to be used in dual circular polarization applications. The overall return loss is better than 20 dB for the simulated frequency range of 23.5 to 28.5 GHz, showing an excellent impedance match over NASA’s frequency allocation. The port isolation between the ports has not been optimized, but can be optimized for the application at hand.

For the current design, the isolation for the standalone feed antenna and polarizer combination varies between –15 to –18 dB over the frequency band. For a standalone application (no reflector), this could be improved by improving the internal wave impedance match between the polarizer and feed antenna. For a reflector application, it can be optimized to partially cancel the reflection from the reflector back into the feed, which is typically of the same order of magnitude.

This work was done by Cornelis du Toit of QSS and Kenneth Hersey of MEI Technologies for Goddard Space Flight Center. GSC-16773-1