https://www.subseatechnologies.com/hemisphere-gnss/antennas/a21/
A21 Antenna
The A21™ antenna is designed to help maintain tracking of GPS and differential correction signals in challenging environments. Sometimes keeping the antenna level and away from electrical noise is just not possible. With a metal base, lower profile, improved multi-path mitigation, and ability to filter out an additional 30 decibels of radio band frequencies, A21 offers superior noise rejection. The A21 is designed for use with Hemisphere GNSS Crescent® and Crescent Vector™ II receivers.
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