Dual Antennas simply perform better
 

Avidyne's dual-antenna TAS600A Systems detect other aircraft sooner and more accurately, so you avoid surprises due to the shadowing effects inherent with single-antenna systems. Using top and bottom directional antennas, TAS600A systems actively interrogate other aircraft, providing timely audible and visual alerts with the precise location of conflicting traffic. The dual-antenna TAS600A Series provides full-time protection and higher performance. Because when it comes to safety, you want to see the whole picture.
 

About ADS-B
 

ADS-B, which stands for “Automatic Dependent Surveillance-Broadcast,” is a new surveillance technique for air traffic control being implemented in the United States (and other countries). With ADS-B, an appropriately-equipped aircraft determines its own position using a WAAS-capable global positioning system (GPS) and periodically broadcasts this position and other relevant information to ground stations for use by Air Traffic Control (ATC) and to other aircraft with “ADS-B In” equipment for display on compatible on-board multi-function displays. ADS-B is automatic in that no pilot or controller action is required for the information to be issued, and it is dependent on all airplanes having compatible equipment (ADS-B Out) that transmits their position and velocity information. A DS-B can be used over several different datalink technologies, but the two most discussed are Mode-S Extended Squitter (1090 ES) and Universal Access Transceivers (UAT). ADS-B provides accurate information and frequent updates to airspace users and controllers, and is intended to improve the use of airspace, reduce ceiling and visibility restrictions, improve surface surveillance, and enhance safety.