Definition
An antenna whose radiation pattern transmits and receives radio energy most strongly in two opposite directions, with reduced sensitivity in the directions perpendicular to that axis.
Plain English
An antenna that sends and receives signals best along a single line, with strong signal going out the front and the back, but weak signal off the sides.
Context Anchor
Seen in discussions of aircraft radio equipment, navigation aids, and antenna radiation patterns.
Derivation
From Latin 'bi-' meaning 'two' and 'directio' meaning 'a pointing or aiming.' The word literally describes something that works in two directions — useful here because the antenna's strongest signal goes out along two opposite headings.
Why Pilots Care
Knowing the radiation pattern of an antenna matters because signal strength and direction-finding accuracy depend on how the antenna is oriented relative to the station being received.
Intuition Check
Bidirectional does not mean the antenna can both transmit and receive. Here it means the antenna’s strongest coverage is in two directions.
Example Sentence 1
The loop antenna in an older ADF system is bidirectional, which is why a sense antenna is added to resolve which of the two directions the station actually lies.
Example Sentence 2
During preflight the pilot confirmed the bidirectional antenna was aligned to cover the forward and aft sectors without side coverage.