I'm asking here because I've gotten different responses from different DMs. My home campaign DM said I had to activate my Knight's defender aura during my first round of an encounter before it was effective. One of the DMs at the D&D Encounters group I attend said that I didn't need to activate the aura because it was assumed that as an at-will aura it was always active unless I choose to deactivate it or become unconscious.
An aura is a continuous effect that emanates from a creature. Unless otherwise noted, an aura uses the following rules.The glossary entry in the compendium states:
Deactivating an Aura: A creature can take a minor action to deactivate or reactivate one of its auras. However, certain auras have set durations and cannot be reactivated after they end.
Death or Unconsciousness Ends: A creature’s auras end immediately when it falls unconscious or dies. - Rules Compendium page 308.
So I assume it should be active unless I say otherwise.