A.T. Robertson Commentary Matthew 14:1

"At that season Herod the tetrarch heard the report concerning Jesus," — Matthew 14:1 (ASV)

Herod the tetrarch (Hηρωιδης τετρααρχης). Herod Antipas ruler of Galilee and Perea, one-fourth of the dominion of Herod the Great.

The report concerning Jesus (την ακουην Ιησου). See on 4:24. Cognate accusative, heard the hearing (rumour), objective genitive. It is rather surprising that he had not heard of Jesus before.