Here's why they aren't antonyms or homophones:
* Antonyms are words with opposite meanings. For example, "hot" and "cold".
* Homophones are words that sound the same but have different meanings and spellings. For example, "to" and "too".
"Frequent" and "often" share the same meaning, so they can't be antonyms. They are spelled differently and have different pronunciations, so they aren't homophones.