Total posts : 45366
1. There is no “legal” antenna size for Part 15 FM operation. The only criterion is signal strength (max 250 uV/m at 3m from the antenna). Most Part 15 FM boxes are designed for maximum legal signal strength using an integral antenna of a certain size. Upping the antenna without dialing the RF power back puts you over the limit.
2. If you’re getting out clear to 2 miles, I’m pretty sure you’re over the limit. Legal strength should get you out probably no more than 1/4 mile, maybe 3/8 mile. I’d take Josh’s advice and dial your output power down.
3. I have no experience with the Metzo, but your antenna is a big item on Part 15 AM. There’s a “Range” topic regarding Part 15 AM started by broadcastman that has 40 replies, and is worth a look. Also, the antenna with the big coil (obviously AM) that’s shown on the Part15.US home page has attracted a lot of interest.