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You can also just put the inReach outside the tent/cabin since you are likely using your phone to actually send and receive messages. I did this at one camp, there wasn't reception in my tent, but there was where my truck was parked so I just left it on the dash while I was in the tent.
I can't imagine actually trying to type a message on a Mini directly without a phone. Assuming you just want to send a predefined message, leaving it outside a cabin or ten for 10 or 15 minutes should give it enough time to connect to satellites.
yeah i dont know about any antenna hacks. however everyone i talked to with the full size inreach says they work alot better then the minis. I have the mini and do not like it one bit. if your hunting the timber you better not need it cuz i texted my wife i killed a buck and might need help at noon and she didnt get the text till 6.
https://www.utvsource.com/garmin-external-inreach-antenna/?gclid=CjwKCAjwiOCgBhAgEiwAjv5whDcTPMO3bwgWn6ezv_sjWDNxU69uHW_NVyqCxkE53IOapBjYhCkf1BoC7N0QAvD_BwE