It can be a hard time to figure out what outdoor antenna works for you. It all depends on your location and the helium hotspots around you.
For your convience, the picture below will explain a little more about about the range of outdoor antennas.

3dBi Outdoor Helium Antenna
Are you placing your Helium Hotspot Miner in a dense, low altitude area with alot of Hotspots near you? Than we recommend an outdoor antenna with of 3dBi or below. As you can see in the example picture below the lower the power output the wider the range is.
5.8dBi & 6dBi Outdoor Helium Antenna
You can place your Hotspot on a higher altitude now what? If you can place your Helium Hotspot in a location with higher altitude and need to get some range to connect to other Hotspots a 5.8dBi or 6dBi Helium Antenna is the one to pick.
8dBi Outdoor Helium Antenna
If you placed your Helium Hotspot Miner is a remote location and you placed it on high altitude a 8dBi Helium Outdoor Antenna could be the solution for you. This is a hard one because you might send the signal over other Hotspots and miss the connection.