HI @ClumsyPilot, the following is the mechanical drawing of the Taoglas OMB.868.B12F21 antenna.
I use a lot of the 5dBi equivalent of the 12dBi antenna that you have ordered. Per the mechanical drawings, I always fix them to standard 2inch/50mm telecom masts using the u-bolts that are delivered with the antenna.
You ask about attaching to the side of a “bar”. Do you mean “barn”? If so then don’t attach the antenna direct to the side of the barn, you need the entire antenna to stick up clear of the apex of the roof.
Are you aware that the 12dBi antenna is 2.4m long? You will need to use a substantial mast - like a scaffolding pole. The scaffolding pole is normally attached to the side of the building using standard heavy-duty telecom “T & K” brackets.
Following is a picture of LoRaWAN base-station using a Taoglas 5dBi (1m long) antenna mounted on a barn using T&K brackets, unistrut and a scaffolding pole. Your antenna is 2x the length of this antenna.
