I currently have that cap hooked to one of these large telescoping poles. I want to say the pole is 2" and the adapater cap from Starlink will fit on anything from 1" to 3" I am sure.
Yeh its 2". This is what I have. If you get on Craigslist you can buy antenna masts all day long for 1/10th of the cost. Tons of companies are taking these down because they are getting rid of the two way radios and going cell or satellite for their trucks.