Recently, I have to design structural support bracket for 28 feet height antenna telescopic mast-2"diameter (EZ TM-30)mounted to our building 13 feet overall height.
However, I'm wondering how to compute the wind load applied.
Check with the manufacturer they may sell a bracket for your specific antenna and have sealed calcs with reaction forces. This depends on the manufacturer and whether the product is frequently used. You can use ASCE 7 but if you are going to be doing a lot of antennas then get familiar with TIA-222-H Standard For Antennas And The Supporting Structures For Antennas And Small Wind Turbines. Its one of the industry groups for the antenna industry