Why most of the antenna has an input impedance of 50 ohm

I am currently designing a bowtie antenna using ansoft hfss for my final year project. Can anyone help me to explain why does most of the antenna has an input impedance of 50ohm. Is 50ohm consider as an effective impedance matching?If yes why?

http://www.microwaves101.com/encyclopedia/why50ohms.cfm

thank you