Bandwidth of Antennas

Hi, I am new in Antenna Engineering, a senior student in college. I am reading Balanis' book, but I couldn't find an answer of the question:
How an Antenna bandwidth is designed. For example, I want to design an Antenna, and consumer says "it should have 1.1:1 bandwidth around 4 GHz". So How an Antenna bandwidth is designed?

Also, which books do you advise to me for advanced level?

Balanis's book is a good starting point for the undergraduate senior / graduate first year.

Generally, a ratio like 1.1 : 1 refers to the ratio between the extreme bandwidths. So if you take the centre frequency as 4 GHz, then the appropriate band would be from 3.81 GHz to 4.18 GHz.

It is more common to request a bandwidth of 10% at (say) 4 GHz. Which would mean from 3.8 GHz - 4.2 GHz.