excuse me.I started learning the new antenna design.I do not know exactly what to ask why. I have to design omni-directional and dual-band antenna for 868/915 MHz.I am try to loop antenna using CST for 868MHz. But I do not know how dual-band. I hope you can help.
Omni-directional combined with high gain implies a tall colinear array or similar. Omni gain. Not a Yagi, not a log periodic.
868 to 915 (nominally) is only about a 2.5% bandwidth around the middle 891.5 MHz. So, maybe treat it as a wide bandwidth antenna problem, which should be easier.