hfss log-periodic

i want to make a criss cross log periodic antenna in HFSS.The shape of radiation pattern and 3D polar plot is exactly right.But the required gain in not achieved.So what should i do to increase gain.?

Gain and radiation pattern shape describe the same characteristic. The only way gain could be down is if there is excessive insertion loss in the antenna. You may be able to increase gain a little without changing the patterns by using more efficient feed components or decreasing the resistance of the elements by using better materials.

I suspect this is not the case and if you need to increase the gain you probably need to make the antenna longer. That will narrow the radiation patterns.