Archimedean Spiral antenna....unbalanced....Vswr problem

Hi,

I'm finding difficulty in designing unbalanced Archimedean Spiral antenna in HFSS....I designed for 1 - 10 Ghz...

but it is working at one frequency 1Ghz( i.e. poor return loss at other frequencies)
Behaving like narrow band antenna.

I excited with lumped port between two arms.

What will be the problem ?

What impedance did you use for your lumped port? A complementary Archimedean Spiral should exhibit an input impedance of 188 ohms.

Did try exciting between centre and Gnd plane?
Where is your design?. What dielectric properties?

I used 188.5 ohms. and im getting BW of 700MHz only. but it should be around 8-10 GHz.

What do you mean by unbalanced? Are you using an unbalanced feed, or is there some aspect of the design that causes this to be a non-complementary antenna design? Can you post a picture of the layout?

yah. It was unbalanced feed only. Substrate is kept hidden in the image.