planar antenna exponential profile

I want to realize a palanaire antenna which radiated exponential format, but I can not determine the right size:
- Minimum width entered
maximum-width output
- The length of the line

knowing that I work the 9 GHz frequency and use a dielectric substrate constente equal 4.9 and 1.45 mm thick

I try with
L = 10mm and 3mm = Wmin and Wmax = 20mm but I get a Result of S11 =-9dB and G = 2 dB
can I use an adaptation quert wave