How to calculate spacing in a dual band microstrip antenna using LPA technique?

hi.....

I want to design a dual band microstrip antenna operating at frequency 2.4 and 5.5 GHz.

i want to use log periodic antenna technique.......but how to calculate the spacing between two of the elements..........
is it λ/2 ?

plzzz help

here's all you need to know about LPA's for microwave bands ....

http://www.w1ghz.org/antbook/conf/WA5VJB_LPA_feed.pdf

I have several of these antennae, they work very well

Cheers
Dave
VK2TDN