interdigital filter formula

Hi all,
I'm trying to realize an interdigital filter but I need to know which is the formula that link the length of the external rod (first and last) with external Q of the filter.
Can someone help me?

Thank's in advance.

you can extracte parameter Q, K from the length of rod. As you choose the Q and K method to design filter, u know the Q and k of the ideal lumped filter.then you can adjust the length of rod until you get the Q and K that you wished.