How to select "λ/4" gap for a frequency band?

i am designing a PIN diode based SPDT switch in the frequency band of 30-500MHz. (i need minimum isolation of 45dB) i went through some design articles, they are suggesting "λ/4" spacing is required for better isolation.
how to select "λ/4" gap for a frequency band?
is it possible to replace "λ/4" gap with lumped elements?
please help me.............................

i need RF chokes for the same frequency? what is the best suited ferrite material for this band?

Dear Friend:

While at UHF and microwaves we talk about "quarter-wave" sections, you should know from a good textbook that a lumped-element equivalent is a simple LC resonator, a parallel L and C resonant circuit.
Without a basic knowledge like this I would not try to design any devices like PIN switches.
Please study first, then try to design something. Otherwise you can create a car with one wheel only.

It is always good to ask for advice; please get a basic knowledge first, to be able to ask qualified questions.