band-reject question?

Hi all!
I'm want to ask a question about band-reject design?
If i used a slit to generate a band-reject characteristic on antenna design, why is the length of slit usually design at quarter-wavelength of rejection frequency?
What's the phenomenon means?

it really depend on your design. The bottom line is that the slot should be quarter wavelength if it is open from one end. If the slot is closed on both sides then it should be half wavelength. This has to do with the resonance modes of a slot line resonator.