As we know, the resonance frequency of a slot antenna is mainly determined by the length of slot.
In general, the length of the slot is about half resonant wavelength. In order to miniature the size of
slot, a dumbbell slot is adopted. Besides, the resonant frequency of the slot antenna will decrease with
an increasing width of slot, which also means a miniaturization. However, the wide slot will lead to
higher cross polarization. So the slot width should be properly chosen.
The microstrip feed line in Fig. 1 can be considered as a microstrip monopole with a length about
one quarter wavelength, which introduces another resonance. According to the theory of monopole
antenna, the wider monopole has wider band and better impedance matching; however, its shade area
over the slot radiation increases, too. Thus the width of feed line should be chosen for a compromise.