Basically slots add extra resonating bands. If the band is near to the main band due to patch then both will combine each other and results to increased bandwidth.
Note that the position of slot is also very important.
Generally, when a slot is added, the resonant frequency changes from a single band to dual-band and even multi-band. That's why, when the design requirements are dual-band or more, then slots could be an option to achieve it.
Hello,
Slots are actually used to increase bandwidth, but it should be in proper position, that is, it should be first checked by its surface current. It is also being used to increased gain.
Some papers are attached.
1) Increasing Microstrip Patch Antenna Bandwidth by Inserting Ground Slots, Journal of Electromagnetic Analysis and Applications, 2018, 10, 1-11 .
2)As the coupling power gets stronger by changing the slot shape, thicker substrate for the radiating layer can be used leading to wider impedance bandwidth. On the other hand, as the slot size decreases, the backward radiation which is dominantly caused by the slot decreases enhancing the antenna efficiency . This is from the book , Bandwidth Enhancement Techniques, By Seyed Ali Razavi Parizi.
Thanks,
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