Recently, I designed a wide band antenna and what I found that by cutting a rectangular slot in ground plane its s11 parameter improved by almost 20 db. Can any body tell me concept behind cutting slot in ground plane. In many ultra wideband antenna design people used wide slot cut in ground plane to improve impedance bandwidth.
The basic concept is that a slot in the ground plane behaves as a load which when added to the antenna can be used to bring the input impedance point closer to the characteristic impedance (e.g. 50ohms). This improves the input impedance match and hence the S11. Ofcourse multiple slots placed in close proximity will couple with each other giving more complicated impedance patterns.
If you are careful to design the slot so that its radiation is negligible, the effect of the slot can be represented by a purely reactive impedance. Long slots would be represented as a network of impedances and can be designed to give broadband responses. Slots which radiate can also be used but then effect on the radiation pattern needs to be considered. Also, the impedance of the slot will then contain a resistive component.
How the slot impedance affects your input impedance depends on the antenna geometry and placement of the slot. In general this is not trivial to calculate but you can approximate its effect by finding the current distribution of your (unslotted) antenna through simulation and applying a perturbation technique. I have seen some literature on this subject but nothing particular comes to mind. Perhaps best to start with a google scholar search.
you can read this paper "Modifying the Ground Plane and its Effect on Planar Inverted-F Antennas (PIFAs) for Mobile Phone Handsets"...
@Christ answer covers almost everything...
Slot on the ground plane actually modifying current pattern...Here i am talking about slotted ground plane...
If it is slot antenna.. slot antenna exhibits high impedance due to Babinet's Principle...High frequency ultra wideband using slot is always possible..because of large ground plane size. Multiple resonance is always possible because someone can cut multiple slots...thats how I believe people are having large BW... However, BW is not always the main issue in terms of application.
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