Antenna directivity and Total efficiency

I am using CST microwave studio for my PCB antenna simulation. The directivity is about 6dBi and Total efficiency is 0.5
When i try to simulate the PCB antenna inside a casing, the directivity becomes 4dBi and total efficiency become 0.8
Can anyone help to explaine this phenomena? Thanks

i cant understand what u mean? u mean shield ?

Not shield. Meaning if I have a PCB antenna, when I simulate just the bare PCB the directivity is about 6dBi and Total efficiency is 0.5
When I place the PCB antenna inside a casing, the simulated result is the directivity becomes 4dBi and total efficiency become 0.8
Just want to understand how come the PCB antenna inside a casing is having higher efficiency than without casing enclosure?

Environment loads the antenna.

Hi, possible to elaborate on this?

Check complex antenna impedance, with and without casing. If metal casing is radiation however low outside this casing.

hi E kafeman
i cant understand CASING can u help me to i conceive it?

I guess it is some sort of Faradays cage. No idea why placing an antenna inside this cage.

Case means some kind of dielectric enclosure, quite common for a PCB antenna, I think. Definitely not a conducting cage.

It's not clear to me, what efficiency in your simulation means. Does it refer to antenna impedance mismatch or losses inside the antenna? Or is it just an simulation artefact indicating insufficient accuracy? If you see efficiency < 1, a correct simulation should tell you where the energy is gone.

In the first case, you should primarly adjust the impedance matching.

Placing dielectric media near the antenna will affect both antenna directivity and impedance matching. If the latter hasn't be optimal before, it can be improved, otherwise get worse.

i understand the casing by FVM definition if u cover or shell or shield antenna with dielectric or metal the loss energy will store in antenna and so radiate and so the gain and efficiency will increase

Thanks, I stand corrected.
A not too unusual situation is when the bare antenna&PCB is tuned slightly too high in frequency at workbench, because when covered by, for example a plastic cover, is then the antenna self resonance lowered a bit which can reduce antenna reflection at intended frequency. That was the kind of "plastic cage" I had in mind that maybe in a simulation can give similar result as what ccjames had got.