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@Jean-Philippe_Rey
The PCB material (most of the time FR-4) has its influence in term of RF performance, as its dielectric constant is not the same as air. Anyway, in your case, you should not worry about radiation pattern, as the module is meant to be put on a breadboard, which contains several metallic strips inside, which will cause most of the radiation loss.
In term of RF performance, a good practice would be to add a small coaxial connector such as U.FL from Hirose (https://en.wikipedia.org/wiki/File:2_mm_SMD_UFL_Socket.jpg), thus allowing the use of an external antenna.
That said, for indoor Wifi, it seems that there is not so many guys complaining about any poor RF performance of a PCB trace antenna on ESP modules.