Thanks for the explanation.
However, I do see a case with PIFA antenna design, you have a feed point from the antenna connect to rx/tx, but you also have a short point near the feed point short the antenna to ground? In such case my understanding is that the whole antenna is short to ground, and then it can not be working as a antenna any more.
I know my understanding above is wrong, because in real case the design works, but how will that work? Where my understanding get wrong?