A common, but understandable, misconception is that increasing the size of the antenna on WiFi equipment will make the signal much better; unfortunately it often has the opposite effect and makes things worse!
Put simply, all an antenna does is focus a signal, much like a torch might have a long narrow beam or a short wide one.
With directional antennas the higher the gain (more dBs) the more distance but to the detriment of the width of the signal.
In the case of an omni-directional antenna which sends the signal all round (think of fat doughnut) as you increase the gain, the flatter it becomes (imagine squashed doughnut). So if you put a higher gain antenna on your router downstairs you may discover the signal upstairs has disappeared!
So when choosing an antenna give careful consideration to where you want the signal to reach, and remember bigger isn’t necessarily better! It is also worth noting that not all access points allow for changing the antennas, do check this before you buy!