I was trying from many days on how to get strong wifi signal because I receive a wifi signal which has no security and free to use for an organisation. I used lan cable near about 5 meter long connected to lan port and on other end made a coil. but it didn't worked so much then I used modem.
I have a huwaei usb modem which I was using before, Just inserted a SIM and plugged in (Setting kept Network : UMTS Only, No need to connect but there should be available network ) then I observed that signal strength was better 25 - 30 percent.
Now I suggest you If you have usb modem then buy a usb extension cable 3 meter or 5 meter depends , and use usb modem via usb extension and put it on a good height. It will Surely Increase the Wifi Signal Strength.