with rf modul you can sometimes (usually) regulate output power but that has nothing to do with antenna. if we use two antennas on same module, if output (W) of radio module is same for both antenna, good antenna sucesfully transmit more (available) power in the air and therefore you get better reception (a little bit simplifed answer). So, perfectly constructed module will suck with bad antenna. Also, you have to construct module in right way. There you have similar problem like in high speed design (i.e 2.4 Ghz). You have to adjust impendance from IC chip to one that you use (usually 50 Ohms), transmit signal so that lines have that impendance to the antenna. If you fail to do this on hardware part you also get less power on output and good antenna does not help. Ones you think you done it all right you measure output and you can fine tune whole circuit. This is because specific hardware in combination with specific antenna never gives same result and you have to fine tune system. Add to this enviroment, for example you hold tablet with hand and hand is positioned close to antenna (this is really usual situation), your hand could give another problem to all this and more measuring and fine tuning might be needed or even moving antenna.