I just purchased a 2 Watt wifi router and a rp-sma T connector. I have a 9dBi antenna and it comes with a 5dBi antenna. If I decide to use the connector, how will it affect the antennas? Will it average them? Add them? Explode?
No. At best you would get about 10 or 11dBi You cannot just hook two via a T connector. The cable is 50 ohms impedance (resistance to RF). If you put two lines in parallel (via a T connector), you are making 25 ohm impedance, creating a mis-match in feedline and antenna impedance. Any extra gain you would get from the antenna, is simply lost in reflected signal back to the wireless device - you will not gain. In fact, the extra reflected power may cause the amplifier to overheat, or it will detect it, and reduce power. In either case, you may end up with a weaker signal instead of stronger, as well as risk damaging your amplifier. That said, if you get the proper phasing harness - matched lengths of cable, cut to 1/4 or 1/2 wavelengths - I can't remember off my head which one (electrical wavelengths of the wireless frequency - account for the velocity factor in the cable), you could phase two antennas together. However, even after all that, your net gain is 3dB (double the power gain), so two 9dBi antennas phased together would make an equivalent 12dBi antenna - hardly worth all that effort with phasing and cost, it would be cheaper to go buy a new 12dBi antenna. Your net effect of phasing those two antennas would be like maybe around 10dBi net effect, after you spend all that money phasing them properly - not worth the effort. A 3dB gain is barely noticeable in wireless terms. To really make some noticeable impact in signal strength, you need about a 6 or 10dB improvement Remember that 3dB is double whatever you are measuring. Doubling up two antennas gives 3dB improvement (actually around 2.7-2.8), 4 antennas would give 6dB improvement over one antenna. dB is a logarithmic scale, you cannot just add the numbers.
