What is VSWR?
VSWR is the ratio between the maximum and minimum values of voltage on the transmission line connected to the antenna. If everything, from the radio end connector to the base of the antenna is perfect, and if the antenna is perfectly tuned to the testing frequency, VSWR will be 1:1. That is, every bit of power that the radio has to deliver is leaving the radio and being transmitted or received by the antenna. For frequencies below 200 MHz (VHF, etc), VSWR below 2.5:1 is acceptable. Granted, lower VSWR means more power leaving the radio but at 2.5:1 the loss does not amount to more than 0.9 dB, even on very long transmission lines. A loss of 0.9 dB is not a substantial change in actual signal strength.
VSWR stands for Voltage Standing Wave Ratio. It is the ratio of the maximum/minimum values of standing wave pattern along a transmission line to which a load is connected. VSWR value ranges from 1 (matched load) to infinity for a short or an open load. For most wireless LAN antennas the maximum acceptable value of VSWR is 2.0. VSWR of 1.5 or less is excellent. This is approximately the same as a Return Loss of 14.5 dB. What this means is that most of the signal from the transmitter to the wireless antenna is being radiated. (96% radiated and 4% reflected) A VSWR of 2.0 (return loss of 9.5 dB) means that 90% is radiated and 10% reflected.