Here is a short list of some key antenna parameters. Antenna impedance is the ratio of the voltage over the current at the antenna input terminals. It should be matched to the system impedance, which is typically 50 Ω (ohms). Directivity is defined as the ability of the antenna to focus or concentrate energy in any given direction. Antenna efficiency is by far the most important antenna parameter for small wireless embedded devices. It is defined as the power radiated over the power input to the antenna. Basically the higher antenna efficiency, the better the antenna. Gain is defined as the efficiency times the directivity. So, Gain is always going to be lower than the directivity. As mentioned before the efficiency, is targeted at 50% but its really dependent on the antenna design. Another key takeaway is that a low gain antenna can still be a very well performing antenna. All of these parameters are only usable over a finite frequency range and that frequency range is the bandwidth of the antenna. So there is going to be impedance bandwidth, an efficiency bandwidth, and a gain bandwidth. Basically one can not use a GPS antenna for Bluetooth. One can not use a Wi-Fi antenna for cellular. One will need to pick the right antenna for the application because they all have a finite frequency range that they can operate over.