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As is evident, directivity can vary greatly with type of antenna. Hence it's important to understand and know directivity before selecting antenna for your application. Like, if you are going to receive/transmit signal from/to any direction, it is better to use antenna with low directivity. Similarly in an application where the direction of signal is fixed, highly directed antennas work way better.
=== Half Power Beamwidth ===
[[File:HPB.jpg|center|frame]]
As we have directional antennas, one obvious question we can get for what angle we can get good radiation power? Although the answer to this question also depends on receiving setup, it is generally defined as the point where the power output drops to half its maximum value. When converted to dB it amounts to approx 3 dB loss, hence it is also called 3 dB point. In one plane, there are going to be two such points. The angular separation between the two points is half power beamwidth.
=== Antenna Efficiency ===
Antenna efficiency is basically ratio of power radiated from the antenna to the power delivered to antenna. In short, it denotes the losses due to antenna. The losses could be attributed to three major factors:
* '''Conduction losses:''' These are losses due to finite conductivity of elements of antenna
* '''Dielectric losses:''' The electric fields induce current in the nearby dielectrics reducing the energy carried by fields. These are dielectric losses
* '''Impedance mismatch:''' Some part of power delivered to antenna by transmitter is reflected back due to mismatch in impedance of antenna driving circuit(transmitter) and antenna and hence results in loss of power. We will talk about impedance mismatch and matching later.
=== Voltage Standing Wave Ratio (VSWR) ===
Impedance matching, which is described in above section, is quantified using the VSWR. It basically describes the power reflected back from the antenna-transmitter junction. More reflected power signifies greater impedance mismatch loss. VSWR is related to reflection coefficient (more commonly known as s11, and represents how much power is reflected back from antenna) by the following formula
[[File:VSWR.JPG|center|frame]]
Lower the VSWR, better is the antenna impedance matched. VSWR can be easily found out using Vector Network Analyser (VNA).