Antennas for wireless devices are as varied as the devices themselves. Possibilities include external versus embedded, printed on flex printed-circuit boards (PCBs), formed from thin sheet metal, created on the product housing using sprayed-on conductive paint, embedded in materials with a high-dielectric constant for size reduction, and so forth. Regardless of the type and configuration of the antenna, performance can be characterized by the same metrics:
Impedance Bandwidth Efficiency Directive Gain Polarization Radiation Pattern
The following discussion of antenna performance parameters uses a transmit (Tx) perspective, but exactly the same parameters can be applied to the antenna for receive (Rx) operation. The impedance, efficiency, gain, and radiation patterns of the antenna will likely be different for receive and transmit modes because the two modes are separated in frequency, but the interpretation of the performance parameters is exactly the same.
3.1 Antenna Impedance and Bandwidth
The antenna is essentially a transducer between the characteristic impedance of the radio system (nominally 50 ohms) and the impedance of free space. As such, the antenna impedance and the radio frequencies over which that impedance is maintained are critical. It is essential that the antenna present an acceptable impedance match over the frequency band(s) of operation. Antenna impedance and the quality of the impedance match are most commonly characterized by either return loss (represented by the scattering parameter S11) or Voltage Standing Wave Ratio (VSWR) – these two parameters are simply different formats of exactly the same impedance data. As shown in 2, S11 is typically measured on a logarithmic (dB) scale. VSWR is a unit-less ratio. (Return
Antenna Fundamentals – Technical Brief