Terms + Definitions of Basic Antenna Design II

Building off of Part I, this paper covers common antenna definitions for antenna design and RF design.  Return loss, S11, antenna efficiency, and impedance bandwidth.

S₁₁ is a measure of how much power is reflected back at the antenna port due to mismatch from the transmission line.  When connected to a network analyzer, S₁₁ measures the amount of energy returning to the analyzer – not what’s delivered to the antenna.  The amount of energy that returns to the analyzer is directly affected by how well the antenna is matched to the transmission line.  A small S₁₁ indicates a significant amount of energy has been delivered to the antenna.  S₁₁ values are measured in dB and are negative, ex: -10 dB.  S₁₁ is also sometimes referred to as return loss, which is simply S₁₁ but made positive instead (Return Loss = - S₁₁).  So if the antenna Return Loss is 8 dB, S₁₁ is -8 dB.  The third and final method to measure an antenna’s ability to accept power is VSWR (voltage standing wave ratio).  VSWR evaluates the ratio of the peak amplitude of the voltage of the wave on the transmission line versus the minimum amplitude of the voltage of the wave.  A VSWR of 1 is ideal; this indicates that there is no reflected power at the antenna port.  When the antenna and transmission line are not perfectly matched, reflections at the antenna port travel back towards the source and cause a standing wave to form.  The worse the mismatch, the larger the amplitude of these reflections.   

A more in-depth look into Return Loss, S₁₁, VSWR and how they are related to the antenna impedance is provided in ‘A Concise Definition of Commonly Used Antenna Terms: Part I.’  Provided below is a chart correlating Return Loss, VSWR, and dB loss due to reflection at the antenna port.

Table 1: Return Loss, VSWR, and Reflection Loss

Radiation Efficiency is a measure of how well an antenna converts accepted power to radiated power.  An efficient antenna will radiated most of the power it accepts from the transmission line.  All antennas have losses associated with them, such as conductor and dielectric losses.  Radiation Efficiency takes into account the loss associated in the antenna only, it should not to be confused Return Loss.  The antenna accepts a certain amount of energy delivered to it by the transmission line (characterized by S11, VSWR, or return loss), and Antenna Efficiency characterizes the amount of energy lost ‘in’ the antenna after this.  It is often expressed in terms of a percentage, or in dB.  For example, an efficiency of 0.8 is 80%, or -0.97dB.

Efficiency=  P_radiated/P_input 

Sometimes, the term Antenna Efficiency is used instead of Radiation Efficiency. 

Bandwidth (sometimes just referred to as impedance bandwidth) refers to the range of frequencies a given Return Loss can be maintained.  Since Return Loss is a measurement of how much power the antenna accepts from the transmission line, the impedance of the antenna must match the impedance of the transmission line for maximum power transfer.  However the impedance of the antenna changes with frequency, resulting in a limited range that the antenna can be matched to the transmission line.  The Bandwidth is a measure of this range.  It is typically paired with a given Return Loss or VSWR value; for example the figure below has a 10 dB Return Loss Bandwidth of 260 MHz. 

 

Figure 1: Impedance Bandwidth Measurement