What is Impedance or Characteristic Impedance?
The characteristic impedance of a transmission line or antenna can be defined in terms of the ratio of the voltage and current amplitudes, and the incident and reflected waves. VSWR has a similar general relationship in its definition. For communication systems, the 50-ohm characteristic impedance has become the adopted standard. It is the result of an industry compromise between maximum and minimum voltage and current handling of a transmission line. Most aviation communications antennas are matched to 50-ohm systems. The VSWR is an indication of how well the antenna is matched to a 50-ohm characteristic impedance system. A VSWR of 2.0:1 (9.5dB return loss) corresponds to approximately 90% of the RF power being transmitted/received. With a 3.0:1 VSWR (6.0dB return loss) approximately 75% of the RF power is being delivered to and from the antenna.
