Antenna Impedance (50 Ω)
In RF systems the transmitter output, the coaxial cable and the antenna are designed at the same characteristic impedance, commonly 50 ohms. The aim is for power to reach the antenna without reflections along the line. An impedance mismatch causes part of the power to return to the transmitter and is measured as SWR.
50 ohms is a compromise: the lowest loss in a coaxial cable is obtained at about 77 ohms, while the highest power handling capacity is at about 30 ohms. 50 ohms is a practical middle point between these two ends and has become the industry standard.
- The whole chain must be the same: transmitter, connectors, cable and antenna.
- A 75 ohm television cable creates a mismatch if used in an RF link; even if it looks mechanically similar, it is electrically different.
- Antenna impedance is not fixed: it changes with frequency, with the mounting surface and with objects in the near field.
The practical consequence is this: when choosing an antenna, impedance match and operating band must be verified just as much as gain. Even a 50 ohm antenna made for the wrong band will behave as a mismatch. In AXI sets the receiver comes with its own antenna and cable; working without stepping outside that chain is the simplest way to preserve the match.
Context in which this term is used: 433 MHz or 2.4 GHz?