The Standing Wave Ratio characterizes the degree of matching between the antenna and the feeder (it is also said to characterize the matching between the transmitter output and the feeder). In practice, a portion of the transmitted energy is always reflected back into the transmitter. Reflected energy degrades transmitter performance and can cause damage.
A standing wave is formed in a waveguide as a result of the superposition of the incident wave and the wave reflected from the load (antenna) at the end of the waveguide. The standing wave mode occurs under conditions of maximum signal reflection at the end of the transmitter-antenna line. Due to this superposition, static maxima and minima of field strength are formed in the waveguide; in other words, the combination of the reflected and incident signal powers creates a non-uniform field strength distribution along the entire length of the waveguide (cable).
It is important to distinguish between SWR and VSWR (Voltage Standing Wave Ratio): the first is calculated by power, the second — by voltage amplitude, and is more commonly used in practice; in general, these concepts are equivalent.
The Standing Wave Ratio is calculated as follows:
SWR = (Winc + Wref) / (Winc - Wref)
where Winc is the incident power and Wref is the reflected power. Ideally, SWR = 1; values up to 1.5 are considered acceptable in the VHF range.
The SWR value depends on many factors, for example:
- The characteristic impedance of the RF cable and RF signal source;
- Inhomogeneities, splices in cables or waveguides;
- Quality of cable termination in RF connectors;
- Presence of adapter connectors;
- Antenna impedance at the cable connection point;
- Quality of manufacturing and tuning of the signal source and load (antenna, etc.).
Traveling Wave Ratio, Cable Shortening Factor, Characteristic Impedance, Reactive Power.