Amateur Radio Station
Amateur Radio Education
VSWR or Voltage Standing Wave Ratio (SWR for short), is a subject often discussed in amateur radio circles. It is therefore surprising how many radio hams misunderstand the meaning of the value shown on an SWR meter.
An SWR of 1:1 is theoreticaly imossible. This value would indicate 100% efficiency in a transmitter, transmission line and antenna system, with no reflected signal whatsoever.
An SWR of 1:infinity is the very worst case, with 100% of the transmitted signal being reflected back towards the transmitter.
The VSWR ratio is given by Z/Z or Z/Z, whichever is greater. Z and Z being the two impedances at the interface. i.e. the impedance of the coax cable and that of the filter or antenna etc.
What is an acceptable VSWR?
Most modern transceivers detect a poor VSWR and reduce the output power in order to keep the reverse power down to a safe level. A medium SWR of 1:1.5 - 1:3.0 will therefore do no harm to the transceiver, however the output power will be reduced.
Reducing your reflected power
The Antenna Tuner
An antenna tuner, transmatch, or antenna tuning unit (ATU) is a device connected between a radio transmitter or receiver and its antenna to improve the efficiency of the power transfer between them by matching the impedanne of the equipment to the antenna. An antenna tuner matches a transceiver with a fixed impedance (typically 50 ohms for modern transceivers) to a load (feed line and antenna) impedance which is unknown, complex or otherwise does not match. This mismatch is usually caused when using a non-resonant antenna (one whose electrical length as compared to the wavelength of the signal does not result in a purely resistive impedance). An ATU allows the use of one antenna for a broad range of frequencies. An antenna plus matcher is never as efficient as a naturally resonant antenna due to additional induced losses on the feed line due to the SWR (multiple reflections), and losses in the ATU itself, although issues of pattern and capture area may outweigh this in practice.
Strictly speaking the 'ATU' is only an antenna matching unit, as it is unable to change the resonant frequency of the antenna system.
The Isolator (VHF and above)
More of a protective device for the transceiver than a ATU
The Isolator does not remove or reduce a poor VSWR, It bacically 'dumps' all of the reverse power into a 50 Ohm load so the transmitter 'sees' a perfect transmission line and therefore increases the power to 100%. However, 100% of the signal will not leave the antenna as there is still a VSWR present on the coaxial cable and some of the signal is lost in the system. As you can see from the table below, a VSWR of 1:1.4 causes a loss of less than 0.15dB or 3%, this is likely to be less than the coax cable loss. Trying to reduce such an SWR still further will be expensive and pointless as the increase in effective radiated power will be totally undetectable by other stations. It should also be noted that a transmitter is seen as a bad load in reverse, the majority of reverse power will therefore be reflected at the coax / transmitter interface.
Reverse power is given by the equation P = P x (S-1) / (S+1), where P is the reverse power, P is the transmitter power andS is the VSWR presented to the transmitter.
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