What is SWR and how do you use an SWR Meter ?
Standing Wave Ratio (or to be more correct, VSWR – Voltage Standing Wave Ratio), is the measured ratio of transmitted power travelling up the feed line from a transmitter to the antenna, as opposed to the quantity of reflected power returned down the feed line because the antenna is not at resonance.
SWR is measured with an SWR meter. An SWR meter is essentially a Wheatstone Bridge device with the meter in one side and the detection line in the opposite side. This allows power readings to be taken for power in both directions on the detection line.
An SWR meter is used with coaxial feed line and often consists of 2 antenna sockets on the rear (usually marked up as “TX” and “ANT”) for connection to the transmitter and the antenna to be checked. On the front it is usual to find a sensitivity adjustment in the form of a variable resistor, a switch to switch between “Forward” (transmitter to antenna) and “Reflected” (antenna to transmitter) power, and last but not least a moving coil meter to display the readings.
To use the meter, the antenna is plugged into the rear socket marked ANT, and the transmitter is plugged into the socket marked “TX”. The SWR measurements should always be initially taken using the minimum output power of the transmitter that will allow the unit to show full scale on the moving coil meter.
Firstly, switch the meter to “FWD” position on the front and key the transmitter. With the sensitivity control, ensure that the moving coil meter reads full scale. Then un-key the transmitter, switch the unit to “REV” and key the transmitter again.
In an ideal world, the meter should read no reflected power, but it is more common to find that there is some level of reflected power displayed. If the reading is less than 1.5, then it is reasonably safe to use the transmitter for longer periods on that frequency with out the likelihood of damage occurring to the output stages. If the reading is around 2, then approximately 10% of the transmitted power is being returned to the transmitter and causing heating in the output stages. This heating will eventually lead to permanent damage, and possibly an expensive repair bill, so it is wise to look further into the reasons why this is happening.
If the reading is 3 or above, stop transmitting immediately!! A reading of 3 or more means that 25% (or more) of the transmitter power is being returned and WILL cause damage to the output stages. A reading this high is often indicative of either a short-circuit or an open circuit on the feed line.
Go back and check the route of the installed coax, the connection to the bottom of the antenna and the installation of the coax plug where it goes into the SWR Meter. Once the culprit of the high reading has been established and cured, the above procedure can be repeated.
