- VSWR at work

When the radiation resistance (voltage to current ratio) of an antenna is the same value as the characteristic impedance of the transmission line (voltage to current ratio in the line), all of the electrical energy transmitted down the line is taken by the antenna and radiated into space in the form of a radio wave.

If the antenna does not match the transmission line perfectly, wave interference is set up at the junction of the line and the antenna, and a portion of the energy is reflected by this mismatch point back down the transmission line towards the transmitter. High VSWR indicates mismatch, which means increased power losses and less radiated signal, making it impossible for your transmitter to perform as well as it should.

The greater the mismatch, the greater amount of energy is reflected. And the longer the cable run, the more significant the power losses will be. The worst case is when the forward and reflected waves are equal in strength, indicating a short circuit or open connection in the antenna system - or your VSWR meter needs calibrating badly!

Cheaper VSWR meters measure the forward and reflected line voltage between the antenna and transmitter to determine an approximate VSWR measurement and this is adequate for most installation purposes. Where more accurate reading are required (as in antenna design or accurate measurement of bandwidth performance, etc.), these can be obtained using higher quality meters that measure current rather than voltage. These days, however, many transmitters have a built-in bridge for measuring VSWR.

With the variable quality of some less expensive VSWR meters it is generally safer not to leave a VSWR meter in line when operating as they can create harmonic radiation. Moreover, some VSWR meters are not truly 50 ohms and can create an impedance hump if left in circuit.

If you are caught without a VSWR meter you can still get a rough idea of the VSWR with an in line power meter. By changing the connections round the meter will read the reflected power.

Using a simple formula, it is then possible to calculate an approximate VSWR. (RP = reflected power, FP = forward power)

VSWR formula

Bandwidth is usually determined by a specific VSWR figure. Normally an antenna is matched to the line at the centre frequency of the required bandwidth. Moving away from the optimum VSWR, higher or lower in frequency, will raise the VSWR. The rate of change will depend on the antenna design and generally will be faster with antennas which have been reduced in length from the optimum for the required frequency.

At 2:1 VSWR the system efficiency is still 89%, having a loss of around 0.5dB. At 3:1 VSWR, system efficiency is reduced to 75%, but the decrease in received signal strength is undetectable by the human ear. Aircraft antennas operating in the VHF band usually have a specified VSWR upper limit as high as 2.5:1. Therefore the 2:1 VSWR 0.5dB loss would seem to be an acceptable value to determine bandwidth for most antennas.

Above 3:1 VSWR indicates a poorer performance often coming from a need to provide extended bandwidth, or a fault in the antenna system.

When checking the bandwidth of an antenna, the VSWR will vary across the band from frequency to frequency.

Plotting your results should result in a smooth curve with the lowest point at centre frequency. With broadband antennas, the VSWR will vary in cycles throughout the band but should still be within normal specifications. It is often not a bad idea to record performance initially which gives a benchmark to compare with if problems occur later.

VSWR curves

If you find different VSWR readings depending on where you insert the meter in the line, this normally is indicative of standing waves on the line and a mismatched antenna. However, sometimes VSWR readings are not all they seem. Where there are devices such as antenna switches, etc., in the line, it is important to look at the antenna on either side of the device. Different readings from transmitter and antenna sides of the device could indicate a mismatch in the device.

Antenna currents can flow on the outside of the coaxial cable due to poor antenna design or poor installation technique (see Feeder Radiation - Striking a Balance 98/4-6), so that the line constitutes an extra load on the line and becomes part of the antenna system. These parallel currents will often cause indicated VSWR to change. A simple way to detect this is by physically moving the transmission line around which will result in changes in VSWR readings if this is the case.

Moreover, with maladjusted or faulty transmitters, it is possible for harmonics and low frequency sub-harmonics to be fed through the final stages of the transmitter driving the VSWR meter, introducing considerable error due to mismatch. This is because, although the antenna may be matched at operating frequencies, it is unlikely to be matched at the frequencies of these spurious emissions. The resultant error can be considerable, depending on the amplitude of the emissions, and can lead to other EMC problems!

The value of the VSWR/power meter in testing equipment should not be underrated. To quote William I. Orr, W6SAI & Stuart D. Cowan, W2LX "Making ... antenna adjustments without the use of an SWR meter is like washing your feet with your socks on - you can do it, but it ain't easy!". It is even possible to check out a whole system with the following procedure: To check the load, connect VSWR/power meter to the transmitter with a short lead and key the transmitter. The VSWR looking into the load should be close to 1:1. Note the output power of the transmitter.

To check the line, now install the VSWR meter and load at the far end of the coaxial line, key the transmitter and read the power output on the meter. The difference, if any, is power being lost in the line. A high loss, could be attributed to contaminated or faulty coaxial cable, poor connectors, switches or anything between the two ends of the line.

Using a VSWR meter for testing the line

To test an in line device, put a 50 ohm load on the VSWR meter and make sure it reads close to 1:1. Then put the load on the far side of the switch so that the device is between the VSWR meter and the load. When powering up the transmitter the VSWR should still read close to 1:1.