The more things change, the more some things stay the same. This is true where VSWR is concerned. Even as today's wireless communications systems advance, they still rely on the basic premise of transmission and reception of RF signals between subscriber units and base stations. No matter how many new technologies and acronyms weemploy, we still need to pay attention to one of the constant acronyms: VSWR.

VSWR, or voltage standing wave ratio, is a measure of how well the components of the RF network are matched in impedance. When the impedances are improperly matched, you lose signal power, which results in weak transmissions, poor reception or both. If large mismatches exist in the RF section of a wireless base station, it doesn't matter if you have the latest digital technology; the system will be inferior.

Another reason you should monitor VSWR closely is that an extreme mismatch between a transmitter and an antenna can cause permanent damage to the transmitter.

Maximum power transfer between two system components occurs when their respective impedances are matched. If the impedances are not identical, some RF power will be reflected back, resulting in a reduction in the amount of power delivered to the load. (See Table 1 on page 62.) These reflections cause voltage standing waves.

Textbook VSWR VSWR is defined as the ratio of the maximum voltage to the minimum voltage in the standing wave. The larger the impedance mismatch, the larger the amplitude of the standing wave.

A perfect impedance match would cause no voltage standing wave, so the ratio of the maximum voltage to the minimum would be 1.

VSWR represents the ratio of the system's characteristic impedance to the impedance of the load (e.g. antenna). You can approximate it as follows:

VSWR = Zdevice / Zsystem

A 100? antenna connected to a 50? cell-site transmitter results in a VSWR of 2. This is commonly expressed as 2:1, which means the antenna impedance is different from the system impedance (50?) by a factor of two.

When the antenna impedance is less than the system impedance, the VSWR equation is flipped:

VSWR = Zsystem / Zdevice

This means that a 25? antenna connected to a 50? system will produce the same 2:1 VSWR as a 100? antenna connected to a 50? system will produce.

Most RF systems have a characteristic impedance of 50?. All of the devices in the radio transmitter and receiver sections are designed to have input and output impedances of 50?. This includes the coaxial cables that are used to interconnect the devices. These devices include multiplexers, bandpass filters, duplexers, low-noise amplifiers, combiners, power amplifiers and, of course, the antennas. The closer an antenna is matched to 50?, the more RF power the antenna radiates into the atmosphere vs. reflecting the RF power down the antenna cables.

Although the concept of VSWR is easy to comprehend, it is extremely difficult to measure directly. For that reason, you need to measure other parameters, such as return loss and reflection coefficient.

Return loss is the difference in power (expressed in dB) between the incident power and the power reflected back by the load due to a mismatch. It is expressed as:

Return loss = 10log(Preflected /Pincident)

A perfect match would result in no reflected power (as it is all delivered to the load), so the return loss would be infinite. Conversely, an open circuit would reflect back all power, so the return loss would be zero. When dealing with return loss, the higher the value, the better the impedance match.

Reflection coefficient (p) is the square root of the ratio between the incident and reflected power:

? = ?(Preflected /Pincident)

More simply, it is the ratio between the incident and reflected voltage. The reflection coefficient is mathematically related to return loss. Return loss is simply the reflection coefficient expressed in dB:

? = 10 -(return loss/20) or return loss = -20 log /?/

The reflection coefficient is a voltage ratio and must be squared to be used for power calculations. It is sometimes easier to think of reflected power in terms of reflection coefficient than in return loss. Reflected power is equal to the incident power multiplied by the reflection coefficient squared.

For example, suppose your antenna has a reflection coefficient of 0.1. Squaring this equals 0.01 or 1%. If your PA is outputting 10W, 1% of that power (0.1W) is being reflected back by the antenna. Another way to think of it is that 99% of the PA power (9.9W) is being delivered to the antenna.

VSWR can be calculated using the following formula once the value of the reflection coefficient is known:

VSWR = (1+/?/)/(1-/?/)

Hands-on VSWR It is relatively easy to measure return loss by using a common spectrum analyzer/tracking generator and then calculating the value of VSWR. You also can use a time domain reflectometer to measure the reflection coefficient and apply that value to calculate VSWR.

Many wireless field engineers carry service monitors; the discussion below describes how to use a spectrum analyzer to make the measurement. For the sake of discussion, it is assumed that the VSWR being measured is that of an antenna, although the same approach will work for any RF device.

Return loss is measured in two steps. The first step is to disconnect the antenna and take a reference-level measurement. The reference-level measurement essentially "zeros out" the effect of the cables and test setup.

When making a reference-level measurement, you inject an RF signal into a test cable. This signal travels through the cable and is reflected back by the open circuit end, where the antenna eventually will be connected. The amount of energy coming back down the cable is measured with the help of a directional coupler that separates it from the energy going forward.

The signal level measured in the reverse direction is always less than the level that was put into the cable due to attenuation through the test cables up to the open circuit and then back down again. This measurement is made at several frequencies, with the reflected power level recorded at each frequency.

The second step in measuring return loss is to connect the antenna to the test cable and rerun the reference-level-measurement procedure at the same test frequencies as before.

This time, however, the energy coming back down the test cable is much lower because a good portion of the forward power actually is being delivered to the antenna.

Some reflected power still exists because the antenna is not perfectly matched to 50W. The difference in dB between the reference level measurement andthe measurement with the antenna attached is the measured return loss.Once you determine the value of return loss, you can calcu late VSWR using the formulas shown here.

Considering the potential for reduced power delivery, and even permanent equipment damage, performing VSWR sweeps on antenna systems is not only a good idea, it is essential to saving both time and money in the long run.