Wireless network: Field testing benefits

05/23/2013


Troubleshooting networks

No matter how carefully you planned and installed your wireless network, some problems can still exist. Some potential problems to look for are interference and the voltage standing wave ratio (VSWR).

Sometimes correcting these issues is as simple as tightening a connector. In more advanced cases, you will need to perform a spectrum analysis to ensure the intended wireless data is not being saturated by noise. Next, test the cabling by testing the VSWR.

VSWR

VSWR refers to a mismatch of impedance between the transmitter and the load impedance (antenna). To achieve maximum power, the load impedance (antenna) and the generator impedance (radio) must match. Ideally, the VSWR is 1:1, or a perfect impedance match. This would allow for a maximum power transfer from the radio to the antenna. Unfortunately, this ideal case is physically impossible.

The next best solution is matching the line to antenna impedance as closely as possible. This will at least minimize power losses. Any impedance mismatch will cause a standing wave in the opposite direction of the forward power on the transmission line causing interference to the forward power. This will result in a voltage maximum and minimum at one-quarter wavelength increments. The greater the amplitude of these maximums and minimums, the greater the signal attenuation will be.

A watt meter can measure the forward and reflected power of a transmission line. Most watt meters measure forward power as the sum of both the forward and reflected power. This is typically done by having the tuning element pointed in the direction of the antenna. To measure reflected power, turn the element so that it points toward the transmitter. A maximum VSWR of 1.5:1, or 5% return, is recommended for optimal system performance. A ratio greater than 1.5:1 indicates a problem in the cable-to-antenna terminations.

Figure 3 displays three wave forms:

  • Incident wave
  • Transmitted wave
  • Reflected wave

Figure 3: Waveforms present in the coaxial cable and transmitted from the antenna display what happens when VSWR is present. Vertical red line represents impedance of cable or other components on the line. Courtesy: Phoenix ContactMost VSWR problems are due to loose connections that are not fastened correctly or are not weather-proofed. An easy way to prevent problems is to seal each junction with weatherized, vulcanizing tape. This simple step not only keeps water from getting into the cable line, but will also keep the connectors fastened.

After cables are installed, the engineer should run a VSWR check on all cables. If the cables were connected in the field, they must be tested to ensure proper connections.

Testing differences

After the initial planning and selection of the best radio system and components for the intended site, conducting a field test is very important. Most suppliers can assist in this step to ensure a final installation is the best and most reliable up-front, thus reducing troubleshooting efforts later.

A field test can also assist you in learning more about the products’ features and functions. For example, radios from all manufacturers are programmed slightly differently, so ensuring that you know how to program the specific radio being installed makes installations much simpler.

Performing a thorough field test can ensure long-term communications success.

- David Burrell is wireless product specialist, Phoenix Contact. Edited by Mark T. Hoske, content manager CFE Media, Control Engineering, Plant Engineering, and Consulting-Specifying Engineer, mhoske(at)cfemedia.com

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