Wireless network: Field testing benefits
After extensive research, you have selected the best wireless technology for your application, and you have selected the accessories you need.
Before you install that equipment, however, there is one final step: testing the network. Careful testing before installation can prevent many potential problems down the road and reduce troubleshooting time following an installation.
Proof of concept
Proof of concept helps to ensure that you have selected the correct radio for the job. In most cases, the first step is to conduct a software path loss study. Distances less than 305 m (1,000 ft) usually do not require such a study. A simple test can be done with a functional radio set to the desired wireless mode, transmit data rate, and transmit power setting.
The first step in performing a path study is to plan where the remote sites will be constructed. This can be done either by traveling to each site with a handheld GPS unit and collecting the GPS coordinates, or by using maps and other tools, such as Google Earth. A path simulation uses topographic maps to plan the network virtually. It can show which objects might obstruct communications. This will help determine the ideal height of the antenna mast, as well as the necessary distance between radio links. With today’s advanced software, it is possible to ensure proper Fresnel zone clearance in line-of-sight wireless systems.
Fresnel zone is the area around the direct line connecting the transmitting and receiving antennas. When terrain or other obstacles are in this area, they can disturb the wireless connection.
Figure 1 shows an ideal installation with undisturbed connection. Figure 2 shows a zone obstructed by terrain. While the antennas still have line of sight, the antenna masts are set at a low level and the Fresnel zone is not completely clear. This can result in a disturbed communication path.
The transmission frequency and the distance between the transmitting and receiving antennas help to determine the radius of the Fresnel zone. For a reliable path, at least 60% of the Fresnel zone should be unobstructed (also known as the 0.6 Fresnel zone). Increasing antenna heights is generally the only way to keep the 0.6 Fresnel zone clear of obstructions. The longer the distance the radios need to communicate, the clearer the Fresnel zone must be.
While path studies are an important step in planning the wireless network, they are not flawless. They might not be able to account for manmade obstructions or foliage growth within the zone. An on-site field test using temporary equipment can prevent such issues.
If the installation is more than 1,000 ft, a field test is the best way to ensure that the correct components have been selected. Recommended equipment for an effective field test includes:
- Coaxial cable
- Portable antenna masts with tripods
- Spectrum analyzer
- Radios under test
- Watt meter
- Power supply/battery
Watt meters and spectrum analyzers are rather expensive to purchase, but they can be rented for short-term use.
A spectrum analysis measures the noise floor in the intended installation area. This is an important item to consider. If a frequency band is in an area prone to high interference, it might be wise to avoid this spectrum completely. Also, some higher-quality radios can block channels in the band. If you perform a spectrum analysis before installation, you can locate potential peaks of interference. The radio might be able to block these channels directly, thus avoiding potential problems.
At each remote site, you will need to test communications back to the master site using a variety of antennas. This will help you find the optimal signal strength. Radio performance is directly related to how the antenna is mounted, raised, and polarized.
Most omnidirectional antennas are vertically polarized. All directional Yagi antennas should match the vertical polarization by having all elements perpendicular to the ground.
Using the site map and software path study created earlier, locate each remote site to be constructed and measured under test. It is best to set up the master location first, and then work outward, from the relative closest site out to the farthest. To determine the height of the master antenna, again refer to the software study. Usually the farthest slave is the weakest link, so you will set the master antenna to the height that is required by that slave.
After you’ve constructed the master location and installed the radio, move to the nearest remote site. Use the mast and tripod to raise the antenna to the predicted height where it achieves a 20 dB fade margin. Using a voltmeter or the radio’s software measurement tool, record the received signal strength to the master and compare the reading to the software simulation’s prediction. If the radio did not achieve a 20 dB fade margin above the receive sensitivity, raise the remote antenna height until it reaches that level. Make note of this height for final installation. Also, make sure that the Yagi antenna azimuth points directly to the master, and that it is polarized correctly. If a 20 dB fade margin cannot be achieved, a higher gain antenna may need to be used.
Once the radio has received an acceptable signal, you need to verify how the RF link will transport the data it must carry. A typical test consists of pushing data through the same (or comparable) link that will be used in the final installation. While conducting this data test, check for packet errors that result from the wireless link. Higher-quality radios will track and display the transmitted and received frames.
Once you have established a proven RF link from master to slave, perform the same tests at all remote sites. Each will have different requirements for antennas, cable, height, and bandwidth.
Thorough testing can be time consuming, but before commissioning a system you should ensure that all remote sites have reliable and effective communication. This will prevent costly and time-consuming troubleshooting later on. A good system design and installation will sustain network performance in the future.
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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 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
Most 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.
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, email@example.com.