How to choose industrial wireless components

For industrial wireless networks, choosing the best accessories helps ensure a high-quality wireless network. Most problems in newly installed wireless networks result from poorly chosen or poor-quality accessories. Don’t make selection of wireless accessories an afterthought.

04/25/2013

Over the past decade, wireless technology has become one of the most efficient ways for industrial professionals to reduce operational expenses. The labor and material expenses of a cabled system can cost up to 500% more than a wireless system. Thanks largely to these potential savings, the industrial market has witnessed a rapid increase in the market acceptance of wireless.

While the radio device itself is obviously a key piece of equipment, installation practices and auxiliary equipment also affect the reliability of a wireless network. Poor installation practices, coupled with low-quality accessories, can lead to failure in a wireless network.

Power measurements

Wireless systems measure all radios’ output powers and components’ gains and losses with a common unit factor. This common factor is known as a decibel. A decibel (dB) is an abbreviation for the power ratio calculated using the formula below. Note that both P1 and P2 refer to the same power unit.

dB=10log⁡(P1/P2)

In wireless communication, power is referred to as dBm. A dBm is calculated as a dB, but P2 always equals 1mW.

dBm=10log⁡(P/(1 mW))

Table 1 shows some power levels commonly found in wireless communications.

Power levels common for wireless communications

Courtesy: Phoenix Contact

For every 3 dBm gain, the effect of power doubles. An increase of 6 dBm will double the effective line-of-sight range for a wireless link; dBi refers to the gain of isotropic antennas. An isotropic antenna radiates equally in all directions. dBi and dBm can be added, and the resulting sum is expressed in dBm.

The sum of all antenna gains, cable losses, radio transmission power, receiver sensitivity, and path loss is the “link budget.” The remaining single level is called the fade margin. This is the cushion between the received signal and the minimum receive threshold of the radio device.

Environmental changes can attenuate the transmitted signal, so when designing a wireless network, it is important to calculate a 10-20 dB fade margin into the link budget of the path to compensate for any such changes. Using higher gain antennas and lower loss coaxial cable, or increasing the transmission power (when possible), are two ways to increase the fade margin. Figure 1 shows an overview of the calculations.

Antenna selection

Proper antenna installation is critical and will affect the entire system’s performance. Choosing an antenna designed for use at the proper frequency and with matching impedance are the first steps. Select an antenna with an appropriate gain for the path it will be used for.

Polarization of antennas:

Polarization refers to the direction in which the radio emits energy through space. Antennas can be polarized in three ways: vertically, horizontally, and radially.

Cross-polarizing antennas mean that the receiver will accept only a fraction of the transmission power. The antenna’s angle determines what that fraction of emitted power is. For example, no power will be received by an antenna 90 degrees out of phase. An antenna 45 or 130 degrees out of phase will receive only half of the power.

However, cross-polarization is not necessarily a bad thing. Two neighboring or overlapping networks that are operating in the same frequency can result in interference. In this case, changing the polarization of one of the networks can overcome the intrusive signals between the two networks.

Universal antenna characteristics

Omni-directional antennas cover a 360-degree plane with nearly uniform characteristics across all directions (Figure 2). Some common applications for omni-directional antennas include cases where the position between the transmitter and receiver can change, moving applications or a multipoint network. They are also ideal when line of sight is obstructed, because the reflections can be used to send the signal from the transmitter to the receiver.

The best place to install an omni-directional antenna is on top of a mast or on a control cabinet. This allows it as much free space in all directions as possible. Unfortunately, however, it is not always possible. If an omni-directional antenna must be mounted on the side of a mast, the installer must observe specific measurements and distances to mount the antenna away from the mast for best possible signal.

If the omni-directional antenna is mounted to conductive material, such as a master control cabinet, its directional characteristics will be affected. The diameter and distance between the antenna and conductive material can alter the antenna’s coverage area significantly. Wall mounting should be avoided, as the wall has a great impact on the antenna’s transmission properties. If wall mounting is the only option, the installation should have a minimum of half a wavelength of the respective operating frequency of distance between the wall and antenna.

Yagi antennas

Yagi antennas (Figure 3) radiate power in a specific direction. This increases the range and reduces the chance of interference.

As the gain of a Yagi antenna increases, the energy becomes more focused. This causes the beam width to decrease, so proper alignment becomes even more critical. Aiming these antennas in the desired direction of communication (such as at the master station) is very important. Remote, fixed stations with line of sight that cover large distances should use directional antennas. The end of the antenna (farthest from the support mast) should face the associated station.

A master location with multiple slave radios must always have an omni-directional antenna, and the slave radios can have Yagi antennas to increase distance possibilities. During final alignment of the antenna heading, it should be oriented for maximum signal strength.

Directional antennas must be mounted securely. Strong winds might sway or wobble an unstable antenna, which could lead to serious misalignment.

Antenna cables

Many installers neglect the antenna coaxial cable. Using a low-quality cable can reduce efficiency or even damage the radio.

Every 3 dB of coaxial cable loss cuts the transmission distance in half. Choosing the correct coaxial cable depends on:

• The length of cable required to span the distance between transmitter and the antenna
• The amount of signal loss that can be tolerated
• Cost considerations.

Long-range transmission paths are likely to be weaker. A low-loss cable type will work best, especially if the cable must be more than 50 ft long. If you have a short-range system or only require a short coaxial cable, a less efficient (and costlier) cable might suffice.

The cable’s loss depends on the frequency. As the radio operating frequency increases, the cable loss also increases. Consult the coaxial cable datasheets to determine if it will work with your intended radio frequency.

The cable’s bending radius is another important consideration during the installation process. The bending radius is the minimum ratio to the inside curvature of the cable without kinking, damaging, or shortening its life span (see Figure 4). The cable’s datasheet typically includes the bending radius.

Power supplies

A radio will not operate properly if it does not have a sufficient power source. This can result in intermittent radio links, dropped data, or the need to reset devices. Yet many designers overlook power supplies when designing a wireless network.

Many radios will have operating modes or times that will demand a high amount of power. The power supply must provide enough current to all of the connected loads. It should also have at least 30% overhead to compensate for expansion and loads that draw variable power.

Installing a UPS as a backup power supply is also a good practice, especially for high-integrity sites. The UPS will act as a backup power supply if power at the site fails.

Choose the right wireless accessories

A high-quality wireless network needs to provide reliable communication, usually for many years of service. Most problems in newly installed wireless networks result from poorly chosen or poor-quality accessories, yet for many people, accessories are often an afterthought when planning their network. From planning to installation, choosing the right components for your wireless system is equally as important as choosing the main system.

- 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@cfemedia.com.

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