In addition to the physical antenna being a critical component in the wireless network, the installation and connection of the antenna to the wireless transceiver is critical. If the antenna is not properly connected and installed, any benefit that the antenna introduces to the network can be instantly wiped out.
Three key components associated with the proper installation of the antenna are voltage standing wave ratio (VSWR), return loss, and the actual mounting of the antenna.
Voltage Standing Wave Ratio (VSWR)
Voltage standing wave ratio (VSWR) is a measurement of the change in impedances to an AC signal. Voltage standing waves exist due to impedance mismatches or variations between devices in an RF communications system. Impedance is a value of ohms of electrical resistance to an AC signal.
A standard unit of measurement of electrical resistance is the ohm, named after German physicist Georg Ohm. When the transmitter generates the AC radio signal, the signal travels along the cable to the antenna. Some of this incident (or forward) energy is reflected back toward the transmitter because of impedance mismatch.
Mismatches may occur anywhere but are usually due to abrupt impedance changes between the radio transmitter and cable and between the cable and the antenna. The amount of energy reflected depends on the level of mismatch between the transmitter, cable, and antenna.
The ratio between the voltage of the reflected wave and the voltage of the incident wave, at the same point along the cable, is called the voltage reflection coefficient, usually designated by the Greek letter ρ (rho). When this quantity is expressed in dB, it is called return loss.
So in an ideal system, where there are no mismatches (the impedance is the same everywhere), all of the incident energy will be delivered to the antenna (except for resistive losses in the cable) and there will be no reflected energy. The cable is said to be matched, and the voltage reflection coefficient is exactly zero and the return loss, in dB, is infinite.
The combination of incident and reflected waves traveling back and forth along the cable creates a resulting standing wave pattern along the length of the line. The standing wave pattern is periodic (it repeats) and exhibits multiple peaks and troughs of voltage, current, and power.
VSWR is a numerical relationship between the measurement of the maximum voltage along the line (what is generated by the transmitter) and the measurement of the minimum voltage along the line (what is received by the antenna).
VSWR is therefore a ratio of impedance mismatch, with 1:1 (no impedance) being optimal but unobtainable and typical values from 1.1:1 to as much as 1.5:1. VSWR military specs are 1.1:1.
VSWR = Vmax ÷ Vmin
When the transmitter, cable, and antenna impedances are matched (i.e., there are no standing waves), the voltage along the cable will be constant. This matched cable is also referred to as a flat line since there are no peaks and troughs of voltage along the length of the cable.
In this case, VSWR is 1:1. As the degree of mismatch increases, the VSWR increases with a corresponding decrease in the power delivered to the antenna. Table 4.2 shows this effect.
|VSWR||Radiated Power||Lost Power||dB power loss|
|1.5:1||96%||4%||Nearly 0 dB|
If VSWR is large, this means that there is a large amount of voltage that is being reflected back toward the transmitter. This of course means a decrease in power or amplitude (loss) of the signal that is supposed to be transmitted. This loss of forward amplitude is known as return loss and can be measured in dB.
Additionally, the power that is being reflected back is then directed back into the transmitter. If the transmitter is not protected from excessive reflected power or large voltage peaks, it can overheat and fail. Understand that VSWR may cause decreased signal strength, erratic signal strength, or even transmitter failure.
The first thing that can be done to minimize VSWR is to make sure that the impedance of all of the wireless networking equipment is matched. Most wireless networking equipment has an impedance of 50 ohms; however, you should check the manuals to confirm this. When attaching the different components, make sure that all connectors are installed and crimped properly and that they are snugly tightened.
As was stated earlier in this chapter, proper installation of the antenna is one of the most important tasks to ensure an optimally functioning network. The following are key areas to be concerned with when installing antennas:
- Appropriate use
The proper placement of an antenna is dependent upon the type of antenna. When installing omni-directional antennas, it is important to place the antenna at the center of the area where you want coverage.
Remember that lower-gain omni-directional antennas provide broader vertical coverage while higher-gain omni-directional antennas provide a wider but much flatter coverage.
Be careful not to place high-gain omni-directional antennas too high above the ground because the narrow vertical coverage may cause the antenna to provide insufficient signal to clients located on the ground.
When installing directional antennas, make sure that you know both the horizontal and vertical beamwidths so that you can properly aim the antennas.
Also make sure that you are aware of the amount of gain that the antenna is adding to the transmission. If the signal is too strong, it will overshoot the area that you are looking to provide coverage to.
This is a security risk, and you should decrease the amount of power that the transceiver is generating to reduce the coverage area. Not only is it a security risk, overshooting your coverage area is considered rude.
If you are installing an outdoor directional antenna, in addition to concerns regarding the horizontal and vertical beamwidths, make sure that you have correctly calculated the Fresnel zone and mounted the antenna accordingly.
After deciding where to place the antenna, the next step is to decide how to mount it. Many antennas, especially outdoor antennas, are mounted on masts or towers. It is common to use mounting clamps and U-bolts to attach the antennas to the masts.
For mounting directional antennas, specially designed tilt-and-swivel mounting kits are available to make it easier to aim and secure the antenna. If the antenna is being installed in a windy location (what rooftop or tower isn’t windy?), make sure that you take into consideration wind load and that you properly secure the antenna.
There are numerous ways of mounting antennas indoors. Two common concerns are aesthetics and security. Many organizations, particularly ones that provide hospitality-oriented services such as hotels and hospitals, are concerned about the aesthetics of the installation of the antennas.
Specialty enclosures and ceiling tiles can help to hide the installation of the access points and antennas. Other organizations, particularly schools and public environments, are concerned with securing the access points and antennas from theft or vandalism.
An access point can be locked in a secure enclosure, with a short cable connecting it to the antenna. There are even ceiling tiles with antennas built into them, invisible to anyone walking by. If security is a concern, mounting the antenna high on the wall or ceiling can also minimize unauthorized access.
Use Make sure that indoor antennas are not used for outdoor communications. Outdoor antennas are specifically built to withstand a wide range of temperatures that they may be exposed to.
Outdoor antennas are also built to stand up to other elements, such as rain, snow, and fog. In addition to installing the proper antenna, make sure that the mounts that you use are designed for the environment in which you are installing them.
Before installing an antenna, make sure you read the manufacturer’s recommendations for mounting it. This suggestion is particularly important when installing directional antennas.
Since directional antennas may have different horizontal and vertical beamwidths, and since directional antennas can be installed with different polarization, proper alignment can make the difference between being able to communicate or not.
The first step is to make sure you have decided on a polarization. Next, decide on the mounting technique and ensure that it is compatible with the mounting location. Then the antennas can be aligned. Once that occurs, the cables and connectors can be weatherproofed and secured from movement.
We can’t emphasize enough the importance of being careful when installing antennas. Most of the time, the installation of an antenna involves climbing ladders, towers, or rooftops.
Gravity and wind have a way of making an installation difficult for both the climber and the people below helping. Plan the installation before you begin, making sure you have all of the tools and equipment that you will need to install the antenna.
Unplanned stoppages of the installation and relaying forgotten equipment up and down the ladder add to the risk of injury. Be careful when working with your antenna or near other antennas.
Highly-directional antennas are focusing high concentrations of RF energy. This large amount of energy can be dangerous to your health. Do not power on your antenna while you are working on it, and do not stand in front of other antennas that are near where you are installing your antenna.
When installing antennas (or any device) on ceilings, rafters, or masts, make sure they are properly secured. Even a 1-pound antenna can be deadly if it falls from the rafters of a warehouse. If you will be installing antennas as part of your job, we recommend that you take an RF health and safety course.
These courses will teach you the FCC and the U.S. Department of Labor Occupational Safety and Health Administration (OSHA) regulations and how to be safe and compliant with the standards. If you need an antenna installed on any elevated structure, such as a pole, tower, or even a roof, consider hiring a professional installer.
Professional climbers and installers are trained and in some places certified to perform these types of installations. In addition to the training, they have the necessary safety equipment and proper insurance for the job.
If you are planning to install wireless equipment as a profession, you should develop a safety policy that is blessed by your local occupational safety representative. You should also receive certified training on climbing safety in addition to RF safety training. First aid and CPR training is also highly recommended.
There are two types of maintenance: preventative and diagnostic. When installing an antenna, it is important to prevent problems from occurring in the future. This seems like simple advice, but since antennas are often difficult to get to after they have been installed, it is especially prudent advice.
Two key problems that can be minimized with proper preventative measures are wind damage and water damage. When installing the antenna, make sure all of the nuts, bolts, screws, and so on are tightened. Also make sure all of the cables are properly secured so that they are not thrashed about in the wind.
To help prevent water damage, cold shrink tubing or coaxial sealant can be used to minimize the risk of water getting into the cable or connectors. Another cabling technique is the drip loop. A drip loop prevents water from flowing down the cable and onto a connector or into the hole where a cable exits the building.
Any water that is flowing down the cable will continue to the bottom of the loop and then drip off. Antennas are typically installed and forgotten about until they break.
It is advisable to periodically perform a visual inspection of the antenna. If the antenna is not easily accessible, a pair of binoculars or a camera with a very high zoom lens can make this a simple task.