In earlier, the components of RF communications were introduced, and the main components were reviewed. However, there are other components that are either not as significant or not always installed as part of the communications link.
Important specifications for all antenna accessories include frequency response, impedance, VSWR, maximum input power, and insertion loss. In here, will discuss some of these components and accessories.
Cables
Improper installation or selection of cables can detrimentally affect the RF communications more than just about any other component or outside influence. It is important to remember this when installing antenna cables. The following list addresses some concerns when selecting and installing cables:
- Make sure you select the correct cable. The impedance of the cable needs to match the impedance of the antenna and transceiver. If there is an impedance mismatch, the return loss from VSWR will affect the link.
- Attenuation increases with frequency. If you convert from 802.11b to 802.11a, the loss caused by the cable will be greater.
- Either purchase the cables precut and preinstalled with the connectors or hire a professional cabler to install the connections.
Improperly installed connectors will add more loss to the communications link, which can nullify the extra money you just spent for the better-quality cable. It can also introduce return loss in the cable due to reflections.
- Make sure the cable you select will support the frequencies that you will be using. Typically, cable manufacturers will list cutoff frequencies, which are the lowest and highest frequencies that the cable supports.
This is often referred to as frequency response. For instance, LMR 1200 will not work with 5 GHz transmissions. LMR 900 is the highest you can use. However, you can use LMR 1200 for 2.4 GHz operations.
- Cables introduce signal loss into the communications link. To determine how much loss, cable vendors provide charts or calculators to assist you. Table below is an attenuation chart for a type of cable produced by Times Microwave Systems. LMR cable is a popular brand of coaxial cable used in RF communications.
| LMR Cable | 30 | 50 | 150 | 220 | 450 | 900 | 1,500 | 1,800 | 2,000 | 2,500 | 5,800 |
| 100A | 3.94 | 5.10 | 8.95 | 10.90 | 15.83 | 22.84 | 30.08 | 33.22 | 35.19 | 39.81 | 64.10 |
| 195 | 1.97 | 2.55 | 4.44 | 5.40 | 7.78 | 11.13 | 14.53 | 15.99 | 16.90 | 19.02 | 29.90 |
| 195UF | 2.34 | 3.03 | 5.28 | 6.42 | 9.25 | 13.23 | 17.28 | 19.01 | 20.10 | 22.62 | 35.57 |
| 200 | 1.77 | 2.29 | 3.98 | 4.83 | 6.96 | 9.92 | 12.92 | 14.21 | 15.01 | 16.87 | 26.35 |
| 200UF | 2.12 | 2.74 | 4.78 | 5.80 | 8.35 | 11.91 | 15.51 | 17.05 | 18.01 | 20.24 | 31.62 |
| 240 | 1.34 | 1.73 | 3.01 | 3.66 | 5.28 | 7.56 | 9.87 | 10.87 | 11.49 | 12.93 | 20.35 |
| 240UF | 1.60 | 2.07 | 3.62 | 4.40 | 6.34 | 9.07 | 11.85 | 13.04 | 13.78 | 15.52 | 24.42 |
| 300 | 1.06 | 1.37 | 2.40 | 2.92 | 4.22 | 6.06 | 7.93 | 8.74 | 9.24 | 10.42 | 16.53 |
| 300UF | 1.27 | 1.65 | 2.88 | 3.50 | 5.06 | 7.26 | 9.51 | 10.48 | 11.08 | 12.50 | 19.81 |
| 400 | 0.68 | 0.88 | 1.54 | 1.87 | 2.71 | 3.90 | 5.13 | 5.66 | 5.99 | 6.76 | 10.82 |
| 400UF | 0.81 | 1.05 | 1.84 | 2.25 | 3.25 | 4.68 | 6.15 | 6.79 | 7.19 | 8.12 | 12.99 |
| 500 | 0.54 | 0.70 | 1.22 | 1.49 | 2.17 | 3.13 | 4.13 | 4.57 | 4.84 | 5.48 | 8.86 |
| 500UF | 0.64 | 0.84 | 1.47 | 1.79 | 2.60 | 3.76 | 4.96 | 5.48 | 5.81 | 6.58 | 10.64 |
| 600 | 0.42 | 0.55 | 0.96 | 1.18 | 1.72 | 2.50 | 3.32 | 3.67 | 3.90 | 4.43 | 7.26 |
| 600UF | 0.51 | 0.66 | 1.16 | 1.41 | 2.06 | 3.00 | 3.98 | 4.41 | 4.68 | 5.31 | 8.71 |
| 900 | 0.29 | 0.37 | 0.66 | 0.80 | 1.17 | 1.70 | 2.25 | 2.48 | 2.64 | 2.99 | 4.87 |
| 900UF | 0.35 | 0.45 | 0.79 | 0.96 | 1.41 | 2.04 | 2.70 | 2.98 | 3.16 | 3.59 | 5.85 |
| 1200 | 0.21 | 0.27 | 0.48 | 0.59 | 0.87 | 1.27 | 1.69 | 1.87 | 1.99 | 2.27 | not supported |
| 1700 | 0.15 | 0.20 | 0.35 | 0.43 | 0.63 | 0.94 | 1.27 | 1.41 | 1.50 | 1.72 | not supported |
- The left side of the chart lists different types of LMR cable. The farther you move down the list, the better the cable is. The better cable is typically thicker, stiffer, more difficult to work with, and of course, more expensive. The chart shows how much decibel loss the cable will add to the communications link.
The column headers list the frequencies that may be used with the cable. For example, 100 feet of LMR-400 cable used on a 2.5 GHz network (2,500 MHz) would decrease the signal by 6 dB.
- Attenuation increases with frequency. If you convert from 802.11b to 802.11a, the loss caused by the cable will be greater.
- Either purchase the cables precut and preinstalled with the connectors or hire a professional cabler to install the connections.
Improperly installed connectors will add more loss to the communications link, which can nullify the extra money you just spent for the better-quality cable. It can also introduce return loss in the cable due to reflections.
Connectors
There are many types of connectors that are used to connect antennas to 802.11 equipment. Part of the reason for this is that the FCC Report & Order 04-165 requires that amplifiers have either unique connectors or electronic identification systems to prevent the use of noncertified antennas.
This was done to prevent people from connecting higher-gain antennas, either intentionally or unintentionally, to a transceiver. An unauthorized high-gain antenna could exceed the maximum EIRP that is allowed by the FCC or other regulatory body.
In response to this regulation, cable manufacturers sell “pigtail” adapter cables. These pigtail cables are usually short segments of cable (typically about 2 feet long) with different connectors on each end.
They act as adapters, changing the connector, and allowing a different antenna to be used. The use of these adapter cables typically violates the rules of the local regulatory body.
They are typically used by Wi-Fi hobbyists or network installers for testing purposes. Remember that these pigtails usually violate RF regulations and are not recommended or condoned.
Many of the same principles of cables apply to the connectors and also many of the other accessories. RF connectors need to be of the correct impedance to match the other RF equipment.
They also support specific ranges of frequencies. The connectors add signal loss to the RF link, and lower-quality connectors are more likely to cause connection or VSWR problems. RF connectors on average add about 1/2 dB of insertion loss.
Splitters
Splitters are also known as signal splitters, RF splitters, power splitters, and power dividers. A splitter takes an RF signal and divides it into two or more separate signals. Only under an unusually special or unique situation would you have a need to use an RF splitter.
One such situation would be if you were connecting sector antennas to one transceiver. If you had three 120 degree antennas aimed away from a central point to provide 360 degree coverage, you could connect each antenna to its own transceiver or you could use a three-way splitter and equal-length cables to connect the antennas to a single transceiver.
When you’re installing a splitter in this type of configuration, not only will the signal be degraded because it is being split three times, known as through loss, but also each connector will add its own insertion loss to the signal.
There are so many variables and potential problems with this configuration that we would recommend only that this type of installation be attempted by a very RF knowledgeable person and for temporary installations.
A more practical, but again rare, use of a splitter is to monitor the power that is being transmitted. The splitter can be connected to the transceiver and then split to the antenna and a power meter. This would allow you to actively monitor the power that is being sent to the antenna.
Amplifiers
An RF amplifier takes the signal that is generated by the transceiver, increases it, and sends it to the antenna. Unlike the antenna providing an increase in gain by focusing the signal, an amplifier provides an overall increase in power by adding electrical energy to the signal, which is referred to as active gain.
Amplifiers can be purchased as either unidirectional or bidirectional devices. Unidirectional amplifiers perform the amplification in only one direction, either when transmitting or when receiving. Bidirectional amplifiers perform the amplification in both directions.
The amplifier’s increase in power is created using one of two methods: fixed gain or fixed output. With the fixed-gain method, the output of the transceiver is increased by the amount of the amplifier. A fixed-output amplifier does not add to the output of the transceiver.
The fixed-output amplifier simply generates a signal equal to the output of the amplifier regardless of the power generated by the transceiver. Adjustable variable-gain amplifiers also exist, but it is a recommended practice not to use adjustable-gain amplifiers.
Unauthorized adjustment of a variable-rate amplifier may result either in violation of power regulations or insufficient transmission amplitude.
Since most regulatory bodies have a maximum power regulation of 1 watt or less at the Intentional Radiator (IR), the main purpose of using amplifiers is to compensate for cable loss as opposed to boosting the signal for range.
Therefore, when installing an amplifier, install it as close to the antenna as possible. Since the antenna cable adds loss to the signal, the shorter antenna cable will produce less loss and allow more signal to the antenna.
Attenuators
In some situations, it may be necessary to decrease the amount of signal that is radiating from the antenna. You could be installing a short point-to-point link and want to reduce the output to minimize interference to other RF equipment in the area.
In some instances, even the lowest power setting of the transceiver may generate more signal than you want. In this situation, you can add a fixed-loss or a variable-loss attenuator.
Attenuators are small devices about the size of a C-cell battery, with cable connectors on both sides. Attenuators absorb energy, decreasing the signal as it travels through. A variable-loss attenuator has a dial on it that allows you to adjust the amount of energy that is absorbed.
Fixed-loss attenuators provide a set amount of loss. Variable-loss attenuators are also often used during outdoor site surveys to simulate loss caused by various grades of cabling and different lengths.
Another interesting use of a variable attenuator is to test the actual fade margin on a point-to-point link. By gradually increasing the attenuation until there is no more link, you can use that number to determine the actual fade margin of the link.
Lightning Arrestors
The purpose of a lightning arrestor is to redirect (shunt) transient currents caused by nearby lightning strikes or ambient static away from your electronic equipment and into the ground. Lightning arrestors are used to protect electronic equipment from the sudden surge of power that a nearby lightning strike or static buildup can cause.
You may have noticed the use of the phrase “nearby lightning strike.” This wording is used because lightning arrestors are not capable of protecting against a direct lightning strike. Lightning arrestors can typically protect against surges of up to 5,000 amperes at up to 50 volts.
The IEEE specifies that lightning arrestors should be capable of redirecting the transient current in less than 8 microseconds. Most lightning arrestors are capable of doing it in less than 2 microseconds.
The lightning arrestor is installed between the transceiver and the antenna. Any devices that are installed between the lightning arrestor and the antenna will not be protected the lightning arrestor.
Therefore, the lightning arrestor is typically placed closer to the antenna, with all other communications devices (amplifiers, attenuators, etc.) installed between the lightning arrestor and the transceiver.
Figure 1 shows a properly grounded radio, cabling, and antenna.
After a lightning arrestor has performed its job by protecting the equipment from an electrical surge, it will have to be replaced, or it may have a replaceable gas discharge tube (like a fuse). Fiber-optic cable can also be used to provide additional lightning protection.
A short piece of fiber-optic cable can be inserted into the Ethernet cable that connects the wireless bridge to the rest of the network. Ethernet-to-fiber adapters, known as transceivers, convert the electrical Ethernet signal to a light-based fiber signal and then back to Ethernet.
Since fiber-optic cable is constructed of glass and it uses light and not electricity to transmit data, it does not conduct electricity. The fiber-optic cable acts as a kind of safety net should the lightning arrestor fail due to a much higher transient current or even a direct lightning strike.
Realize that if there is a direct lightning strike to the antenna, you can plan on replacing all of the components from the fiberoptic cable to the antenna.
Furthermore, a direct lightning strike may also arc over the fiber link and still cause damage to equipment on the opposite side of the fiber link. Grounding the RF cables as well can help prevent this from happening.
Grounding Rods and Wires
When lightning strikes an object, it is looking for the path of least resistance, or more specifically, the path of least impedance. This is where lightning protection and grounding equipment come in to play. A grounding system, which is made up of a grounding rod and wires, provides a low-impedance path to the ground.
This low-impedance path is installed to encourage the lightning to travel through it instead of through your expensive electronic equipment. Grounding rods and wires are also used to create what is referred to as a common ground.
One way of creating a common ground is to drive a copper rod into the ground and connect your electrical and electronic equipment to this rod using wires or straps (grounding wires).
The grounding rod should be at least 6 feet long and should be fully driven into the ground, leaving enough of the rod accessible to attach the ground wires to it. By creating a common ground, you have created a path of least impedance for all of your equipment should lightning cause an electrical surge.
On tower structures, a ground rod should be placed off of each leg with a No. 2 tinned copper wire. These connections should be exothermically welded to the tower legs. A No. 2 tinned copper wire should also form a ring around the grounding rods.
The following diagram illustrates a proper grounding ring.
The dashed lines are No. 2 tinned copper wire and the circles are grounding rods. Ice bridges and building grounds should also be bonded to this ring to provide equal grounding potentials.
