RF Splitters

An RF Splitter is a device that has a single input connector and multiple output connectors. An RF Splitter is used for the purpose of splitting a single signal into multiple independent RF signals. Use of splitters in everyday implementations of wireless LANs is not recommended. Sometimes two 120-degree panel antennas or two 90-degree panel antennas may be combined with a splitter and equal-length cables when the antennas are pointing in opposite directions. This configuration will produce a bidirectional coverage area, which may be ideal for covering the area along a river or major highway. Back-to-back 90 degree panels may be separated by as little as 10 inches or as much as 40 inches on either side of the mast or tower. Each panel in this configuration may have a mechanical down tilt. The resultant gain in each of the main radiation lobes is reduced by 3 - 4 dB in these configurations.

When installing an RF splitter, the input connector should always face the source of the RF signal. The output connectors (sometimes called "taps") are connected facing the destination of the RF signal (the antenna). Figure 5.26 shows two examples of RF splitters. Figure 5.27 illustrates how an RF splitter would be used in a wireless LAN installation.

Splitters may be used to keep track of power output on a wireless LAN link. By hooking a power meter to one output of the splitter and the RF antenna to the other, an administrator can actively monitor the output at any given time. In this scenario, the power meter, the antenna, and the splitter must all have equal impedance. Although not a common practice, removing the power meter from one output of the splitter and replacing it with a 50 ohm dummy load would allow the administrator to move the power meter from one connection point to another throughout the wireless LAN while making output power measurements.


RF Connectors

RF connectors are specific types of connection devices used to connect cables to devices or devices to devices. Traditionally, N, F, SMA, BNC, & TNC connectors (or derivatives) have been used for RF connectors on wireless LANs.

In 1994, the FCC & DOC (Canadian Department of Communications) ruled that connectors for use with wireless LAN devices should be proprietary between manufacturers. For this reason, many variations on each connector type exist such as:

• N-type
• Reverse polarity N-type
• Reverse threaded N-type

Choosing an RF Connector

There are five things that should be considered when purchasing and installing any RF
connector, and they are similar in nature to the criteria for choosing RF amplifiers and
attenuators.

1. The RF connector should match the impedance of all other wireless LAN components (generally 50 ohms).
2. Know how much insertion loss each connector inserted into the signal path causes. The amount of loss caused will factor into your calculations for signal strength required and distance allowed.
3. Know the upper frequency limit (frequency response) specified for the particular connectors. This point will be very important as 5 Ghz wireless LANs become more and more common. Some connectors are rated only as high as 3 GHz, which is fine for use with 2.4 GHz wireless LANs, but will not work for 5 GHz wireless LANs. Some connectors are rated only up to 1 GHz and will not work with wireless LANs at all, other than legacy 900 MHz wireless LANs.
4. Beware of bad quality connectors. First, always consider purchasing from a reputable company. Second, purchase only high-quality connectors made by name-brand manufacturers. This kind of purchasing particularity will help eliminate many problems with sporadic RF signals, VSWR, and bad connections.
5. Make sure you know both the type of connector (N, F, SMA, etc.) that you need and the sex of the connector. Connectors come in male and female. Male connectors have a center pin, and female connectors have a center receptacle.

Wireless LAN Accessories

When the time comes to connect all of your wireless LAN devices together, you will need to purchase the appropriate cables and accessories that will maximize your throughput, minimize your signal loss, and, most importantly, allow you to make the connections correctly. This section will discuss the different types of accessories and where they fit into a wireless LAN design. The following types of accessories are discussed in this section:

• RF Amplifiers
• RF Attenuators
• Lightning Arrestors
• RF Connectors
• RF Cables
• RF Splitters

Each of these devices is important to building a successful wireless LAN. Some items are used more than others, and some items are mandatory whereas others are optional. It is likely that an administrator will have to install and use all of these items multiple times while implementing and managing a wireless LAN.


RF Amplifiers
As its name suggests, an RF amplifier is used to amplify, or increase the amplitude of, an RF signal, which is measured in +dB. An amplifier will be used when compensating for the loss incurred by the RF signal, either due to the distance between antennas or the length of cable from a wireless infrastructure device to its antenna. Most RF amplifiers used with wireless LANs are powered using DC voltage fed onto the RF cable with a DC injector near the RF signal source (such as the access point or bridge).

Sometimes this DC voltage used to power RF amplifiers is called "phantom voltage" because the RF amplifier seems to magically power up. This DC injector is powered using AC voltage from a wall outlet, so it might be located in a wiring closet. In this scenario, the RF cable carries both the high frequency RF signal and the DC voltage necessary to power the in-line amplifier, which, in turn, boosts the RF signal amplitude. Figure 5.20 shows an example of an RF amplifier (left), and how an RF amplifier is mounted on a pole (right) between the access point and its antenna.

RF amplifiers come in two types: unidirectional and bi-directional. Unidirectional amplifiers compensate for the signal loss incurred over long RF cables by increasing the signal level before it is injected into the transmitting antenna. Bi-directional amplifiers boost the effective sensitivity of the receiving antenna by amplifying the received signal before it is fed into the access point, bridge, or client device.


RF Attenuators

An RF attenuator is a device that causes precisely measured loss (in –dB) in an RF signal. While an amplifier will increase the RF signal, an attenuator will decrease it. Why would you need or want to decrease your RF signal? Consider the case where an access point has a fixed output of 100mW, and the only antenna available is an omni-directional antenna with +20 dBi gain. Using this equipment together would violate FCC rules for power output, so an attenuator could be added to decrease the RF signal down to 30mW before it entered the antenna. This configuration would put the power output within FCC parameters. Figure 5.22 shows examples of fixed-loss RF attenuators with BNC connectors (left) and SMA connectors (right). Figure 5.23 shows an example of an RF step attenuator.

Power over Ethernet (PoE) Devices

Power over Ethernet (PoE) is a method of delivering DC voltage to an access point, wireless bridge, or wireless workgroup bridge over the Cat5 Ethernet cable for the purpose of powering the unit. PoE is used when AC power receptacles are not available where wireless LAN infrastructure devices are to be installed. The Ethernet cable is used to carry both the power and the data to the units.

Consider a warehouse where the access points need to be installed in the ceiling of the building. The labor costs that would be incurred to install electrical outlets throughout the ceiling of the building to power the access points would be considerable. Hiring an electrician to do this type of work would be very expensive and time consuming. Remember that Ethernet cables can only carry data reliably for 100 meters and, for any distance more than 100 meters, PoE is not a viable solution. The following figure illustrates how a PoE device would provide power to an access point.


Common PoE Options
PoE devices are available in several types.

• Single-port DC voltage injectors
• Multi-port DC voltage injectors
• Ethernet switches designed to inject DC voltage on each port on a given pair of pins

Single-port DC Voltage Injectors

Access points and bridges that specify mandatory use of PoE include single-port DC voltage injectors for the purpose of powering the unit. See Figure 5.17 below for an example of a single-port DC voltage injector. These single-port injectors are acceptable when used with a small number of wireless infrastructure devices, but quickly become a burden, cluttering wiring closets, when building medium or large wireless networks.


Multi-port DC Voltage Injectors

Several manufacturers offer multi-port injectors including 4, 6, or 12-port models. These models may be more economical or convenient for installations where many devices are to be powered through the Cat5 cable originating in a single wiring closet or from a single switch. Multi-port DC voltage injectors typically operate in exactly the same manner as their single-port counterparts. See Figure 5.18 for an example of a multi-port PoE injector. A multi-port DC voltage injector looks like an Ethernet switch with twice as many ports. A multi-port DC voltage injector is a pass-through device to which you connect the Ethernet switch (or hub) to the input port, and then connect the PoE client device to the output device, both via Cat5 cable. The PoE injector connects to an AC power source in the wiring closet. These multi-port injectors are appropriate for mediumsized wireless network installations where up to 50 access points are required, but in large enterprise rollouts, even the most dense multi-port DC voltage injectors combined with Ethernet hubs or switches can become cluttered when installed in a wiring closet.


Active Ethernet Switches

The next step up for large enterprise installations of access points is the implementation of active Ethernet switches. These devices incorporate DC voltage injection into the Ethernet switch itself allowing for large numbers of PoE devices without any additional hardware in the network. See Figure 5.19 for an example of an Active Ethernet switch. Wiring closets will not have any additional hardware other than the Ethernet switches that would already be there for a non-PoE network. Several manufacturers make these switches in many different configurations (number of ports). In many Active Ethernet switches, the switch can auto-sense PoE client devices on the network. If the switch does not detect a PoE device on the line, the DC voltage is switched off for that port. As you can see from the picture, an Active Ethernet switch looks no different from an ordinary Ethernet switch. The only difference is the added internal functionality of supplying DC voltage to each port.

RF Antenna Concepts

There are several concepts that are essential knowledge when implementing solutions that require RF antennas. Among those that will be described are:

• Polarization
• Gain
• Beamwidth
• Free Space Path Loss

The above list is by no means a comprehensive list of all RF antenna concepts, but rather a set of must-have fundamentals that allow an administrator to understand how wireless LAN equipment functions over the wireless medium. A solid understanding of basic antenna functionality is the key to moving forward in learning more advanced RF concepts.

Knowing where to place antennas, how to position them, how much power they are radiating, the distance that radiated power is likely to travel, and how much of that power can be picked up by receivers is, many times, the most complex part of an administrator's job.


Polarization
A radio wave is actually made of up two fields, one electric and one magnetic. These two fields are on planes perpendicular to each other, as shown in the following figure.


The sum of the two fields is called the electro-magnetic field. Energy is transferred back and forth from one field to the other, in the process known as "oscillation." The plane that is parallel with the antenna element is referred to as the "E-plane" whereas the plane that is perpendicular to the antenna element is referred to as the "H-plane." We are interested primarily in the electric field since its position and direction with reference to the Earth's surface (the ground) determines wave polarization.

Polarization is the physical orientation of the antenna in a horizontal or vertical position. The electric field is parallel to the radiating elements (the antenna element is the metal part of the antenna that is doing the radiating) so, if the antenna is vertical, then the polarization is vertical.

• Horizontal polarization - the electric field is parallel to the ground
• Vertical polarization - the electric field is perpendicular to the ground

Vertical polarization, which is typically used in wireless LANs, is perpendicular to the Earth’s plane. Notice the dual antennas sticking up vertically from most any access point - these antennas are vertically polarized in that position. Horizontal polarization is parallel to the Earth. In the foolowing figure illustrates the effects polarization can have when antennas are not aligned correctly. Antennas that are not polarized in the same way are not able to communicate with each other effectively.


Gain
Antenna gain is specified in dBi, which means decibels referenced to an isotropic radiator. An isotropic radiator is a sphere that radiates power equally in all directions simultaneously. We haven't the ability to make an isotropic radiator, but instead we can make omni-directional antennas such as a dipole that radiates power in a 360-degree horizontal fashion, but not 360 degrees vertically. RF signal radiation in this fashion gives us a doughnut pattern. The more we horizontally squeeze this doughnut, the flatter it becomes, forming more of a pancake shape when the gain is very high. Antennas have passive gain, which means they do not increase the power that is input into them, but rather shape the radiation field to lengthen or shorten the distance the propagated wave will travel. The higher the antenna gain, the farther the wave will travel, concentrating its output wave more tightly so that more of the power is delivered to the destination (the receiving antenna) at long distances. As was shown in the figure, the coverage has been squeezed vertically so that the coverage pattern is elongated, reaching further.


Beamwidth
As we've discussed previously, narrowing, or focusing antenna beams increases the antenna’s gain (measured in dBi). An antenna’s beamwidth means just what it sounds like: the “width” of the RF signal beam that the antenna transmits. The following figure illustrates the term
beamwidth.

There are two vectors to consider when discussing an antenna’s beamwidths: the vertical and the horizontal. The vertical beamwidth is measure in degrees and is perpendicular to the Earth's surface. The horizontal beamwidth is measured in degrees and is parallel to the Earth's surface. Beamwidth is important for you to know because each type of antenna has different beamwidth specifications. The chart below can be used as a quick reference guide for beamwidths.


Free Space Path Loss
Free Space Path Loss (or just Path Loss) refers to the loss incurred by an RF signal due largely to "signal dispersion" which is a natural broadening of the wave front. The wider the wave front, the less power can be induced into the receiving antenna. As the transmitted signal traverses the atmosphere, its power level decreases at a rate inversely proportional to the distance traveled and proportional to the wavelength of the signal. The power level becomes a very important factor when considering link viability.
The Path Loss equation is one of the foundations of link budget calculations. Path Loss represents the single greatest source of loss in a wireless system. Below is the formula
for Path Loss.