Cisco AIR-ANT1728, AIR-ANT5959, AIR-ANT4941, AIR-ANT3549, AIR-ANT1729 Reference Manual

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Cisco Aironet Antennas

Overview

Executive Overview

This antenna reference guide is intended to provide information to assist in understanding the issues and concerns ofantennasusedwithaCiscoAironet Bridge system. It details deployment and design, limitations, and capabilities, and basic theories of antennas. This document also contains information on the Cisco antennas and accessories as well as installation scenarios, regulatory information, and technical speciﬁcations and diagrams of the available antennas.

Overview of Antennas

Each Cisco Aironet radio product is designed to perform in a variety of environments. Implementingtheantennasystemcan greatly improvecoverage and performance.Tooptimize the overall performance of a Cisco wireless LAN, it is important to understand how to maximize radio coverage with the appropriate antenna selection and placement. An antenna system comprisesnumerous components,including theantenna, mountinghardware, connectors,antenna cabling,and in some cases, alightning arrestor.For additionalconsultation, please contact your authorized Cisco Aironet partner. Cisco Partners can provide onsite engineering assistance for complex requirements.

Reference Guide

WirelessLANsystem,orWireless

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Radio Technologies

In the mid 1980s, the FCC modiﬁed Part 15 of the radio spectrum regulation, which governs unlicensed devices. The modiﬁcation-authorized wireless network products to operate in the Industrial, Scientiﬁc, and Medical (ISM) bands using spread spectrum modulation. This type of modulation had formerly been classiﬁed and permitted only in military products. The ISM frequencies are in three different bands, located at 900 MHz, 2.4 GHz, and 5 GHz. This document covers only the

2.4GHz band. The ISM bands typically allow users to operate wireless products without requiring speciﬁc licenses, but this will vary in some

countries. In the U.S., there is no requirement for FCC licenses. The products themselves must meet certain requirements to be certiﬁed for sale, such as operation under 1 watt transmitter output power (in the U.S.) and maximum antenna gain or EIRP ratings.

Radio Frequency Fundamentals

Radio Frequency (RF) engineering is a very complex ﬁeld of study and far too involved to explain in detail here.

2.4GHz Spectrum

The Cisco Aironet 350 Series uses RF spectrum in the 2.4 GHz unlicensed ISM band. In the U.S., three bands are deﬁned as unlicensed and known as the ISM bands (Industrial, Scientiﬁc, and Medical). The ISM bands are as follows:

• 900 MHz (902-928MHz)

• 2.4 GHz (2.4 - 2.4835 GHz)—IEEE 802.11b

• 5 GHz (5.15-5.35 and 5.725-5.825 GHz)—IEEE 802.11a, HIPERLAN/1 and HIPERLAN/2. This band is also known as the UNII band.

Each range has different characteristics. The lower frequencies exhibit better range, but with limited bandwidth and hence lower data rates. The higher frequencies have less range and subject to greater attenuation from solid objects.

Direct Sequence Spread Spectrum

The Direct Sequence (DS) Spread Spectrum approach involves encoding redundant information into the RF signal. Every data bit is expanded to a string of chips called a chipping sequence or Barker sequence. The chipping rate as mandated by the U.S. FCC is 10 chips at the 1 and 2 Mbps rates and 8 chips at the 11 Mbps rate. So, at 11 Mbps, 8 bits are transmitted for every one bit of data. The chipping sequence is transmitted in parallel across the spread spectrum frequency channel.

Frequency Hopping Spread Spectrum

Frequency Hopping (FH) Spread Spectrum uses a radio that moves or hops from one frequency to another at predetermined times and channels. The regulations require that the maximum time spent on any one channel is 400mS. For the1- and 2-Mb FH systems, the hopping pattern must include 75 different channels, and must use every channel before reusing any one. For the Wide Band Frequency Hopping (WBFH) systems, that permit up to 10-Mb data rates, the rules require use of at least 15 channels, and they cannot overlap. With only 83MHz of spectrum, it limits the systems to 15 channels, thereby causing scalability issues.

In every case, for the same transmitter power and antennas, a DS system will have greater range, scalability and throughput than an FH system. For this reason Cisco has chosen to support only DS systems in the Spread Spectrum products.

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Antenna Properties and Ratings

An antenna gives the wireless system three fundamental properties—gain, direction, and polarization. Gain is a measure of increase in power. Direction is the shape of the transmission pattern. The good analogy for an antenna is the reﬂector in a ﬂashlight. The reﬂector concentrates and intensiﬁes the light beam in a particular direction similar to what a parabolic dish antenna would to a RF source in a radio system.

Antennas are rated in comparison to isotropic or dipole antennas. An isotropic antenna is a theoretical antenna with a uniform three-dimensional radiation pattern (similar to a light bulb with no reﬂector). dBi is used to compare the power level of a given antenna to the theoretical isotropic antenna. The FCC in the U.S. uses dBi in its calculations. An isotropic antenna is said to have a power rating of 0 dB, for example, zero gain/loss when compared to itself.

Unlike isotropic antennas, dipole antennas are real antennas (dipole antennas are standard on Aironet Access Points, Base Stations and WorkgroupBridges). Dipole antennas have a different radiation pattern compared to isotropic antennas. The dipole radiation patternis 360 degreesin the horizontalplane and 75degrees in the vertical plane (assuming the dipole antenna is standing vertically) and resembles a donut in shape. Because the beam is “slightly” concentrated, dipole antennas have a gain over isotropic antennas of 2.14 dB in the horizontal plane. Dipole antennas are said to have a gain of 2.14 dBi (in comparison to an isotropic antenna).

Some antennas are rated in comparison to dipole antennas. This is denoted by the sufﬁx dBd. Hence, dipole antennas have a gain of 0 dBd (= 2.14 dBi).

Note that the majority of documentation refers to dipole antennas as having a gain of 2.2 dBi. The actual ﬁgure is 2.14 dBi, but is often rounded up.

Type of Antennas

Ciscooffers several different styles of antennas foruse in the 2.4GHz ranges. Every antenna offered for salehas been FCC approved. Each type of antenna will offer different coverage capabilities. As the gain of an antenna increases, there is some tradeoff to its coverage area. Usually gain antennas offer longer coverage distances, but only in a certain direction. The radiation patterns below will help to show the coverage areas of the styles of antennas that Cisco offers, omni-directional, yagis and patch antennas.

Omni-Directional Antennas

An omni-directional antenna is designed to provide a 360 degree radiation pattern. This type of antenna is used when coverage in all directions from the antenna is required. The standard 2.14 dBi “Rubber Duck” is one style of an omni-directional antenna.

Figure 1 Omni-Directional Antenna

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Directional Antennas

Directionalantennas come in many differentstyles and shapes. An antennadoes not offerany added power to thesignal, and instead simply redirects the energy it received from the transmitter. By redirecting this energy, it has the effect of providing more energy in one direction, and less energy in all other directions. As the gain of a directional antenna increases, the angle of radiation usually decreases, providing a greater coverage distance, but with a reduced coverage angle. Directional antennas include yagis, patch antennas, and parabolic dishes. Parabolic dishes have a very narrow RF energy path and the installer must be accurate in aiming these at each other.

Figure 2 Directional Patch Antenna

Figure 3 YAGI Antenna

28 – 80 degrees at 2.4 GHz

68 – 78 degrees at 900 MHz

Directional Yagi

Diversity Antenna Systems

Diversity antenna systems are used to overcome a phenomenon known as multipath distortion of multipath fading. It uses two identical antennas, located a small distance apart, to provide coverage to the same physical area.

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Multipath Distortion

Multipath interference occurs when an RF signal has more than one path between a receiver and a transmitter. This occurs in sites that have a large amount of metallic or other RF reﬂective surfaces.

Just as light and sound bounce off of objects, so does RF. This means there can be more than one path that RF takes when going from a TX to and RX antenna. These multiple signals combine in the RX antenna and receiver to cause distortion of the signal.

Multipath interference can cause the RF energy of an antenna to be very high, but the data would be unrecoverable. Changing the type of antenna, and location of the antenna can eliminate multipath interference.

Figure 4 Multipath Distortion

Ceiling

Received Signals

TX RX

Time

Combined Results

Obstruction

Floor

You can relate this to a common occurrence in your car. As you pull up to a stop, you may notice static on the radio. But as you move forward a few inches or feet, the station starts to come in more clearly.By rolling forward, you move the antenna slightly, out of the point where the multiple signals converge.

A diversity antenna system can be compared to a switch that selects one antenna or another,never both at the same time. The radio in receive mode will continually switch between antennas listening for a valid radio packet. After the beginning sync of a valid packet is heard, the radio will evaluate the sync signal of the packet, on one antenna, then switch to the other antenna and evaluate. Then the radio will select the best antenna, and use only that antenna for the remaining portion of that packet.

On transmit, the radio will select the same antenna it used the last time it communicated to that given radio. If a packet fails, it will switch to the other antenna and retry the packet.

One caution with diversity, it is not designed for using two antennas, covering two different coverage cells. The problem in using it this way, is that if antenna #1 is communicating to device #1, while device #2 (which is in the antenna #2 cell) tries to communicate, antenna #2 is not connected (due to the position of the switch), and the communication fails. Diversity antennas should cover the same area, from only a slightly different location.

Time

With the introduction of the latest DS physical layer chips, and the use of diversity antenna systems, DS systems have equaled or surpassed FH in areas of how Multipath interference is handled. While the introduction of WBFH does increase the bandwidth of theFH systems, it drastically effects the ability tohandle multipath issues,further reducing its range compared to present DSsystems in high RF reﬂective sites.

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Wireless LAN Design

Before the physical environment is examined, it is critical to identify the mobility of the application, the means for coverage and system redundancy. An application such as point-to-point which connects two or more stationary users may be best served by a directional antenna, while mobile users will generally require a number of omni-directional micro cells. These individual micro cells can be linked together seamlessly via the wired LAN infrastructure or by using the wireless repeater functionality built into every Cisco Aironet Access Point. All Cisco Aironet Wireless LAN products are designed to support complex multicell environments transparently through the Cisco patented MicroCellular Architecture.

The Physical Environment

Once mobility issues are resolved, the physical environment must be examined. While the area of coverage is the most important determining factor for antenna selection, it is not the sole decision criteria. Building construction, ceiling height, and internal obstructions, available mounting locations and customers aesthetic desires must be considered. Cement and steel construction have different radio propagation characteristics. Internal obstructions such as product inventory and racking in warehousing environments are factors. In outdoor environments, many objects can affect antenna patterns, such as trees, vehicles, buildings, and trains, to name just a few.

The Network Connections

The access points use a 10/100 Mb Ethernet connection. Typically the access point is in the same location as the antenna. While it may seem that the best place to put the access point is in a wiring closet with the other networks components such as switches, hubs and routers, this is not the case. Placement of the antenna in an area that provides the best coverage (determined by a site survey) is required. Therefore many people new to wireless LANs want to locate the access points in the wiring closet and connect the antenna via RF coax. Antenna cable introduces losses in the antenna system on both the transmitter and the receiver. As the length of cable increases, so does the amount of loss introduced. To operate at optimum efﬁciency, cable runs should be kept as short as possible. (See the section on cabling later in this document).

Building Construction

The density of the materials used in a building’s construction determines the number of walls the RF signal can pass through and still maintain adequate coverage. Here are just a few examples. Actual effect on the RF must be tested at the site, and therefore a site survey is suggested.

Paper and vinyl walls have very little affect on signal penetration. Solid walls and ﬂoors and pre-cast concrete walls can limit signal penetration to one or two walls without degrading coverage. This may vary widely based any steel reinforcing within the concrete. Concreteand concrete block walls may limit signal penetration to three or four walls. Wood or drywall typically allows for adequate penetrationof ﬁve or six walls. A thick metal wall causes signalsto reﬂect off,resulting in poorpenetration. Steel reinforced concrete ﬂooring will restrict coverage between ﬂoors to perhaps one or two ﬂoors.

Recommendations for some common installation environments are outlined below:

• Warehousing/Manufacturing—In most cases, these installations require a large coverage area. Experience has shown that an omni-directional antenna mounted at about 20 to 25 feet, typically provides the best overall coverage. Of course this will also depend upon the height of the racking, material on the rack and ability to locate the antenna at this height. Mounting the antenna higher will sometimes actually reduce coverage as the angle of radiation from the antenna is more outward than down. The antenna should be placed in the center of the desired coverage cell and in an open area for best performance. In cases where the radio unit will be located against a wall, a directional antenna such as a patch or yagi can be used for better penetration of the area. The coverage angle of the antenna will affect the coverage area.

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• Small Ofﬁce/Small Retail—The standard dipole may provide adequate coverage in these areas depending upon the location of the radio device. However, in a back corner ofﬁce a patch antenna may provide better coverage. It can be mounted to the wall above most obstructions for best performance. Coverage of this type antenna depends upon the surrounding environment.

• Enterprise/ Large Retail—In most cases, these installations require a large coverage area. Experience has shown that omni-directional antennas mounted just below the ceiling girders or just below the drop ceiling typically provide the best coverage (this will vary with stocking, type of material and building construction). The antenna should be placed in the center of the desired coverage cell and in an open area for best performance. In cases where the radio unit will be located in a corner, or off to one end of the building a directional antenna such as a patch or yagi can be used for better penetration of the area. Also for areas that are long and narrow, such as long rows of racking, a directional antenna, used at one end may provide better coverage. The radiation angle of the antennas will also affect the coverage area.

• Point-to-Point—When connecting two points together (such as an Ethernet bridge), the distance, obstructions, and antenna location must be considered. If the antennas can be mounted indoors and the distance is very short (several hundred feet), the standard dipole or mast mount 5.2 dBi omni-directional may be used. An alternative is to use two patch antennas. For very long distances (1/2 mi. or more) directional high gain antennas must be used. These antennas should be installed as high as possible, and above obstructions such as trees, buildings, and so on; and if the directional antennas are used, they must be aligned so that their main radiated power lobes are directed at each other. With a line-of-site conﬁguration, distances of up to 25 miles at

2.4GHz can be reached using parabolic dish antennas, if a clear line-of-site is maintained. With the use of directional antennas, you are subject to fewer interference possibilities, and less likely to cause interference to anyone else.

• Point-to-Multipoint Bridge—In this case (in which a single point is communicating to several remote points) the use of an omni-directional antenna at the main communication point must be considered. The remote sites can use a directional antenna that is directed at the main point antenna.

Cabling

As stated above, cabling introduces losses into the system, negating some of the gain an antenna introduces, and reducing range of the RF coverage.

Interconnect Cable

Attached to all antennas (except the standard dipoles), this cable provides a 50 Ohm impedance to the radio and antenna, with a ﬂexible connection between the two items. It has a high loss factor and should not be used except forvery short connections (usually less than 10 feet). Typical length on all antennas is 36" (or 12" on some outdoor antennas).

Low Loss/Ultra Low Loss Cable

This cable provides a much lower loss factor than the interconnect cable, and it is used when the antenna must be placed at any distance from the radio device. While it is a low loss cable, it should still be kept to a minimum length. This cable is the only cable type supplied by Cisco for mounting the antenna away from the radio unit. It is offered in four different lengths with one RTNC plug and one RTNC jack connector attached. This allows for connection to the radio unit and to the interconnect cable supplied on the antennas.

Connectors

Connectors used on equipment manufactured after June 1994 must be unique, nonstandard connectors (per FCC and DOC regulations).Therefore, Cisco Aironet products use a connector known as Reverse-TNC (RTNC)connectors. Whilethey are similar to the normal TNC connectors, they cannot be mated to the standard connectors. To ensure compatibility with Cisco Aironet products, use antennas and cabling from Cisco.

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Mounting Hardware

Each antenna requires some type of mounting. The standard dipole antenna simply connects to the RTNC connector on the back of the unit. The 5.2 dBi mast mount omni and the yagi antennas are designed to mount to a mast up to 1.5 inches, and each comes with mounting hardware for attachment. The 13.5 dBi yagi has an articulating mount option listed in the back of this document. For most indoor applications, a .75" or 1" electrical conduit provides a suitable mounting. For outdoor application, a heavy galvanized or aluminum wall mast should be used that will withstand the wind loading rating of the selected antenna. Patch antennas are designed to mount ﬂat against a wall orceiling. Ceiling mount antennas are equipped with a drop ceiling cross member attachment. The 21 dBi Parabolic Dish mounts to a 1.625" to a 2.375" mast. Fine threaded turnbuckles allow accurate aiming of the antenna.

Lightning Arrestors

When using outdoor antenna installations, it is always possible that an antenna will suffer damage from potential charges developing on the antenna and cable, or surges induced from nearby lightning strikes. The Aironet Lightning Arrestor is designed to protect radio equipment from static electricity and lightning induced surges that travel on coaxial transmission lines. It protects equipment from surges up to 5,000 Amperes. This will not prevent damage in the event of a direct lightning hit.

Theory of Operation

The Cisco Aironet Lightning Arrestor prevents energy surges from reaching the RF equipment by the shunting effect of the device. Surges are limited to less than 50 volts, in about .0000001 seconds (100 nano seconds). A typical lightning surge is about .000002 (2 micro seconds).

Figure 5 Cisco Aironet Lightning Arrester

Lug

Lockwasher

Nut

To Antenna

Ground Wire

To RF Device

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The accepted IEEE transient (surge) suppression is .000008 seconds (8 micro seconds). The Lightning Arrestor is a 50 ohm transmission line with a gas discharge tube positioned between the center conductor and ground. This gas discharge tube changes from an open circuit to a short circuit almost instantaneously in the presence of voltage and energy surges, providing a path to ground for the energy surge.

Installation

This Arrestor is designed to be installed between your outdoor antenna cable and the Aironet Wireless Device. Installation should be indoors, or inside a protected area. A good ground must be attached to the Arrestor. This can be accomplished by use of a ground lug attached to the Arrestor, and a heavy wire (#6 solid copper) connecting the lug to a good earth ground. See Figure 5.

Understanding RF Power Values

Radio Frequency signals are subject to various losses and gains as they pass from transmitter through cable to antenna, through air (or solid obstruction), to receiving antenna, cable and receiving radio. With the exceptionof solid obstructions, most of these ﬁgures and factors are known and can be used in the design process to determine whether an RF system such as a WLAN will work.

Decibels

The Decibel (dB) scale is a logarithmic scale used to denote the ratio of one power value to another—for example:

• dB = 10 log10 (Power A/Power B)

An increase of 3 dB indicates a doubling (2x) of power. An increase of 6 dB indicates a quadrupling (4x) of power. Conversely, a decrease of 3 dB is a halving (1/2) of power, and a decrease of 6 dB is a quarter (1/4) the power. Some examples are shown below in Table 1.

Table 1 Decibel Values and Corresponding Factors

Increase Factor Decrease Factor

0 dB 1 x (same) 0 dB 1 x (same) 1 dB 1.25 x -1 dB 0.8 x 3 dB 2 x -3 dB 0.5 x 6 dB 4 x -6 dB 0.25 x 10 dB 10 x -10 dB 0.10 x 12 dB 16 x -12 dB 0.06 x 20 dB 100 x -20 dB 0.01 x 30 dB 1000 x -30 dB 0.001 x 40 dB 10,000 x -40 dB 0.0001 x

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Cisco AIR-ANT1728, AIR-ANT5959, AIR-ANT4941, AIR-ANT3549, AIR-ANT1729 Reference Manual

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