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27-07-2005, 04:00 PM
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    #1  

Antenna Concepts and Building a "Coffee Can" Antenna


The following is an complete tutorial from the book "

Wireless Hacking: Projects for Wi-fi Enthusiasts" by Syngress Publishing

Whether your wireless system is a simple home office setup or a large-scale outdoor wireless network* the antenna system is the most important* but often most overlooked* aspect of the system. The antenna system is often the “make or break” factor for a successful wireless transmission. Poor antenna selection or design can lead to frustration and intermittent connectivity problems. This translates into poor throughput performance and frustrated wireless users.

In this article* we explore the important issues surrounding antenna selection for any 2.4 or 5 GHz unlicensed wireless system. You’ll get all the information you need to achieve the best performance possible. We’ll examine many types of commercially available antennas* and you’ll see how you can build your own antenna as an alternative* using inexpensive materials from the local hardware store.
Topics in this Article:
  • Before You Start: Basic Concepts and Definitions
  • Building a Waveguide “Coffee Can” Antenna
  • The Future of Antennas
At the conclusion of this article* you should:
  • Understand the different decibel (dB) measurements and how to use them correctlych
  • Understand the Federal Communications Commission (FCC) rules and power output requirements for operation in the 2.4 and 5 GHz bands
  • Understand the different types of antennas and use Figures of Merit (FOMs) to determine the best choice
  • Understand the “3 dB” rule and its importance in determining system performance
  • Understand “wavelengths” and use a formula to determine how long an antenna needs to be for a given frequency
  • Determine system performance requirements using the link budget calculation
  • Estimate the “fresnel zone” to determine antenna height and orientation
  • Understand how different building materials can affect performance
  • Be aware of safety precautions such as lightning arrestors* proper grounding techniques* and building code adherence
Before You Start: Basic Concepts and Definitions

Before you can install and/or construct any antenna* there are several terms and calculations with which you should be familiar. While a degree in physics is not necessary* a basic understanding of physics is helpful.

An antenna is simply a passive transducer that radiates energy (gain) into space. Antennas do not actually amplify the signal; they simply change the shape of the energy pattern being radiated. You should be able to select or construct a basic antenna for your use once you understand the basics of antenna design* construction* and operation.

The decibel is the most important unit of measurement when looking at antenna performance. The decibel (or dB) is the basic unit used for radio frequency (RF) power measurement. Table 10.1 lists decibel power levels in relation to wattage levels.

Table 10.1 Transmit Power in Decibels

Watts Decibels

1/1000 0 dB

1/100 1 0dB

1/10 20 dB

¼ 24 dB

½ 27 dB

1 30 dB

2 33 dB

5 37 dB

We use decibel measurements because signal strengths vary logarithmically* not linearly. A logarithmic scale allows simple numbers to represent large variations in signal levels. You’ll see it’s also very useful in calculating system gains and losses. In the following sections* we’ve included brief definitions of all the terms we’ll be using in this article:
  • dB Decibel. The basic unit of measurement that represents the ratio of two signal levels.
  • dBm/dBW Decibel milliWatt. This measurement is used to represent power* with 0 dBm defined as 1 milliWatt. For larger signals* there is also dBW* a reference to 1W. Small signals are represented as negative numbers* (for example* –95 dBm). When referencing commercial Wi-Fi devices* power output is normally given in dBm. Many WLAN PCMCIA cards and some Access Points (APs) have a power output of +17dBm (50mW). There is also usually a Receive Signal Sensitivity Indicator (RSSI) measurement listed in dBm (for example* –95dBm).
  • dBd Decibel dipole. The output power (gain) an antenna has over a dipole antenna at the same frequency. A dipole (two-pole) antenna is a ½ wave antenna used as a reference against all other antennas. It is a reference known as 0 dBd (zero decibel referenced to dipole). The dBd measurement is usually used only with antennas below 1 GHz.
  • dBi Decibel isotropic. This measurement is used for antennas above 1 GHz .A dipole antenna has 2.14 dB higher gain than the 0 dBi dipole reference. If antenna gain is given in dBd* not dBi* add 2.14 to convert to the dBi rating.
Need to Know…RF Power

There are several basic rules that you should know when working with antennas* RF power* and expected signal strength. The “3 dB” rule is perhaps the most important rule when dealing with RF (signal) power. It states that for every 3 dB increase in level* the power is doubled. For every 3 dB decrease* the power is cut in half. Similarly* every 10 dB increase in level is 10 times the power* and every 10 dB decrease in level results in 1/10 the power. This is sometimes referred to as the “rule of 3s and 10s.”

Once you understand the different decibel measurements* it is easy to understand Figures of Merit (FoMs) when working with antennas. FOMs are attributes that describe an antenna’s performance characteristics. The FoMs are listed as part of every antenna’s specifications. Important FoM attributes like gain and front-to-back ratio are listed in dB or dBm. There are many other RF terms and figures that use decibel reference and values (these terms are explained in greater detail later in this article). Once you are familiar with FoMs in general* it will be easy to recognize the important features of antennas and choose the best antenna for your application.

Effective Isotropic Radiated Power (EIRP) is defined as the power found in the main lobe of the antenna relative to an Isotropic radiator with 0 dB of gain. The EIRP is calculated by taking the antenna gain (in dBi) plus the power (in dBm) inbound from the transmitter. For example* a 9dBi antenna fed with 26 dBm of power would have an EIRP of 35 dBm.

9 dBi + 26 dBm = 35 dBm (3.2W)

The chart on the left in Figure 10.1* known as a Smith chart* shows the propagation area of a Yagi antenna (image on the right of the figure). A Smith chart is included with any antenna specification and represents the radiation pattern of the antenna. It also shows the front-to-back ratio* and the “side lobes*” which are the smaller* less powerful radiation patterns on each side of the main lobe.

Figure 10.1 Representation of a Unidirectional Yagi Antenna Radiation Pattern

Antenna Concepts Building "Coffee Can"

The top pattern represents the main lobe and transmit gain. The lower pattern the back lobe. The difference (in dB) between the front and back lobe is called the front-to-back ratio.

A Word about Antenna Gain and Coverage

Since the EIRP is in the main antenna lobe only* antenna selection is critical.

When using a high-gain omni antenna (8–12 dBi)* the propagation angle is very flat and narrow. Placing the antenna too high will cause the main lobe to pass over the intended target antenna. The irony here is that height is required to clear obstructions* a.k.a. Line-of-Sight* from the Wireless Point of Presence (WiPoP) path to the receivers. Higher gain omni antennas have a flatter* “pancake” shape* while lower gain omni antennas tend to have a wider “donut” shaped pattern.

It may be necessary to use a unidirectional antenna and “down tilt” that concentrates the energy (signal) in a more focused area. Unidirectional antennas direct energy in one direction by radiating the entire signal in a concentrated area instead of 360 degrees like an omni. Table 10.2 lists antenna types and associated values in dBi (gain). Figures 10.2 through 10.6 are images of these antenna types.

Table 10.2 Typical Antenna Types and Gain Values for Off-the-Shelf Antennas

Antenna Type Gain (dBi as we’re dealing with >1GHz) Freq.

Unity gain Omni 0 dBi

Low Gain Omni 2–6 dBi

High Gain Omni 8–12 dBi

4 x 6″ Panel (Unidirectional) 7 dBi

Small Yagi 10 dBi

8″– 10″ Panel (Uni) 13 dBi

12″ Panel (Uni) 16 dBi

Long Yagi 16 dBi

18″ Parabolic Dish 19 dBi

18″ Diagonal Mesh/Grid Antenna 21 dBi

24″ Diagonal Mesh/Grid Antenna 24 dBi

Figure 10.2 8 dBi Omni

Antenna Concepts Building "Coffee Can"

Figure 10.3 8 dBi Uni (Panel)

Antenna Concepts Building "Coffee Can"

Figure 10.4 Large Omni

Antenna Concepts Building "Coffee Can"

Figure 10.5 24″ x 36″ Mesh Grid Antenna (21 dBi)

Antenna Concepts Building "Coffee Can"

Figure 10.6 Yagi (12 dBi)

Antenna Concepts Building "Coffee Can"

Note… Interesting Antenna

An interesting antenna type has been developed by cantenna.com. This “super cantenna” resembles a Pringles can antenna* is linearly polarized* and features a gain of 12 dBi and a beam width of 30 degrees. You can learn more about this innovative* low-cost product at

www.cantenna.com.

Antenna Concepts Building "Coffee Can"

Federal Communications Commission

A common misconception when using ‘unlicensed’ equipment is that there are no rules covering the operation of such equipment. While there are no license requirements* the FCC does have some regulations with respect to the maximum power output levels when using unlicensed equipment. Part 15 of the FCC’s rules for radio equipment lists the specific power requirements. We discuss the pertinent limitations in this section.

The FCC has relaxed the rules on EIRP limits for Point-to-Point (PtP) systems. This has increased the choices of antennas and extended the range of PtP systems. The EIRP for a 2.4–2.5 GHz PtP system is now 36dBm (an amazing 4 watts!) We must calculate a link budget to determine the total EIRP* and remain in FCC compliance. The FCC allows only 30 dBm (1W) EIRP for Point-to-Multipoint (PtMP) communications. This limits the antenna choices and makes the calculation of system output very important. However* for most off-the-shelf commercial equipment using attached antennas* the output is 50–200 mW. This coupled with a 6 dBi antenna is well below FCC limits. Using the previous charts and remembering the rules will help you calculate power levels and remain in compliance. A good rule to remember for 2.4 GHz PtP systems is that for every 3 dBi of antenna gain over 6 dBi* the transmitter power output must be reduced by 1 dB. For 2.4 GHz PtMP* every 3 dBi of antenna gain over 6 dBi must be met with a 3 dB reduction in transmitter power.

The 5 GHz band has various output power limits. The limits depend upon the sub-band within the 5 GHz band in which you’re operating. The lower portions of the 5 GHz unlicensed band are between 5.15 and 5.25 GHz The output for these devices is fixed at a maximum of 50 mW. The 5.25–5.35 GHz middle sub-band has a power limit of 250 mW.

The 5.725–5.825 GHz upper band is normally used for high bandwidth (T-1 > OC-3) transmissions associated with microwave radio. This band has most recently been adopted by many Wireless Internet Service Providers (WISPs) as a high data rate “backhaul” solution. This removes congestion from the 2.4 GHz (DSSS) frequency band and allows much more bandwidth (users) to be concentrated for transmission.

The Link Budget is the calculation of the losses and gains (in dB) for the complete RF system* and is determined using a simple formula that combines all the power and gain figures for both sides of a link.

Link Budget = P(t) + TX(G) + Rx(G) + Rx - Path Loss

Where:
  • P(t) = power of transmitter (e.g.* 17 dBm)
  • TX(G) = transmit antenna gain (e.g.* 6 dBi)





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27-07-2005, 08:11 PM
saud alali غير متصل
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    #2  
والله اخي الكريم
ذكرتني ايام زمان تقريبا منذ 8 سنوات عندما كنت ادرس تخصص راديو وتلفزيون
و لكن الآن التخصص غير و الوظيفة غير


 


Antenna Concepts and Building a "Coffee Can" Antenna

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