Do-It-Yourself Antennas

Wireless Hacks, 2nd Edition

By Rob Flickenger, Roger Weeks

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Publisher: O'Reilly

Pub Date: November 2005

ISBN: 0-596-10144-9

Do-It-Yourself Antennas

Introduction

The price of wireless networking hardware has fallen dramatically in a very short time. Wireless adapters now come standard with many computers, and off-the-shelf access points are commonplace, making it easy for just about anyone to set up an off-the-shelf wireless network. However, the prices of antennas and related components have also fallen sharply as high demand and extreme competition have driven the industry to an increasingly high-volume, low-margin business.

These lower prices have enabled do-it-yourselfers to experiment and find out just how little it takes to build a working network. There is something almost magical about radio networking. Tales of war driving (and even war walking) aside, just imagine that today in many cities around the world, dozens of invisible networks exist on any given street corner. As you sit at a cafe eating your lunch, you might be completely unaware of the dozens of people simultaneously using the environment around you to communicate with people around the world. I believe that it is largely this mysterious, intangible aspect of unseen global communications that draws people to embark on their own antenna projects. The deeply rewarding feeling of making something useful out of virtually nothing is worth much more than saving a few dollars on an off-the-shelf network component.

When comparing antenna designs, there are a number of important factors to keep in mind. The first antenna property that people usually refer to is gain. The gain of an antenna is a measurement of how well it radiates in the direction you intend it to, measured in decibels. This measurement is actually the antenna's performance as compared to an imaginary invention called an isotropic radiator (this is the i in dBi).

Imagine an infinitely small light suspended in the vacuum of space. It radiates light equally in all directions, and by definition has no gain in any direction. Now, take this light and place it in the head of a flashlight. Without increasing the brightness of the bulb, you can turn the head of the flashlight to focus its beam in a particular direction. This is gain. By directing the energy in a particular direction, you both make the light cover a smaller area and appear to be brighter in the area it does cover. The higher the gain, the tighter and brighter the beam appears to be. Also note that antenna gain is reciprocal, meaning that it works for both transmission and reception. Adding an antenna to either end of a radio link will help performance for both ends of the link.

Another important property to keep in mind when designing or purchasing an antenna is that it must be tuned to the frequency for which you are using it. An antenna that is well matched to the radio it is connected to is said to have a low standing wave ratio (SWR). The SWR of an antenna is measured using an SWR meter or reflectometer. It is a measurement of how much energy actually leaves the antenna versus how much energy is reflected back at the radio from the antenna itself. At (legal) 802.11-power levels, a badly mismatched antenna with a high SWR simply results in poor performance. At higher power levels, a mismatched SWR can actually damage your radio or amplifier. As you'll see in the antenna designs in this chapter, the antenna is tuned by manipulating a number of factors, including the size of various active components, and their relative distance away from reflective components.

One property of antennas that is frequently overlooked by beginners is their front-to-back (F/B) ratio. This is a measurement of how much energy radiates in the expected direction (at the center of the strongest beam) versus the average amount of energy radiated in the opposite direction. A high F/B ratio means that most of the energy goes in the direction that the antenna is pointed. A low F/B ratio means that more energy is lost in the reverse direction, potentially causing unwanted interference with nearby devices. This is particularly important if you are using two or more antennas adjacent to each other, pointed in different directions. A higher F/B means that it is less likely that adjacent antennas will interfere with each other.

Finally, one last important property of antennas to keep in mind is their polarization. Briefly, this refers to the orientation of the electrical and magnetic parts of the radio wave as they leave the antenna. Polarization is discussed in greater detail in "Take Advantage of Antenna Polarization" . There is also a comparison of the various general types of antennas and their typical uses .

The hacks in this chapter describe a number of inexpensive, highly effective antenna designs that you might find useful for your own wireless networking project.

Federal Communications Commission (FCC) regulations govern the use of wireless antennas, largely to make sure they don't interfere with communications systems of emegency vehicles and keep people from inadvertently disrupting their neighbors' electronics systems. The FCC web site (http://www.fcc.gov) provides a search tool to help you find relevant information on the new FCC regulations, which allow for high-gain certification of the types of antennas covered in this chapter.

Make a Deep Dish Cylindrical Parabolic Reflector

This simple design provides high gain without pigtails or modifying your AP.

We needed a parabolic reflector to focus coverage. This design can reduce signal from some areas while enhancing signal in other areas. The reflector was designed to be installed in outdoor enclosures with WAP-11 access points, but it is becoming quite popular with people building indoor LANs, as well as with people building short point-to-point links. This design offers high performance and easy construction: scissors, tape, cardboard, tin foil, and 20 minutes, and you are in business. The completed project is shown in Figure 6-1.

This antenna is so easy to make, tune, and install, and it performs so well, that you should try one before electing to purchase a commercial antenna. One benefit is that you can cheaply check to see whether you are purchasing enough commercial antenna gain to make the link you want.

Here are some advantages over other antennas:

• No pigtail [Appendix B] required

• No modification to AP (no voiding of warranty)

• No matching (SWR) problems

• No purchased parts

• Trivially easy construction

• Very low probability of error

• As good as or better performance than the Pringles can antenna [Hack #85]

• Superior front-to-back/front-to-rear ratio

• Improves wireless LAN privacy

• Reduces interference

This design can easily complete links up to one kilometer by sitting two WAP-11s in windows at each end of a link with clean line of sight. The 6-inch version of the antenna gives you about 10 to 12 dB of gain over the stock antenna. With a WAP11, this equates to approximately 27 to 33 dB of Effective Isotropically Radiated Power (EIRP). This means you wind up with an apparent power in the favored direction between 500 mW and 2 watts.

Of course, that gain has to come from somewhere. It comes from the back side of the reflector, so power that is normally transmitted in that direction is bounced forward. That feature of this antenna can be used to enhance the privacy of your wireless network, which was my reason for designing it in the first place. The rest is just gravy (but it is real and rather tasty gravy). Figure 6-2 shows the approximate radiation pattern of a 9-inch reflector.

To build this reflector, you can use the sample template in Figure 6-3 or download the original reflector template from http://www.freeantennas.com/projects/template/parabolic.pdf. The drawing can be scaled using a copy machine to make a dish of any reasonable size. The gain computations for various sizes of the dish are also provided on the web site, as well as rough graphs showing beam width and gain/frequency. This reflector is frequency independent, meaning it works with any wireless gear, on any band.

The square drawn on the diagram will help you to ensure that your scaling does not corrupt the aspect ratio of the template. In other words, if the square is still square after you enlarge or reduce the template, you still have a good template.

Approximate radiation pattern for a 9" reflector

Reflector template

Hack 83. Make a Deep Dish Cylindrical Parabolic Reflector
Focal length varies proportionally with the size of the dish, so the focal point is also shown in the drawings. Positioning of the feed point (focal point) is the most critical aspect of a deep dish parabolic. Errors of 1/4" or more are unacceptable at these frequencies. It might help to fiddle with the positioning, as small irregularities (~1/4" or greater) will move the focal point slightly. If the dipole is not in the focal point, you will lose gain. Parabolic reflectors also lose gain if your finished reflector varies much from the correct curve.

The reflector is designed to be fed by a dipole, which is why it is not circular. A dipole is long and cylindrical, while the focal point on a circular dish is circular. The focal point on this design is a cylinder. Many access points (such as the WAP-11) use one or more dipoles as their antenna. This reflector is the optimal shape for such an antenna. Some units, such as the WET-11, do not use dipoles as their antenna. You can download a modified template for the WET-11 at http://www.freeantennas.com/projects/template/index.html.

The reflector should be made from a piece of square material to shape the curve. If you need to reduce height for packaging reasons, a shorter antenna will work but will lose roughly 3 dB for each halving of reflector height. It is also important to try to get the dipole lined up in the center of the reflector.

Front-to-back ratio is a measurement of how well a directional antenna rejects interference from directions other than the desired direction. The front-to-back ratio with this antenna depends upon the size of the wire mesh you use to make the antenna. Finer mesh yields only slightly better gain but yields much better front-to-back ratio. Modeling shows the F/B ratio to be better than ~25 dB if you use 1/4" or smaller mesh. My calculated gain figures presume the reflector is 55 percent efficient. If you use a solid sheet of aluminum or copper as your reflector, your gain figures may be a little bit higher than these. The radiation pattern is narrower in the vertical plane than the horizontal plane.

People have made good reflectors from Pringles cans, large tin cans, wire screen, aluminum sheet, and tin roofing material. Any flat metal surface or screen, such as tinfoil taped to cardboard, will work. You can build one of these in less than a half an hour using an old shoebox and a roll of tin foil.

Michael Erskine


Spider Omni Antenna

The spider omni is possibly the smallest and simplest omnidirectional antenna design around.

This is one of the simplest and smallest homemade antenna designs we've seen for 2.4 GHz. It isn't much larger than a standard N connector because that is exactly what it is made of. It has been dubbed the spider omni, because it looks a bit like a crazed spider crawling up your antenna feed, as shown in Figure 6-4. Technically, it is a ground plane antenna, but practically speaking, it acts like a vertically polarized 3 dB omni.

Figure 6-4. The spider omni

The spider is simple to construct, if you have a good soldering iron and some basic tools. You need a standard N connector and about a foot or so of solid copper 12-2 romex (common 12-gauge electrical wiring). You'll also need a good vice to hold onto the pieces as you solder them, as well as a pair of needle-nose pliers, some good solder, and a bottle of flux.

First, cut five pieces of bare copper romex, each about 3 cm long. Straighten out each piece as best as you can. Using needle nose pliers, make a small 180-degree bend on one end of four of the pieces. Now, tin the bent tip of each piece, as well as one end of the remaining straight piece. This will make your soldering job much easier later.

If you don't know what tinning is, you might want to get the help of a friend who has experience with soldering. To tin means to cover the end of a piece of wire with solder before actually soldering it to your project. This helps the solder to flow better, and ultimately makes a better bond between the metal surfaces.

Next, solder the straight piece to the gold cup on your N connector. Don't use too much solder; there should be just enough to fill the cup without overflowing. Prepare to solder the four legs directly onto the N connector's chassis. You need to use a lot of heat, and liquid flux will help the solder to flow better and bond to the body of the connector. I found it easiest to clamp the straight piece of wire, rather than the threaded bottom of the N connector. This helps to keep the heat from dissipating into your vise while you solder to the chassis.

Take your time, and don't use too much solder on the legs. When you are finished, let the whole thing cool for several minutes, as the chassis will be quite hot.

Now, trim all of the leads to about 20 mm past the edge of the housing. Trim the center lead to about 20 mm past the end of the gold cup. Bend the four radials connected to the housing down at a slight angle. Physically mounting the omni is straightforward if you use heavy feed line, such as LMR 400. Mount the antenna with the center lead pointing up.

The spider omni doesn't provide a tremendous amount of gain, about 3 dB or so, as far as we can tell from informal tests, but it does work quite well for what it is. Higher gain antennas are certainly possible, but they tend to be more complicated and much larger. For many applications, you just can't beat the size and cost of this tiny little antenna.

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