Building a Directional Tin Can Antenna
In Antenna Cble category, you learned how to make a Wi-Fi antenna cable that can be used to connect an external antenna to your Wi-Fi card or access point. It is now time to build an antenna and put the cable from Chapter 1 to good use.While there are many commercial antennas available on the market today, they can be expensive. And hey, let’s face it, attaching a commercial antenna to your Wi-Fi network will not turn heads like making your own will.
There are several different types of antennas that you can build. The most famous Wi-Fi antennas are made from either a coffee can or a Pringles potato chip can. In this chapter, you will learn how to build your own antenna from a regular, metal coffee can. You will be able to build it quickly and cheaply. As an added bonus, you will have lots of coffee, which will come in handy in staying awake for the other projects in this book.
Here are the items you will need for this chapter’s project:
➤The coaxial cable you built in Chapter 1
➤Metal can about 4 inches in diameter and 51⁄2 inches long (100 mm–135 mm)
➤Type N-Connector
➤Long-nosed pliers
➤Small wire cutters
➤Single-sided razor blade
➤Scissors
➤Hammer
➤Drill
➤Soldering iron and solder
➤Copper embossing material (optional)
Types of Can Antennas
There are two popular types of homebrew Wi-Fi can antennas, the Pringles can antenna and the tin can antenna. They both have the same means to an end—increase signal strength in one direction—but they differ radically in operation and construction. The Pringles can antenna is actually a Yagi antenna with a Pringles can covering used to mount the antenna components. You may recall from Chapter 2 that a Yagi antenna uses a single element as a radiator, with additional metallic elements. A single reflector element and multiple director elements help to shape the beam into a directional pattern.
In fact, the Pringles can isn’t really a can, it’s just a cylindrical cardboard container. Figure 3-1 shows the internal components of the Pringles can antenna. The primary components are the radiator and the beam-shaping elements. All other components serve to hold the antenna together in the correct position for best efficiency.
While the Pringles can is merely a shell, the tin can is the actual antenna on a tin can antenna. This is because the tin can antenna is a “waveguide” antenna (see Figure 3-2). That is, the size, shape, and electrical conductivity of the tin can act upon the radio frequency signals.When you place a small radiator in the right location, the dimensions of the can itself will shape the beam and light up the sky.
A waveguide is a type of radio frequency (RF) transmission path. Where low-frequency systems can use copper wires, like that used in your car radio, high-frequency RF will sometimes use waveguides to route high-power, high-frequency signals. Military radar systems often use waveguide transmission lines.
Understanding Waveguides
A waveguide is a type of transmission line, like coaxial cable (see Figure 3-3). But, unlike coaxial cables, waveguides can carry microwave frequencies with almost no loss. RF energy as high as 60 GHz or higher travel easily through a waveguide conduit. A waveguide is constructed from metal in a very specific size and shape, usually rectangular. It is also very costly to manufacture, install, and can be difficult to maintain. Because it’s made from metal, and must be of exact dimensions, waveguide transmission lines are very rigid.
Waveguides exploit a very interesting aspect of electromagnetic RF energy: The duality of electromagnetism. Electromagnetic energy is composed of an electric field and a magnetic field (hence the name). In a coaxial wire, these fields are present along the center conductor and reflected from the outer shield. In a waveguide, these two fields travel along the waveguide
without the need for a center conductor. The inner surface of the waveguide essentially directs the signal through the empty space of the interior itself. Waveguide theory breaks apart all of the elements of radio frequency transmission. The details are quite complicated and can fill volumes. For this book, the important thing to note about waveguides is that size and shape of the waveguide itself is important, and placement of the radiator inside the waveguide is important.
Constructing a waveguide transmission line is difficult.To use a waveguide antenna, however, is a snap. You only need a short portion of the waveguide path to make an antenna. And Wi-Fi frequencies dictate a size and shape that is easily available at any grocery store.
Sizing a Waveguide Antenna
As you know, a waveguide needs to be of specific dimensions. The waveguide antenna, therefore, must be the correct size for the frequency you are working with. In this case, you are working with Wi-Fi operating in the 2.4 GHz band. Let’s size this antenna for the middle of the band at channel 6, which has a frequency of 2.437 GHz (see the frequencies in Table 1-2 in Chapter 1).With proper construction, this antenna should operate well across all Wi-Fi frequencies from channels 1 to 14.
To ensure a can that’s sized well, it should follow the dimensions shown in Figure 3-4.
The dimensions for the can antenna built in this chapter are:
Diameter: ideally 100 mm plus or minus 10 percent (90–110 mm)
Length: about 123 mm or a full wavelength, plus or minus 10 percent
Wedge-shaped radiating element: 24 mm (about 1/5 of a wavelength)
Radiator offset: 27 mm (about 7/32 of a wavelength)
To calculate wavelength, use the formula wavelength in millimeters = 300 divided by the frequency in gigahertz. So, the wavelength for channel 6 is 300 / 2.437 = 123 mm.
Finding the Right Can
For this project, you can use just about any smallish coffee can. There are a couple of things you have to keep in mind: it has to be a metal can and it should be close to the dimensions noted in the previous section. Remarkably, Maxwell House and Folgers Coffee cans are the exact dimensions needed for this project. The ounces (or grams) measurements vary somewhat from 11.5 oz. (368 g) to 13 oz. (326 g) because they are measuring weight. But the can dimensions are identical for these two brands and probably many more.
To find the right can, go to your local grocery store with a measuring tape or ruler and measure the cans on the shelf. The store personnel might look at you funny. Just tell them you are buying some coffee to make your Internet access go further.
In choosing your coffee, remember that one can make many cups of coffee to drink. You really have two choices; you can dump the coffee or keep a pot of coffee ready at all times for the other projects in this book. For this chapter, we used an 11.5 oz. Maxwell House coffee can.
Preparing the Can
It’s time to get the can ready to be converted into a directional antenna. You can do this in two steps: preparation and cleaning.
Step 1: Preparing the Can Opening
The coffee can you purchased probably has a plastic cover on the top and a metal cover protecting the coffee freshness. Remove the plastic lid and put it aside; you will use it later. You will also want to make sure that the can itself is intact with no indentations. Most cans will have ridges around the circumference of the can which are okay; you just want to make sure that it has not been dropped or mishandled. These indentations or dents can affect the efficiency of the can.
The coffee can will be sealed in one of two ways.With many of the older coffee cans, you needed to open the can with a can opener and discard the removed lid. If this is the coffee can you have, make sure that you grind down or file the inside edge of the can so that it is smooth.
If it’s a newer can, it will have a thick tin foil covering with a ring to pull the cover off. Simply remove the cover and discard it.
Step 2: Cleaning the Can
While having coffee grounds in the antenna will not affect its operation much, it sure can make a mess of things, so make sure that you clean the can out well. Also make sure that you clean the opening of any foreign objects, such as glue, pieces of the original tin or tin foil cover. The coffee can should now look similar to that shown in Figure 3-5.
