Building a Classic Paperclip Antenna

Have you ever had this survivor fantasy? You’re stranded on a desert island (with your laptop of course!) and desperately need to connect to the outside world. You empty your pockets on the ground, and find nothing but commonplace objects. But then... in a frenzy of creative brilliance you whip all the ordinary pocket-objects together to come up with a jerry-rigged wireless antenna. Within minutes you are e-mailing potential rescuers from the beach, and surfing your favorite sites to kill time.

Of course, that is only a fantasy. But, this chapter offers you a project which comes pretty close to the fantasy in both results and ingenuity. This chapter will show you how to put together the ultimate “homebrew” antenna—a working Yagi antenna for 2.4 GHz Wi-Fi out of little more than paperclips stuck together. This model is commonly called the Frisko antenna, after the French Frisko brand of ice cream cups whose wooden spoons were used in the first prototypes. Figure 2-1 shows a completed paperclip antenna.

If you do any research on antennas, you will notice that there are several different types of antennas around.Two common types are directional and omnidirectional. The difference between these two types of antenna is a simple but important one. A directional antenna transmits its information in a single direction, while an omni antenna transmits the information in all horizontal directions.

In addition, you need to understand how antenna efficiency, or gain, works as related to other antennas. Antenna gain is measured in decibels, isotropic (dBi), defined as the strength of an antenna as related to a theoretical sphere around an imaginary antenna. dBi is a logarithmic measurement, so every 3 dBi is a doubling of gain.What you need to know is the higher the dBi, the more sensitive and focused the antenna.

An omni antenna sends and receives signals equally in front, behind, to the left, or to the right of the antenna. However, when you go above or below the antenna, signal strength drops off significantly. The trade-off you make when choosing a high-gain antenna is this focusing, or thinning, of the above and below energy. The low-gain omni works better vertically than a high-gain omni, but it won’t extend as far horizontally. Figure 2-2 shows a diagram of two antennas viewed from the top. The directional antenna is most sensitive in one direction, meaning signals being sent and received by the antenna will be strongest in the direction the antenna is pointing. The omnidirectional antenna sends and receives signals in all directions equally. This is a generalization, but it’s mostly accurate. Later chapters will delve further into the specifics of antenna operation.

Since directional antennas direct their information at a specific target (or at least in the direction of the target), they require less power to transmit, but more precision in their placement. Omnidirectional antennas need little precision in their placement, but require more power to send and receive signals. You are probably familiar with these different types of antennas, because you see them almost every day. A satellite dish would be considered highly directional, looking up into space, while

the antenna on your car is omnidirectional, listening to radio no matter which direction your car is facing. Let’s take a closer look at these different types of antennas.

Omni Antennas

The omnidirectional antenna is probably the most common Wi-Fi antenna available. Just about every Wi-Fi device you can buy comes with an omni antenna. This is because the omni is so easy to set up, and generally works in consumer environments without much planning. There are a few different types of omni antennas. Omni signals spread out sideways, but not vertically (see Figure 2-3).

Even though an omni antenna does not work very well above and below, it is not considered a directional antenna. Wi-Fi antennas are generally rated in two-dimensional space that assumes it is mounted parallel to the Earth’s surface. Knowing how the beam is shaped, and that an antenna is not truly omnidirectional will help you choose the right antenna for your Wi-Fi toys.

Dipole Antenna

The dipole antenna is just about the simplest antenna there is. The dipole is a half-wave antenna that consists of two opposing radiating elements. It’s made up of two quarter-wavelength poles

 

Figure 2-4 shows a simple dipole made from steel paperclips. Each arm of the dipole is 31 mm in length, or 1/4 of a wavelength for Wi-Fi channel 6. The center conductor is soldered to the right arm, while the shield is soldered to the left arm. It doesn’t matter to which side you solder. The dipole antenna is unique in that it can be mounted vertically or horizontally.When standing vertically, the dipole antenna is omnidirectional.When horizontal, this antenna will radiate outward in two directions off the sides (and slightly upwards), like turning a donut on its edge.

A coaxial is another common antenna used in Wi-Fi. It’s used on most wireless access points you can buy. If the access point has a stubby little antenna on it, chances are it’s a coaxial antenna. The coaxial antenna works in much the same way as a dipole antenna. The construction is slightly different though. The antenna feed comes up through the bottom with a metal casing around the shield-connected arm. Coaxial antennas are usually a total of half a wavelength with each arm being one quarter wavelength of the frequency.

The antenna in Figure 2-5 is from a common access point with two antennas. The plastic on one of the antennas is removed to show you the actual antenna element. You can see that the cable runs through the base of the antenna. The center conductor extends to the top while the shield is soldered to the metal cylinder that becomes the base radiator.

The driven array antenna is often used for mobile applications. This is a vertical antenna with gain created by multiple segments of half-wavelength elements arranged vertically end-to-end to achieve gain. An array is simply more than one element working together. The driven array means that each element has an electrical connection with the one next to it. The signal is driven into each radiating element via an antenna coil that maximizes the transfer of energy between adjacent elements.