Jeff Duntemanns Drive-By Wi-Fi Guide
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How Antennas Work
One way I like to put things to my students is that coaxial cable (more on which later) is a hose for radio waves, and antennas are nozzles. The hose transports the radio energy from a transmitter to a different location (like someplace outside and up high) and an antenna sprays that radio energy out into the air. And just as you can have many kinds of nozzles that spray water in lots of different patterns, from a straight, strong stream in one direction to a uniform spray in all directions, so it is with antennas: They can 'spray' radio energy in different patterns, either in every direction at once or in a very narrowly focused beam.
The water metaphor pretty much ends there, but it's a good start. Different types of antennas emit radio waves in fields of different shapes, and that makes all the difference in the world. For the moment let's set aside discussion of the physical form and materials of which antennas are made. (I'll get back to that.) The crucial issue is the shape of the radio field.
The Mythical Isotropic Antenna
With Wi-Fi work we'll be most interested in the shape of the radio field produced by particular antennas, so let's start by positing an antenna that emits radio waves equally in all directions. If we could somehow magically see the radio field around the antenna, it would be in the shape of a sphere, with the antenna at the center. Such an antenna doesn't actually exist, but it has a name nonetheless-an isotropic antenna.It radiates equally in all directions. It's a mathematical abstraction, created specifically to give us a sort of 'base point' against which to measure the performance of real-world antennas. The closest we come in real Wi-Fi work is in the omnidirectional antenna, which creates a somewhat lumpy field spread out mostly evenly in the horizontal plane. 'Omni' here doesn't necessarily include 'up' and 'down.' (More on this a little later.)
The idealized isotropic antenna is important in Wi-Fi work because antennas that you buy for your Wi-Fi equipment are rated in units called 'dBi,' which I will explain in more detail a little later in this chapter. Quite simply, the dBi unit is a measurement of antenna 'reach' relative to the mythical isotropic antenna, which has the shortest 'reach' possible of any antenna, given the same amount of input radio energy. The more dBi, the greater the gain of the antenna, and the farther it can reach.
To understand this notion of 'reach' (which is my coinage and not a technical term!), recall that the mission of an antenna is to get a readable radio signal from point A (the antenna's location) to point B. The longest distance from A to B is the antenna's reach, and a good measure of its effectiveness as an antenna.
An isotropic antenna radiates radio energy in a perfect sphere. Point A is at the ball's center, and Point B is somewhere on the sphere's surface. If you imagine the field of an antenna to be something like a balloon filled with water, a sphere is the most compact form a volume of water in a flexible container can take. The distance from the center of the balloon to its surface is as short as with any shape the balloon can take. On the other hand, a balloon can be pulled or squeezed into other shapes without changing the volume of water it contains. No matter what shape you squeeze the balloon into, if you change its shape into something other than a sphere, the distance from its center to its farthest extent will be greater than what it was when it was a sphere.
The Shape of the Field
If you're careful how you squeeze the balloon, you can change its shape to something long and thin, so that Point A at the center is way farther out than Point B on its edge. That's how antennas work: They change the shape of the radio field they emit from a sphere (as an isotropic antenna would) to something else, something that moves the edge of the field farther away from the center, or something that has a shape that more nearly 'fills' a building or other area.
Figure 8.1 is a view from above, looking down on three different antennas and their fields. Antenna A is our mythical isotropic antenna, which has a spherical field, seen here as a circle. Antenna B is a 'picture frame' antenna, which changes the shape of the field to something nearly all to one side of the antenna, with a corresponding increase in the distance from A to B. And Antenna C is a parabolic dish antenna, which squeezes almost all of the field into one long arm, pushing A and B much farther apart.
The picture frame antenna's mission isn't to get a signal across long distances, but to concentrate the field almost entirely to one side of the antenna. Hanging on a wall (hence the name) a picture frame antenna will 'fill the room' with a Wi-Fi radio field. The parabolic dish antenna, however, is designed to create as narrow a beam as possible, sometimes a beam that can reach across several miles of open space.
If you'll need to evaluate commercial antennas, your best single source of information will be the radiation pattern charts most manufacturers publish. An excellent example is shown in Figure 8.2, which is the horizontal plot of Pacific Wireless' PAWIN24-10 directional panel, or picture frame antenna. In this plot, you're looking at the radio field of the antenna from the side rather than from above. Most manufacturers will publish two such charts, one viewed from the side (horizontal) and the other from above (vertical.)
Note that the field boundaries are not smooth curves, but have bumps and lumps and odd little tucks. These might seem to be problems, but remember that the field boundaries are not walls, but simply plots of equal field strength, like contour lines on a topographic map. A bulge is an area of slightly greater field strength whereas a tuck is an area of less field strength.
I emphasize understanding the shape of an antenna's radio field, because an antenna's gain figure in dBi isn't the whole story. A gain antenna has gain in very specific directions. If you don't understand the directions in which an antenna's gain is distributed, you can buy the wrong antenna for the job. If your Wi-Fi setup doesn't work as it should, you can't blame the antenna. (See my later discussion on picture frame antennas for a detailed example.)
The Threat from the Third Dimension
People who scrutinize the radiation pattern of an antenna as viewed from above (as they're almost always plotted and viewed) often forget that an antenna's radiation pattern has a third dimension as well. Even 'omnidirectional' antennas are omnidirectional only in the horizontal plane. They don't necessarily radiate up (or down) with the same intensity that they radiate outward.
The cheap and very simple rubberized omnidirectional antennas shipped with most access points radiate pretty randomly in most directions, including up and down. The better and more expensive omnidirectional gain antennas get their gain by 'squeezing' the vertical dimension of radiation down into the horizontal plane.
Think again of a completely spherical balloon. Push down on it from above, and it spreads out horizontally. Push it hard enough and it will take on the form of a round sofa cushion or even a doughnut. That's precisely what happens with the radiation pattern of an omnidirectional gain antenna. The energy that would have been radiated up or down is redirected more horizontally.
This becomes an issue in several facets of Wi-Fi work. 'Picture frame' Wi-Fi antennas intended to be hung on office walls often have considerable gain. In part this gain is directional: The bulk of the antenna's energy is radiated in a 'sector' in the shape of a fan that may be 140 degrees or so in angular extent. However, my tests have shown that picture frame antennas are very horizontal in their radiation. They work very well at filling office cube farms on a single floor. Go one floor upstairs or down, and the signal drops like a stone. This may be good-if your competitors rent the floors above or below you. But don't try to use a picture frame antenna to service more than a single floor. Between the inherent shielding of the floor's construction (especially in steel-framed office buildings with jungles of wires and HVAC ducts between floors) and the vertical directionality of the antenna, you won't get the kind of full-bitrate coverage you want.
In my tests I found that picture frame antennas don't work well even in woodframe residential construction, when you intend to service even two floors of an ordinary ranch house. For a side-view look at the radiation pattern of a picture frame antenna, see Figure 8.2.
In a completely different realm, wardrivers (see Chapters 18 and 19) have found that omnidirectional gain antennas miss access points on the higher floors of office buildings. Think of driving down LaSalle Street in Chicago, with 50- and 70-floor office towers on every side. On the higher floors, the access points are almost directly above a cruising vehicle, and with a horizontally directional antenna, anything much above the fifth or sixth floor won't be detected.
A Quick Word on Frequency and Wavelength
Space prevents me from going much more deeply into general antenna theory, but I need to put frequency and wavelength into a Wi-Fi context. Radio signals are electromagnetic waves, and have complementary aspects called frequency and wavelength. The frequency is how many complete waves occur each second. The wavelength is how far in space one of those waves travels at the speed of light. (Radio waves travel at the speed of light, 186,000 miles or 300,000 kilometers per second.)
Lower frequencies have longer wavelengths. The frequency of the center of the AM radio band is 1 megahertz (million cycles per second, abbreviated MHz) and its wavelength is about 300 meters, roughly 1,000 feet. The frequency of the center of the FM broadcast band is 100 MHz, and its wavelength is about 3 meters, roughly 10 feet. The frequency of the Wi-Fi band is 2.4 gigahertz (billion cycles per second, abbreviated GHz) and its wavelength is just under 5 inches. We're in extremely small wavelength territory here, compared to more familiar radio and TV broadcasting. This is why Wi-Fi signals are said to be microwaves. (Though in truth, they're on the big side for microwaves, which go down in wavelength to very small fractions of an inch.) The major advantage of being in microwave territory is that good antennas are quite small and inexpensive, and easy to build if you're handy with tools.
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