This month, we conclude our discussion of antenna propagation patterns. In July, we covered the concept of antenna beamwidth. Then, we discussed different types of omnidirectional antennae and when they might be used. In August, we moved on to directional antennas, covering the patch/panel and yagi types. Last month, we covered sector antennas. If you missed those articles, we recommend that you read them addition to this one, for maximum completeness.
In this issue, we'll conclude our coverage of directional antennae. The major types of directional antennae are: patch/panel, yagi, sector, and dish/grid. This month, we'll cover dish/grid antennas.
The dish antenna is probably the antenna type that you're most familiar with, since dish antennas bring digital satellite television to many homes. The general design of a dish antenna is a concave metallic "dish" with a receiver element mounted at the focal point of the dish. The curve of the dish matches that of a mathematical function called a parabola (which is why these antennas are sometimes called "parabolic dish" antennas). A parabola has the quality that a ray that strikes the interior of the parabola at any point will be reflected towards a single focal point. This makes a parabola perfect for collecting and focusing RF energy over long distances. All RF energy hitting the dish is reflected towards the focal point, which is where the actual antenna element is located.
A grid antenna is functionally equivalent to a dish. Instead of using a parabolic dish of solid metal, a grid antenna uses a metal grid. This allows the antenna to be lighter and more resistant to wind. You might think that a grid would cause the antenna to be less sensitive than a comparable dish, since some of the RF signal will pass through the empty areas of the reflector, but that isn't the case. The spacing of the grid is carefully matched to the wavelength of the signals that the antenna is designed to receive so that the antenna is just as sensitive as an equivalent dish.
Dish antennas create a very long, very narrow coverage area. Their beamwidth may be as small as two or three degrees, and they may have gains of 20 to 30 dBi. Below, you can see the azimuth and elevation chart and a picture of the coverage pattern for a 30 dBi dish antenna.
Dish Antenna Pattern Graph
Dish Antenna (Aimed to the Right) - 3 Dimensional Coverage Volume at -80 dBm
First, notice the small spherical "ball" at the left-hand end of the antenna's coverage (nearest to the antenna). Even dish antennas, the most directional antennas of all, emit some energy in their non-primary directions. This means that, like the other directional antennas that we've discussed, a receiver who is close enough to a dish antenna can get a signal even if the antenna is not pointed directly at the receiver. This means that two dish antennas placed close enough together can interfere with each other even though they're putting the vast majority of their energy in their primary direction. Second, notice the extremely long main lobe. In fact, it's so long that our prediction software cuts it off, which is why it appears to have a pointy, diamond-shaped tip. In reality, the coverage continues until it rounds off at the ends. The graphic above represents a propagation distance of 55,000 feet, or about ten miles! And that's just where our prediction software stops... in reality, this signal goes even farther.
A dish antenna's extremely long, narrow coverage pattern defines its uses. Dish antennas are only used for links between fixed (non-mobile) transmitters. They are most commonly used in long distance point-to-point links, with dishes on either end pointing at each other. In some cases, a point-to-multipoint configuration is created with an omnidirectional antenna in the center and two or more dishes in a "star" configuration pointing back at the central omnidirectional antenna. In even rarer cases, two dish antennae are used on either side of a very thick or very absorptive obstacle--for example, a heavy concrete wall or the metal skin of an aircraft. In this special case, the high gain of the dish antenna is used to "blast" through the obstacle. This setup should only be used as a last resort, as, obviously, it would be preferable to run a short wire! But it's a good technique to have in your hat for when nothing else will do.
Dish antennas are not appropriate for even slightly mobile users because of their very narrow coverage areas. Typically, an equivalent dish or grid antenna will be mounted at either end of a point-to-point link and will be attached to a wireless bridge that will translate packets onto a wired network at the other end. Because these antennas have such a narrow beamwidth and are typically used over such long distances, careful alignment is essential to maximizing signal strength.
There is a basic principle of antennae that is so unexpected (to the uninitiated student) that some people refuse to believe it's true the first time they hear it. The principle is called the Reciprocity Theorem. Its consequences are that, if we are using the same input and output gain, then regardless of differences in our antenna gain, if one I can hear you, you can hear me. This month, we'll explore this concept and its implications.
Consider the case where an AP has a 12 dBi omni antenna attached and a client has a 2 dBi omni antenna on a PCMCIA card. Both the AP and the client are using 15 dBm of transmit power. It might not surprise you that the AP's high-gain antenna can push a signal a long way out to the client, but you might guess that the client's low-gain antenna couldn't get a signal back to the AP. You'd be wrong. Antenna reciprocity basically means that the exact same qualities that make an antenna good at transmitting a signal also make it good at receiving a signal. To put it another way, the Rayleigh-Helmholtz reciprocity theorem states:
If an electromagnetic force of some particular magnitude is applied to the terminals of antenna "A" and the received current is measured at some other antenna "B" then an equal current (in both amplitude and phase) will be obtained at the terminals of antenna "A" if the same electromagnetic force is applied to the terminals of antenna "B".
As an analogy for an RF antenna, imagine a paddle sticking up out of the smooth surface of a lake. Another paddle is sticking up at the opposite end of the lake. One paddle begins to oscillate back and forth, creating waves that push on the other paddle, causing it to move. In our analogy, the paddles are antennas and the waves are RF waves. To carry the analogy further, imagine that one paddle is much bigger than the other--it represents our high-gain antenna. When the big paddle oscillates, it makes much bigger waves, causing the smaller antenna to move more even though it's got a smaller surface area. When the little paddle oscillates, on the other hand, the big paddle's increased surface area causes it to move more as well! The analogy fails somewhat because, in reality the increased mass of the big paddle would give it enough inertia that it wouldn't really move more, but for the sake of the analogy, the antennas are massless.
Antenna reciprocity arises from a property of physics equations called "time-symmetry". Time symmetry means that it doesn't matter whether time runs forwards or backwards, the physics equations should work out the same. Time symmetry is one of the touchstones of new physics theories. Any theory that violates time symmetry is called into serious question. To understand the significance of time symmetry, consider a pool table with a white ball near one end and a black ball in the center. The white pool ball is accelerated by the force of impact with the cue stick and travels towards the center of the pool table. In the center, the white ball strikes the black ball in a straight, center-to-center impact. The inertia of the white ball is transferred to the black ball and it is now accelerated away from the white ball in a straight line, leaving the white ball stationary at the point of impact. If you were to make a movie of the two balls striking and then played the movie backwards, it would show exactly the same thing except now it would be the black ball that starred in the opening scene of the movie. If the mass, velocity, and other characteristics of the Amazing Pool Ball Adventure movie were represented through mathematical equations, the equations would not be time dependent. Time could run forward or backward and the results would be identical.
To some readers, the reciprocity theorem may be new. The implications of antenna reciprocity are far reaching and, if this is the first time you've encountered the concept, the implications may be too hard to accept without proof. In fact, not only is reciprocity demonstrable in the lab and in real-world installations, but the physicists of the world can provide mathematical proof that the theorem holds true. If the antennae and the space between them are replaced with a network of linear, passive, bilateral impedances, then the current through the network can be calculated in accordance with standard practices in electronics theory. Whether on paper or in practice, given the same input power on both ends, "If you can hear me, then I can hear you!"
Next month, we'll discuss some of the real-world implications of antenna reciprocity.
Mr. Joe Bardwell, President and Chief Scientist at Connect802 was interviewed by the New York Times last month as part of an article discussing the growth of Wi-Fi in the residential sector. The article, available by paid subscription to the New York Times archives, was also carried by many other local newspapers. It may be found on-line from the Detroit (Michigan) News.
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