How is Signal Strength Increased When an Antenna Has "Gain"?



When an antenna has 'gain' or is 'directional' it doesn't make the transmitted power any greater. Some particular amount of energy is fed into the antenna, some of that is lost to the impedance of the feed line, some is electrically reflected back down the feed line and lost, and the rest is radiated outwards from the antenna. As we say, "There's no free lunch in the physics department." The way the metal elements of an antenna are constructed (their size, shape, orientation, and relationship to each other) changes the way the electromagnetic signal radiates away from the antenna. The change in the 3-dimensional shape of the propagating wave (and, consequently, the spatial volume of the resulting 3-dimensional area) changes the density of the signal. It's increased energy density in some particular direction that results in increased signal strength for an intended receiver and is the quality of an antenna called 'gain'.







The Isotropic Radiator And The Antenna "dB" Gain Ratio

We begin thinking about gain by considering a theoretical radiating point source called an 'isotropic radiator'. An isotropic antenna would radiate signal outwards equally in all directions, creating a spherical transmission volume. There is no perfect isotropic antenna. A dipole antenna (a straight 'stick') radiates RF in a manner that can be visualized by thinking about a fluorescent light bulb. Signal radiates outwards from the sides, but not from the top or bottom. This produces a torroidal (doughnut) shaped signal volume around a dipole antenna. Since the signal is 'squeezed' into a transmission volume shape that is 'flatter' than the theoretical isotropic pattern the RF energy is compressed into a smaller volume. This results in the electromagnetic energy being more 'dense' in any given area inside the transmission volume than it would have been in the spherical volume of the isotropic radiator. The increased 'density' of signal is referred to as antenna gain and it's measured in decibels relative to the isotropic case (dBi, which is often simply written as 'dB' on an antenna spec sheet).





Where Can I Find Detailed Technical Information About RF Signal Propagation?

If you want the most concise, in-depth technical document describing, in tremendous detail, the way electromagnetic energy propagates, the technical dissertation, "I'm Going To Let My Chauffeur Answer That" (a dissertation written by Connect802's Chief Scientist and President, Joseph Bardwell) delves into the math and physics that explain Maxwell's wave equations and RF signal propagation. The math is well explained and there's plenty of concept information. You can download this dissertation from the Technical Information page.







Understanding How To Interpret Antenna Pattern Graphs

An antenna pattern graph is a graphical representation of the energy density (radiation pattern) in space, around the antenna. An antenna's pattern graph (typically provided in a manufacturer's spec sheet or product data sheet) describes in which directions and to what effectiveness an antenna both radiates energy into space and recieves energy from space. Both transmit and receive characteristics are identical (that is, the directions and effectiveness an antenna manifests when transmitting are the same as when receiving). This is called the "Law Of Reciprocity" and it's a fundamental law of RF physics.





Antennas radiate in 3-dimensions and, to describe the 3D pattern a pair of polar-coordinate graphs are used. Antenna pattern graphs present dB (decibel) loss relative to an arbitrary (albeit realistic) manufacturer's antenna gain specification. For example, if a manufacturer has an antenna that they specify as a "5 dB Gain" antenna then the zero reference level on the antenna pattern graph implies that all dB references on the polar coordinate antenna pattern graph are relative to 5 dB. A plotted loss of 3 dB implies that, at that angle on the polar coordinate graph, the gain would be 2 dB (because 5 dB minus 3 dB equals 2 dB).





There are two views shown in an antenna pattern presentation and the two views represent the pattern looking at the antenna from the side view and looking at the antenna from the top-down view. These two views may be referred to by several different names, depending on who is assigning the names to the graph. The names you'll encounter are:





For the side view:

Vertical Pattern: The name references the fact that you're looking at the pattern as if the antenna were placed vertically in front of you.



Elevation Pattern: The name references the fact that, as with an AutoCAD architectural plan, this is the view that shows an "up/down" representation of the antenna pattern with the antenna pointing up in the middle



E-Plane Pattern: This name refers to the fact that the electric field (as opposed to the magnetic field) is polarized in the direction of the long axis of the antenna. For a vertical dipole antenna, positioned up-and-down, the E-Plane field is considered to be the electric field being modulated vertically. You can remember this name because "E" is in "E-Plane" and "Elevation"

For the top-down view:

Horizontal Pattern: The name refers to the fact that the polar coordinates are on a horizontal plane with the antenna in the middle and you're looking down at this horizontal plane



Azimuth Pattern: The term "azimuth" is taken from the astronomy and navigation world and it refers to the relative angle of an object on the horizon. If you imagine a ship at sea (or an astronomer viewing the stars) they would say, "The azimuth angle to the such-and-such point is x degrees," referring to the view, horizontally, out to the horizon.



H-Plane Pattern: This name refers to the fact that the magnetic field (as opposed to the electric field) is polarized in the direction perpendicular to the long axis of the antenna. For a vertical dipole antenna, positioned up-and-down, the H-Plane is considered to be the magnetic field being modulated horizontally. The letter "H" is the mathematical vector variable name assigned to the magnetic field in, for example, Maxwell's Wave Equations.



You can think of the Elevation Graph as a side view of the antenna. Sometimes this is referred to as the antenna's Vertical Plane antenna graph because you're imagining that the antenna is in front of you in a vertical orientation. The second graph is called the "Azimuth" graph. It represents a horizontal plane with the antenna in the middle. You can think of the Azimuth Graph as a top-down view of the antenna. Sometimes this is referred to as the antenna's Horizontal Plane antenna graph because you're imagining that the antenna is under you and there's a horizontal plane slicing through the antenna. These two "slices", the Azimuth and Elevation graphs, make up an antenna's pattern graphs.







It's important to note that antenna pattern graphs, when referencing Elevation as a vertical plane with the vertical antenna in the middle and Azimuth as the top-down view assume that the antenna is oriented in the orientation in which it's intended to be used. From the perspective of the polar coordinate graph, this means that an external antenna on a wall-mounted access point is aligned vertically and "up" is represented by the 90-degree point on the circular polar coordinate graph. "Down" would be 270-degrees.





On the other hand, if the antenna pattern graph is in a spec sheet for a ceiling mount antenna then "Down" would be 90-degrees. Think about this for a moment because the concept is not obvious. If you sat a ceiling mount antenna on a desk you would agree that 90-degrees would be "up" however this is not the intended installation method for the antenna. Hence, when this ceiling-mount antenna's pattern graph references the 90-degree point it means, in the real-world, this is the "Down" direction.





The Dipole Antenna Pattern Graph

Below is an antenna pattern graph for a typical dipole antenna (the "stick" style antenna used externally on many access points.) To start with, you'll need to know the specified gain of the antenna as listed in the manufacturer's data sheet. They'll indicate that this is a "5 dB Gain", "2 dB Gain", or some other value. The value may be marked as "dBi" (decibels relative to isotropic) instead of "dB" (decibels). For antenna specifications these two abbreviations mean the same thing; "dB" is just a shorthand since "dBi" is understood as the reference ratio. Here are some details to learn about the elements of the graphs, keeping in mind that the intent is that this antenna is installed straight up-and-down: