Overview Of WiFi Antenna Operation

Overview Of WiFi Antenna Operation

Electromagnetic Radiation And Antenna Operation

Antenna Gain and Directivity
Bits are represented by altering the phase of a sine wave (a "carrier frequency") and usually its amplitude (voltage) in a pre-defined manner that represents bits. This process is called "modulation". During transmission, a modulated electrical signal is sent to an antenna where the fluctuating electrical signal creates a magnetic signal that has a corresponding fluctuation. This electromagnetic signal propagates through space where it encounters a receiver's antenna. The magnetic field impresses an electric field on the receiving antenna and the receiver circuitry "demodulates" the altering electromagnetic field by detecting the changes in phase and amplitude. Bits go into the air; bits come out of the air. This is a very generalized explanation of how electromagnetic signals propagate but it will suffice for the present discussion without going any deeper into the RF physics that govern the operation of an antenna.

An antenna transmits and receives electromagnetic signals however the strength of the propagating energy field (the modulated electromagnetic field) is no equal in all directions surrounding an antenna. It can't be inasmuch as there has to be an electrical connection to some point on the antenna and that point isn't going radiate in the same way as the rest of the antenna. Moreover, the laws of physics dictate certain symmetries in electromagnetic field behavior so when there's a deviation in energy propagation in one part of an antenna there's a resultant deviation in some other part of the antenna. The terms "gain" and "directivity" are applied to the way signal energy propagates from an antenna. The size and shape of the antenna and the way it's constructed determine the gain and directivity of the antenna. "Gain" refers to the effective change (increase or decrease) in signal energy density in some particular directions relative to the amount of energy fed into the antenna. Directivity refers to the direction in which the signal energy is most concentrated. Both gain and directivity are present when an antenna transmits signals and when it receives signals. When gain increases the amount of desired signal energy that can be captured increases but the amount of environmental noise and interference that's captured increases by the same amount.

The 1/2-Wave Dipole Antenna
Some 802.11 Wi-Fi wireless LAN access points use external antennas that are like "sticks", sticking up (and adjustable) from the top or sides of the access point. It's common to see a roughly 5-inch long straight antenna as standard equipment. This type of antenna is technically called a "1/2-wave dipole" and, typically, there are multiple dipole antennas on an access point. The standard dipole antenna radiates RF energy around it as if a doughnut had been placed down over the stick of the antenna. This type of antenna is referred to as "omni-directional" because it radiates in all directions around the plane perpendicular to the antenna shaft. 
Dipole Radiation Pattern
RF energy radiates outward from a dipole antenna forming a 3-dimensional volume that resembles a doughnut. The picture to the left presents a visualization of a doughnut-shaped RF signal energy volume surrounding a vertical radiating element. This is a highly conceptual representation since, in reality, energy density is not confined to a fixed shape (like the "doughnut" in the image, with a distinct outside surface.) In reality the energy density becomes more and more diluted as the distance from the radiating element increases. In fact, this dilution of energy density decreases by a factor of 4 every time the distance from the radiator increases by a factor of 2. This relationship, "distance increases by two, signal strength decreases by four" is called the "Inverse Square Law". Signal strength varies with the square of the relative distance from a radiating element.
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:
  • The Vertical graph (sometimes called an Elevation Plot) represents a side view of the antenna
  • The Horizontal graph (sometimes called an Azimuth Plot) represent a top-down view of the antenna
  • The outer circle represents 0 dB loss or gain
  • The light blue inner circle close to the outer circle represents a 3 dB loss in the pattern graph shown
  • The numbers on the horizontal scale (3, 10, 20, 30, 40) represent dB loss
Typical Vertically Mounted Dipole Antenna Pattern Graph
Looking at the Vertical pattern graph above you see that at the 0-degree/180-degree points there is no loss (relative to whatever gain was specified for this antenna). So, if the transmitting/receiving WiFi device were level with the antenna it would get 100% of the available signal. Let's imagine that this access point antenna was mounted 7 feet up on a wall in a hallway or on the outside of a building. If you were standing 7 feet away from the access point you would be looking up at a 45-degree angle to see it. Since 360 degrees minus 45 degrees is 315 degrees you can determine the effectiveness of the antenna at your location. Look at the Vertical antenna pattern graph and find the 315 degree point. Follow the dashed line inwards until you get to the red pattern line. This line is at, roughly, the 7 dB point. You can see this if you look at the 3 and 10 db circles and note that your point is just about half-way between them. Hence, at your location you would experience a 7 dB loss relative to the antenna's rated gain. In this example, if you were using a 5 dB antenna with a 20 dBm (100 mW) access point transmit power then the effective gain of your antenna at the 45-degree down-looking point would be 5-7= -2dB and your 20 dBm signal would be reduced to 18 dBm. Note, in the graph, that if you were standing directly under the access point your 5 dB antenna would introduce roughly 22 dB of loss meaning that your 20 dBm transmit power would be reduced by 17 dB, down to a meager 3 dBm of available signal!

Here's another important thing to consider: This antenna (let's assume) is designed to be mounted vertically, pointing "up" (as is the case for any typical wall-mounted access point with external dipole antennas). If you were to mount this antenna "upside down" on the ceiling you'll note that the 90-degree point has a 40 dB loss. With the antenna upside down the 90-degree reference is now pointing straight down. If you stood under such an installed antenna the 5 dB rating on the antenna would be reduced by 40 dB and the result is that you would have a 35 dB loss. Your 20 dBm transmit power would be reduced to -15 dBm. Because dBm units are logarithmic, a negative value means that the linear numeric value (the non-logarithmic value) is a fraction less than one. In the example, -5 dBm equates to a linear value of 0.3 dBm. (To calculate: raise 10 to the negative 1.5 power).

A Ceiling-Mounted Antenna Pattern
Below is an antenna pattern graph for an access point that's designed to be mounted on the ceiling. This is the style that often is attached to drop-ceiling t-bar. You can see (in the Vertical pattern graph) how the coverage pattern is optimized for overhead mounting. There is not a 40 dB loss directly under the antenna as was the case with the simple vertical dipole discussed above. As before, however, notice that the design has optimized coverage underneath the antenna (from 210-degrees to 330-degrees) however, directly on top (the 90-degree point) there is a "null" where loss increased to 40 dB. Don't install this upside down (which would probably be impossible to do) but, from a practical standpoint, don't count on this antenna to provide coverage to the area on the floor above, directly over the mounting location.

Antenna Pattern Graph For A Typical Ceiling-Mounted Access Point Antenna

A High-Gain, Directional, Parabolic Dish Antenna
Below is the antenna pattern graph for a high-gain, directional, parabolic dish with a manufacturer's specification of a 3.5-degree beamwidth. Antenna beamwidth is specified based on the "half-power beamwidth", the angle within which the signal loss is less than or equal to 3 dB. The half-power beamwidth angle is considered the optimal connectivity zone. Remember that signal is present outside the 3 dB beamwidth but it's greatly diminished in power.

Antenna Pattern Graph For 3.5-degree Beamwidth Parabolic Dish

What Is "Half-Power Beamwidth"?
Antenna specifications often make reference to an angular value called the "Half-Power Beamwidth" or "3dB Beamwidth". This is a reference to the angle on the antenna pattern graph in which signal power is reduced no more than 3 dB. Consider the Ceiling-Mounted Access Point antenna pattern graphs above and you'll notice that a blue circle is inside the outer circle and that blue circle is where 3 dB of loss occurs. If you look to see where the antenna pattern (solid red line) intersects the blue circle (3 dB point) you'll see that it intersects both above and below 0 degrees. Remember that a change of 3 dB means that a ratio has gotten larger by a factor of two (it has doubled) or it has gotten smaller by a factor of two (it has halved). This is because Log 2 = 3. When measurements are made within the angle formed by the two 3 dB points the signal loss will be less than 3 dB.

When you study the Ceiling Mounted AP vertical antenna pattern graph you'll see that the 3 dB points are roughly at 50-degrees and 320-degrees. That means that the 3 dB Half-Power Beamwidth angle is 90-degrees. Now, as an RF system designer using this ceiling-mounted AP you would be much more interested in the 3 dB beamwidth angle below the horizontal (below the ceiling) and, in this case, that angle would be 40-degrees. This is the kind of information that would be used when comparing different vendor's equipment, different antenna options, and different RF engineering, design, and installation options.

The Connect802 RF Design, Engineering, and Consulting Team
Our WiFi 802.11 wireless network RF engineering, design, and consulting services can help you make sense out of the nuances of technical details relating to antenna specification, equipment selection, and your overall 802.11 WiFi wireless system installation. We're ready to be a resource for you; whether it's to roll out a complete, corporate-wide WiFi deployment or simply answer a technical question that you need answered. "We're At Your Service!"

Definitions Of Terms Related To Antenna Pattern Graphs
The following terms are commonly used when discussing antennas and their pattern graphs:
Half-Power Beamwidth
The angle, as determined from an antenna pattern graph, within which the RF signal power decreases by no more than 3 dB. This translates to no more than a 50% reduction in signal power (since a 3 dB loss is mathematically equivalent to dividing by 2). The Half-Power Beamwidth angle defines the zone in which optimal performance can be expected from an antenna. The half-power beamwidth angle (also called the "3 dB beamwidth angle") is a manufacturer's specification and is used for both access point and antenna comparison as well as for the RF engineering and design phase of a project.

Isotropic radiator
An isotropic radiator is a hypothetical lossless antenna that radiates its energy equally in all directions. This imaginary antenna would have a spherical radiation pattern and the principal plane cuts would both be circles (indeed, any plane cut would be a circle).

Gain
The gain of an antenna (in any given direction) is defined as the ratio of the power gain in a given direction to the power gain of a reference antenna in the same direction. It is standard practice to use an isotropic radiator as the reference antenna in this definition. Note that an isotropic radiator would be lossless and that it would radiate its energy equally in all directions. That means that the gain of an isotropic radiator is G = 1 (or 0 dB). It is customary to use the unit dBi (decibels relative to an isotropic radiator) for gain with respect to an isotropic radiator. Gain expressed in dBi is computed using the following formula:

GdBi = 10*Log (GNumeric/GIsotropic) = 10*Log (GNumeric)

Occasionally, a theoretical dipole is used as the reference, so the unit dBd (decibels relative to a dipole) will be used to describe the gain with respect to a dipole. This unit tends to be used when referring to the gain of omnidirectional antennas of higher gain. In the case of these higher gain omnidirectional antennas, their gain in dBd would be an expression of their gain above 2.2 dBi. So if an antenna has a gain of 3 dBd it also has a gain of 5.2 dBi.

Note that when a single number is stated for the gain of an antenna, it is assumed that this is the maximum gain (the gain in the direction of the maximum radiation).

It is important to state that an antenna with gain doesn't create radiated power. The antenna simply directs the way the radiated power is distributed relative to radiating the power equally in all directions and the gain is just a characterization of the way the power is radiated.
3-dB beamwidth. The 3-dB beamwidth (or half-power beamwidth) of an antenna is typically defined for each of the principal planes. The 3-dB beamwidth in each plane is defined as the angle between the points in the main lobe that are down from the maximum gain by 3 dB. This is illustrated in Figure 3. The 3-dB beamwidth in the plot in this figure is shown as the angle between the two blue lines in the polar plot. In this example, the 3-dB beamwidth in this plane is about 37 degrees. Antennas with wide beamwidths typically have low gain and antennas with narrow beamwidths tend to have higher gain. Remember that gain is a measure of how much of the power is radiated in a given direction. So an antenna that directs most of its energy into a narrow beam (at least in one plane) will have a higher gain.

802.11ac

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