ANTENNAS
INTRODUCTION.
Antennas generally fall into the following categories: Omnidirectional and Directional. Although there are many different
antennas, most are just variations of these two basic types.
Omnidirectional antennas
Omnidirectional antennas (omnis) radiate a pattern in all directions, IE: 360 degrees. Omnis are good for large open areas where there is little in the way of obstructions. Warehouses with low racking and high ceilings, and manufacturing areas are
examples.
Omnis need to be mounted out in the clear. Many times an omni is found mounted on a ceiling extending downfrom the support beams.Omnidirectional antennas can vary in shape. Depending on the gain, most are just black or white sticks in varying lengthsOthers look somewhat like smoke detectors or simplified small, flattened hockey pucks.Some omnidirectional antennas have customizable patterns. The radiation pattern can be modified during installation toprovide more coverage in some areas and a less in others. Even the "uptilt" and "downtilt" of the signal can be adjusted. Thisallows the pattern to be customized to the installation. This insures that signal is not wasted in directions where it is notneeded and/or wanted. These kinds of antennas are used outdoors to cover large open areas such as theme parks oroutdoor malls.The gain of the antenna affects the coverage pattern. A low gain omni will have a relatively small coverage area, but it will bevery broad vertically. This is why low gain omnis are used for high bay warehouses where the antenna is mounted in high generally 35 feet or higher. This broad coverage also wraps around racking better. These kinds of antennas work well when the antennas need to be mounted high and the user population is at ground level.
High gain omnis radiate a signal further in a more narrow form. These antennas are deployed for outdoor use where users are not near, but more of a distance away. A good example of this is point to multipoint bridging. The center pointwould use a higher gain omni with the outer areas using directional antennas pointing towards the center point. (Directionalantennas are discussed below.) It would not be wise to use a high gain omnidirectional antenna in a high bay warehouse. Thesignal would be radiating outwards, without enough downward signal to the users at ground level.
Directional antennas
There are varieties of directional antennas. Although these are all directional antennas, a large difference exists among each.The difference is the coverage patterns.
Yagi antennas are the most well known. The Yagi looks a lot like an older television antenna. long boom with horizontalsticks (elements) along its length. The higher the frequency, the smaller the elements. A Yagi for 2.4Ghz has elements less
than 3 inches long. In fact, the most common Yagi antenna for 2.4Ghz looks like a long cylinder. The cylinder is just aweatherproof cover. Yagi antennas work by focusing more signal in one direction like a mirror behind a light bulb. Thehigher the gain of the antenna, the narrower the radiated signal will be. One use for Yagi Antennas is within large
warehouses with high racking and long aisles. An Omni may not fit between the top of the racks and the ceiling, therefore, aseries of Yagis become the antenna of choice. They are used to fire asignal down the aisles. Yagi’s can also be used outdoors as bridge links between two locations over a long distance. In many cases a Yagi may cover up to 3 or more miles.
Sector Antennas are somewhat similar to Yagis, however, they present a much wider coverage. Yagi’s tend to be less than 35 degrees in coverage where as a Sectored Antenna typically is between 60 and 120 degrees in coverage. Sectored Antennas are used mainly outdoors where the antenna may be at the edge or corner of the coverage area.
Patch or Panel Antennas are flat, square, or round and used where a low profile is needed. Many times this is due to esthetics or reducing the risk of an antenna being damaged. Patch and Panel Antennas are much like Sectored Antennas. One type of Patch is a hemispherical. This antenna type has a 180 degree coverage. It’s well recognized for coverage in retail
stores, parking lots, and convention halls. Another style of Patch is the bi-directional. It’s a small antenna that fires a signal in two directions, 180 degrees from each other. These are know for coverage in long hallways such as hospital corridors. The Parabolic is the "big gun" of the directional antennas. The Parabolic is used exclusively for outdoor, long distance point
to point bridging. Typically demanding coverage of 10 to 35 miles.
other types.
Multipolarized Antennas
Antenna development for Wi-Fi (and other wireless technologies) is a hot area right now, with many new developments. For example, a company named WiFi-Plus, Inc., has developed multipolarized antennas. According to the company's chief technology officer, Jack Nilsson, these antennas have the ability to propagate and receive signals that are both horizontal and vertical. These models are better than conventional models for going around obstructions. WiFi-Plus's multipolarized antennas can also be used in situations where Wi-Fi is being broadcast using a directional antenna to a deep valley. A conventional directional antenna might broadcast signals that would overshoot the valley, but a multipolarized antenna is capable of broadcasting signals that travel horizontally following the direction of the RF beam, but also can be received down in the valley.
ANTENNA GAIN.
One item that needs to be discussed in more detail is Gain. Gain is defined as the compressing of the vertical component of the antenna pattern, in effect causing the radiation pattern of the antenna to reach out further toward a base station or cell site Antenna Gain is an antenna’s ability to gather in signal and radiate signal.
Usually the higher the Gain, the larger the antenna will be. For an omnidirectional, that means height. For Yagi Antennas, length. Gain is expressed in dBi-a unit of antenna gain. The dBi measure is referenced to a theoretical, dimensionless point source with a completely spherical radiation pattern (http://www.antenna.com/faqs_theory.html). This is the only way to
compare relative performance when looking at similar antennas. Most manufacturers rate their antennas in dBi. A few still use dB. When comparing gain, the units must be the same for both.
FACTORS TO CONSIDER.
install their own WLAN’s in a small office or home, the antenna is not a major factor since most cards and access pointshave built in antennas. However, for large installations such as warehouses, manufacturing plants, hospitals, airports andsimilar areas, the antenna becomes a critical part of the entire system. The correct selection and usage of antennas maymean the difference between a cost effective installation with robust reliable performance, and a network with areas ofweak coverage, unreliable communications and poor performance. The correct selection and usage of antennas may alsomean the avoidance of potential expenses due to too many access points and regular visits by technicians searching to fix a
network problem.
EXAMPLES.
1) Frequency (MHz): 45 ~ 860
2) Gain (dB):
a) VHF (L): 26
b) VHF (...
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SPECIFICATION: VHF OUTDOOR ANTENNA ELEMENTS: 5 CHANNELS: 1-12 FREQUENCY: 46-230dB GAIN: 3.5-7dB IMPEDANCE: 75/ 300 OHMS PACKING: EACH IN PRINTED BOX
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Impedance matching.As an electro-magnetic wave travels through the different parts of the antenna system (radio, feed line, antenna, free space) it may encounter differences in impedance (E/H, V/I, etc). At each interface, depending on the impedance match, some fraction of the wave's energy will reflect back to the source[, forming a standing wave in the feed line.
The ratio of maximum power to minimum power in the wave can be measured and is called the standing wave ratio (SWR). A SWR of 1:1 is ideal. A SWR of 1.5:1 is considered to be marginally acceptable in low power applications where power loss is more critical, although an SWR as high as 6:1 may still be usable with the right equipment. Minimizing impedance differences at each interface (impedance matching) will reduce SWR and maximize power transfer through each part of the antenna system.
Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed line and radio by adjusting the impedance of the feed line, using the feed line as an impedance transformer. More commonly, the impedance is adjusted at the load (see below) with an antenna tuner, a balun, a matching transformer, matching networks composed of inductors and capacitors, or matching sections such as the gamma match.
Impedance matching is the electronics design practice of setting the output impedance (ZS) of a signal source equal to the input impedance (ZL) of the load to which it is ultimately connected, usually in order to maximize the power transfer and minimize reflections from the load. This only applies when both are linear devices.The concept of impedance matching was originally developed for electrical power, but can be applied to any other field where a form of energy (not just electrical) is transferred between a source and a load.
Matching is obtained when ZL = ZS.
With modern audio electronics, impedance matching degrades audio performance[1] [2], so impedance bridging is used insteadComplex conjugate matchingThis is used in cases in which the source and load are reactive. This form of impedance matching can only maximize the power transfer between a reactive source and a reactive load at a single frequency. In this case,
Zload = Zsource*
(where * indicates the complex conjugate).
If the signals are kept within the narrow frequency range for which the matching network was designed, reflections (in this narrow frequency band only) are also minimized. For the case of purely resistive source and load impedances, all reactance terms are zero and the formula above reduces to
Zload = Zsource
as would be expected.
Zload = Zsource*
(where * indicates the complex conjugate).
If the signals are kept within the narrow frequency range for which the matching network was designed, reflections (in this narrow frequency band only) are also minimized. For the case of purely resistive source and load impedances, all reactance terms are zero and the formula above reduces to
Zload = Zsource
as would be expected.
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