Antenna basics, antenna types, antenna functions
Before we get into the black magic that is wireless signal propagation, we need to understand a vital part of the industrial wireless system: antennas. Antennas are the means for coupling the transmitter to the medium, in this case, free space. An antenna is an electromagnetic radiator; it creates an electromagnetic field that proceeds out from the transmitting antenna to the receiver’s antenna, which then converts the electromagnetic wave into electrical signals that are applied to the receiver’s input stages.
There are several different types of antennas in three broad categories: omni-directional, directional, and semi-directional.
– Omni-directional antennas propagate in all directions.
– Semi-directional antennas propagate in a constricted fashion, defined by a specific angle.
– Directional antennas have a narrow “beam” that allows highly directional propagation; familiar types are the parabolic and Yagi. Each has unique characteristics and applications.
Propagation patterns are shown on a polar chart, the angle of propagation being limited to where the power level drops by 3 dB. Figure 1 shows a polar plot for different antennas; the half-power beamwidth for a Yagi antenna is shown.
Passive gain amplifies the signal
All antennas exhibit passive gain, which serves to amplify the signal. Passive gain is measured by the quantity dBi, which is the gain referenced to a theoretical isotropic antenna; an isotropic antenna transmits energy equally in all directions, and does not exist in nature. The gain of an ideal half-wave dipole antenna is 2.15 dBi. It should also be noted that as directionality increases, so does gain.
EIRP, or equivalent (or effective) isotropic radiated power, is the measure of the maximum power a theoretical isotropic antenna would emit in the direction of maximum antenna gain. EIRP accounts for losses from transmission lines and connectors, and includes actual antenna gain. EIRP allows calculation of real power output and field strength values, if actual antenna gain and transmitter output power are known.
Dipole antennas, rubber ducky
Dipole antennas are the most common type of antenna used and are omni-directional, propagating radio frequency (RF) energy 360 degrees in the horizontal plane. These devices are constructed to be resonant at a half or quarter wavelength of the frequency being applied. This antenna can be as simple as two pieces of wire cut to the proper length or can be encapsulated as shown in the illustration; this configuration is commonly referred to as a “rubber ducky” antenna. The dipole is used in many enterprise and small office and home office (SOHO) Wi-Fi deployments.
An antenna exhibits a typical impedance, allowing for matching of the antenna to the transmitter for maximum power transfer. If the antenna and transmitter are not matched, reflections will occur on the transmission line which will degrade the signal or even damage the transmitter. These reflections are described by the term standing wave ratio (SWR) and indicate the efficiency of the transmission line. SWR of 1:1 would indicate that no power is reflected and lost; 5:1 would indicate a reflection and loss of 44%. SWR is commonly used as a voltage ratio and referred to as VSWR.
Directional and semi-directional antennas focus radiated power into narrow beams, adding a significant amount of gain in the process. Antenna properties are also reciprocal. The characteristics of a transmitting antenna, such as impedance and gain, are also applicable to a receiving antenna. This is why the same antenna can be used for both sending and receiving. The gain of a highly directional parabolic antenna serves to amplify a weak signal; this is one reason why this type of antenna is frequently used for long distance links.
Patch antenna, microstrip antenna
A patch antenna is a semi-directional radiator using a flat metal strip mounted above a ground plane. Radiation from the back of the antenna is effectively cut off by the ground plane, enhancing forward directionality. This type of antenna is also known as a microstrip antenna. It is typically rectangular and enclosed in a plastic enclosure. This type of antenna lends itself to being manufactured by standard printed circuit board methods. Patch antennas are widely used semi-directionals; a patch antenna can have a beamwidth of between 30 to 180 degrees and a typical gain of 9 dB.
Sector antennas are another type of semi-directional antenna. Sector antennas provide a pie-shaped (sector) radiation pattern and are usually installed in what is known as a sectorized array. Beamwidth for a sector antenna can be between 60 to 180 degrees, with 120 degrees being typical. In a sectorized array, antennas are mounted back-to-back to provide full 360-degree coverage. Sector antennas are used extensively for cellular communication.
A commonly used directional antenna is the Yagi-Uda Array, usually just called a Yagi. It was invented by Shintaro Uda and his colleague, Hidetsugu Yagi, in 1926. A Yagi antenna uses several elements to form a directional array. A single driven element, typically a dipole, propagates RF energy; elements placed immediately in front of and behind the driven element re-radiate RF energy in phase and out of phase, enhancing and retarding the signal, respectively. The elements are called parasitic elements; the element behind the driven element is called the reflector, while the elements in front of the driven element are called directors. Yagi antennas have beamwidths in the range of 30 to 80 degrees and can provide well in excess of 10 dBi passive gain. A multi-element high-gain Yagi is shown in Figure 4.
Parabolic or dish antenna
Parabolic, or dish, antennas are the most familiar type of directional antenna. A parabola is a symmetric curve; a parabolic reflector is a surface that describes that curve throughout a 360-degree rotation—a dish or, to use the technical term, a paraboloid. A parabolic reflector has a high degree of directivity and has the ability to focus RF energy into a beam, much like a flashlight. Parabolic antennas have a very narrow beamwidth, usually not exceeding 25 degrees. Gain is dependent on diameter and frequency; at 2.4 GHz, a 1 meter dish will provide about 26 dBi gain, while a 10 meter antenna will provide 46 dBi gain at the same frequency. The antenna is “fed” by either a half wave dipole antenna or a feed horn. Parabolic antennas are used for long distance communication links between buildings or over large geographic areas. Very large parabolic antennas are used for radio astronomy and can provide gain of 10 million or about 70 dBi.
A variation of the dish is the grid antenna. Given that a parabolic reflector will present a large solid surface to the wind, it follows that high or even moderate wind conditions will cause the dish to move out of alignment or deform. To prevent this from happening, the reflector is perforated into a grid. The spacing of the grid elements is frequency dependent; it is inversely proportional to the frequency. Gain and beamwidth are similar to the parabolic antenna.
– Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA). Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, firstname.lastname@example.org.
See other wireless tutorials from Capano on the wireless revolution, radio frequency basics, and comparative modulation: Spread spectrum modulation terms and definitions for wireless networking
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.