NOT for PUBLIC DISSEMINATION Wireless 103: Long
Total Page：16
File Type：pdf, Size：1020Kb
This is a BETA TEST version of this tutorial; NOT FOR PUBLIC DISSEMINATION Wireless 103: Long Links; Part 2: Antenna Theory Terminology: Radiation: You will see the words “radiate” and “radiation” here often. You may find yourself saying these words in public. Small minded people will panic. There are many kinds of radiation; the ones we are concerned with are Ionizing Nuclear Radiation and Non-Ionizing Electromagnetic Radiation. Ionizing Nuclear Radiation is atoms disassembling themselves. This is bad. This is not what we are talking about here. Non-Ionizing Electromagnetic Radiation is the topic of this tutorial. TV, FM, AM, CB, Cell phones and light. No cause for panic. Decibels: In the simplest possible terms, leaving out as much math as possible: decibels = 10 times the logarithm of ( power out now / power out before ) Why we use decibels: You can multiply by adding logarithms. You divide by subtracting. You don't have to keep track of decimal points in every number. • +3 db is twice the previous power, -3 db is half the previous power • +10 db = 10 times the original power, -10 db is .1 times the original power • +20 db = 100 times the original power, -10 db is .01 times the original power • +30 db = 1000 times the original power, -10 db is .001 times the original power • 10 db – 10 to the 1nd power • 20 db – 10 to the 2nd power • 30 db – 10 to the 3nd power +13 db is 20 times the original power, -13 db is .05 Again: multiply by adding. 10 db is times 10, 3 db is double, so +13 db is 2 X 10 the original power 3 dB is half, or double. We multiply by addition, so ( 3 db loss + 3 db loss ) = 6 db loss = ¼th of the original power. Antenna Types: An antenna is: • Omni directional, or • Directional An Omni directional antenna radiates a signal around itself in all directions. Viewed from the top, a 360 degree circle of RF ( Radio Frequencies ) of equal strength in all directions, with the antenna in the middle. Typically shortened to omni. Radiation pattern images courtesy of Trevor Marshall Azimuth means a horizontal circle, with 0 as the direction the antenna is pointed in. Elevation is a vertical circle. The blue line represents the same RF field strength in these images. A Directional antenna radiates a signal that is directed away from the back of the antenna. Viewed from the top, an elliptical shape with the antenna at one end. An Antenna has: • Gain or • No gain Gain is an increase, compared to something else. Gain is measured in decibels. The FCC and other regulatory bodies measure gain in reference to an Isotropic Radiator. All antennas have gain, compared to an Isotropic Radiator. An isotropic radiator is a “theoretical construct”: It can exist only in our minds. An isotropic radiator is: • An infinitely small point • In infinitely empty space • Radiating a single RF signal The signal radiated by an isotropic radiator would be spherical. The simplest antenna, the ¼ wave Marconi, has 2.14 dB gain over an isotropic radiator. Gain over an isotropic radiator is expressed in decibels as dBi. The Marconi has gain over an isotropic radiator because it does not radiate straight up or down. Gain over a ¼ wave Marconi is expressed as dBd. Gain in antennas is passive. Gain antennas do not increase signal power. High gain antennas direct the power applied to them in a more efficient fashion than lower gain antennas. What does increase is the power detected at the other end of the link. Polarization: An antenna can be polarized in four ways: Vertical, Horizontal, Left Hand Circular or Right Hand Circular. Vertical and horizontal polarization are two variations of Linear Polarization. They are common because vertical and horizontal are defined states. Vertical Polarization: You are watering the lawn with a garden hose. You find this boring. You shake the garden hose up and down just to see the resulting water shape in the air. You have created a Vertically Polarized wave: Vertical polarization and omni directional antennas work well together. Car radio antennas are vertically polarized. Horizontal Polarization: You shake the hose horizontally. You have created a Horizontally Polarized wave: Television signals are horizontally polarized. Circular Polarization: Consider a spacecraft in orbit. The spacecraft needs to spin around its axis to remain stable. The antenna is attached to the spacecraft. The polarity of the downlinked signal rotates as the spacecraft rotates. If the signal seen on the ground is rotating counterclockwise, the waveform has Left Circular Polarization: Left Hand Circular Polarization ( Counter clockwise ): If the signal seen on the ground is clockwise, the waveform has Right Hand Circular Polarization: Polarization Loss Factor: Vertically polarized antennas do not get all available signal strength when pointed at horizontally polarized antennas. LCP loses signal strength when connected to RCP: Note the 30 dB isolation between horizontal and vertical. You can use the same channels as your neighbors, without interference between networks, for a long link if you use horizontal polarization on the long link. Frequency and Wavelength: Frequency: Frequency is how often an event happens in a specified period of time. Radio Frequencies are electromagnetic waves that can be detected at a difference. Radio frequencies begin at about 100,000 cycles per second and end at about 300,000,000,000 cycles per second. Radio frequencies are defined by the carrier frequency of the radio transmitter. The carrier starts out as a sine wave. The sine wave is the simplest possible repeating waveform: This image is two cycles of a sine wave. Cycles per second is a phrase that makes sense in English, but not in other languages. The abbreviation Hz was adopted as a universal symbol for the concept of cycles per second. It is an abbreviation of the last name of early researcher Heinrich Hertz. Abbreviations are used to eliminate the zeros. • Kilo Hz = 1000 Hz • Mega Hz = 1000,000 Hz • Giga Hz = 1000,000,000 Hz. Pronounced Gig-uh, not Jig-uh as in the movie. The length of a cycle can be calculated. The length of one cycle is the wavelength of that frequency. Wavelength: Radio Frequency waves travel through space at the speed of light: 299,792,498 meters per second. For practical purposes, we say the speed of radio waves is 300,000,000 meters per second. This may also be expressed as 300,000 kilometers per second If radio waves travel 300,000,000 meters per second, and the wave has a frequency of 150 mHz - 150,000,000 cycles per second – then the length of one wave is 300,000,000 meters per second / 150,000,000 cycles per second = 2 meters We can simplify this formula by eliminating the zeros: Wavelength in meters = 300 / Frequency in mHz The maximum voltage or current of a sine wave occurs at points 180 degrees from each other; half of a cycle. Half of a cycle is half of a wavelength. The optimum length of a driven element – the part of an antenna which receives the RF from the transceiver – is ½ wavelength. The Greek symbol Lambda is used as shorthand for wavelength: 802.11 b, g and n ( lower case is proper ) signals use the 2.4 gHz band. 2.4 gHz is 2,400 mhz. The wavelength of a 2.4 gHz signal is: 300 / 2400 = 1/8 meter. 1 meter = 39.37”. Wavelength is used as a coarse tool to describe a band of frequencies that exhibit similar behavior. For our purposes, we can round that up to: Wavelength of the 2.4 gHz band = ( 1/8 X 40 ) = 5 inches The simplest antenna: The Marconi dipole. The simplest antenna is the Marconi ¼ wave dipole. The Marconi is a vertical metal rod, ¼ wavelength long, fed from the bottom. It is resting just above the earth, or a flat metal surface which functions as a ground plane. In an ideal world, the ground plane would be a circle with a ¼ wavelength or greater radius. The antennas on your car are Marconis. The image on the left shows how Marconis operate. The antenna is one element of a dipole. The earth, or the ground plane, is the other. The image on the right shows the Marconis major shortcoming. The dark blue region shows the shape of the RF pattern leaving the Marconi. Note that the strongest part of the RF field slopes up at a 20 degree angle. This is fine for HF – broadcast band and shortwave. Those signals go far, fly high, and bounce off the ionosphere. This gives good long range communication. 802.11 signals don't go that far. A Marconi would waste RF warming up the sky at 802.11 frequencies. Omni Directional Antennas: The rubber duck antennas on wireless routers are center fed dipoles. Center fed dipoles are ½ wavelength antennas. Dipole means two poles. You will note that rubber ducks usually have a bulge at the bottom. This bulge contains a Decoupling Sleeve. A decoupling sleeve is a copper or brass sleeve, 1/4 wavelength long, electrically connected to the coaxial cable inside the antenna where the coax stops and the driven element begins. This sleeve acts as the other end of a ½ wave dipole. If you see a disassembled wireless router or AP you will see that the antenna screws into a small cube about ½” on a side. There is no ground plane. A Marconi would not work. Omni directional gain antennas are omni antennas that develop a stronger signal than a Marconi. They do this by reshaping the pattern of the RF leaving the antenna, in the horizontal plane.