FIGURE 7-1 At A, the electric (E) and magnetic (M) lines of flux around a straight, current-carrying wire. At B, the flux lines far away from a current-carrying wire.
The proliferation of wireless devices in recent years has not been accompanied by the level of radio-frequency (RF) spectrum management (a fancy term for technological law and order) necessary to ensure compatibility and freedom from mutual interference. Your electric meter might mess up your wireless router. A compact fluorescent lamp (CFL) in a garage-door opener’s motor box can interfere with the link between the remote and the motor. A cordless phone set can make an FM radio buzz when the two devices operate near each other. The list goes on.
The following experiment is easy to do, even though the theory gets sophisticated. You’ll need a computer with an Internet connection. You’ll also need a 12-foot, two-wire audio cord with a 1/8-inch monaural phone plug on one end and spade terminals on the other end (Radio Shack part number 42-2454, for example). If you can’t find that component, you can connect a 12-foot length of two-wire speaker cable to a 1/8-inch monaural phone plug.
The AC electricity in the utility grid produces obvious effects on appliances: glowing lamps, blowing fans, and chattering television sets. This AC also produces electromagnetic (EM) fields that aren’t apparent to the casual observer. The presence of this EM energy causes tiny currents to flow or circulate in any object that conducts electricity, such as a wire, a metal rain gutter, the metal handle of your lawn mower, and your body. You can easily put together a device that will detect these fields and produce a graphic display of their characteristics.
A current-carrying wire is always surrounded by theoretical electric flux lines and magnetic flux lines. Around a straight span of wire, the electric or E flux lines run parallel to the wire, and the magnetic or M flux lines encircle the wire, as shown in Fig. 7-1A. If the wire carries constant DC, the electric and magnetic fields are static, meaning that the E and M fields stay the same all the time. If the wire carries AC or pulsating DC, the fields fluctuate. The varying E field gives rise to a changing M field, which in turn generates another varying E field. As the E and M fields regenerate each other, a “hybrid field” (the EM field) travels away from the wire, perpendicular to both sets of flux lines at every point in space, as shown in Fig. 7-1B.
FIGURE 7-1 At A, the electric (E) and magnetic (M) lines of flux around a straight, current-carrying wire. At B, the flux lines far away from a current-carrying wire.
All EM fields display three independent properties: amplitude, wavelength, and frequency. The amplitude is the intensity or strength of the field. The frequency is the number of full AC or pulsating DC cycles per second. The wavelength is the distance in space between identical points on adjacent waves. At 60 Hz, the AC utility frequency in the United States, EM waves measure 5000 kilometers (approximately 3100 miles) long in free space (air or a vacuum).
You can use your computer to “look at” the EM fields that drench the space all around you. A simple freeware program called DigiPan, available on the Internet, can provide a real-time, moving graphical display of EM field components at frequencies ranging from 0 Hz (that is, DC) up to around 5500 Hz. Here’s the website: www.digipan.net.
DigiPan shows the frequency along a horizontal axis, while time is portrayed as downward movement all across the whole display. Figure 7-2 shows this scheme. The relative intensity at each frequency appears as a color. You can adjust the colors to suit your taste. If there’s no energy at any frequency in the program’s range, the display is black. If there’s a little bit of energy at a particular frequency, you’ll see a thin, vertical blue line creeping straight downward, if you leave the program set for its default color scheme. If there’s a moderate amount of energy, the line turns yellow. If there’s a lot of energy, the line becomes orange or red. The developers and users of DigiPan and similar programs have coined the term waterfall for the display because of its appearance when signals exist at numerous frequencies.
FIGURE 7-2 The DigiPan display system. The horizontal axis portrays frequency. Time “flows” downward across the whole screen. Signals show up as vertical lines. This drawing shows two hypothetical examples.
To observe the EM energy on your computer, you’ll need an antenna. Cut off the U-shaped spade lugs from the audio cord with a scissors or diagonal cutter. Separate the wires by pulling them apart along the entire length of the cord, so that you get a 1/8-inch monaural phone plug with two 12-foot wires attached.
Insert the phone plug into the microphone input of your computer. Arrange the two 12-foot wires so that they run in different directions from the phone plug. You can let the wires lie anywhere, as long as you don’t trip over them! This arrangement will make the audio cord behave as a dipole antenna to pick up EM energy.
Open the audio control program on your computer. If you see a microphone input volume or sensitivity control, set it to maximum. Set your computer to work with an external microphone, not the internal one. If your audio program has a “noise reduction” feature, turn it off. Set the microphone gain (or input sensitivity) to maximum. Then launch DigiPan (it might take 30 seconds or so to load) and follow these three steps, in order.
1. Click on “Options” in the menu bar and uncheck everything except “Rx.”
2. Click on “mode” in the menu bar and select “BPSK31.”
3. Click on “View” in the menu bar and uncheck everything.
Once you’ve carried out these steps, the upper part of your computer display should show a jumble of text characters on a white background. The lower part of the screen should be black with a graduated scale at the top, showing numerals 1000, 2000, 3000, and so on. Using your mouse, place the pointer on the upper border of the black region and drag that border upward until the white region with the distracting text vanishes.
If things work correctly, you should have a real-time panoramic display of EM energy from zero to several thousand hertz. Unless you’re in a remote location far away from the utility grid, you should see vertical lines of various colors. These lines represent EM energy components at specific frequencies. You can read the frequencies from the graduated scale at the top of the screen. Do you notice a pattern?
A pure AC sine wave appears as a single pip or vertical line on the display of a spectrum monitor (Fig. 7-3A). This pip means that all of the energy in the wave is concentrated at one frequency, known as the fundamental frequency. But many, if not most, AC utility waves contain harmonic energy along with the energy at the fundamental frequency.
FIGURE 7-3 At A, a spectral diagram of pure, 60-Hz EM energy. At B, a spectral diagram of 60-Hz energy with significant components at the second, third, and fourth harmonic frequencies.
A harmonic frequency is a whole-number multiple of the fundamental frequency. For example, if 60 Hz is the fundamental frequency, then harmonics can exist at 120 Hz, 180 Hz, 240 Hz, and so on. The 120 Hz wave is at the second harmonic, the 180 Hz wave is at the third harmonic, the 240 Hz wave is at the fourth harmonic, and so on. In general, if a wave has a frequency equal to n times the fundamental where n is some whole number, then that wave is called the nth harmonic. (The fundamental is the first harmonic by definition.) In Fig. 7-3B, a wave is shown along with its second, third, and fourth harmonics, as the entire “signal” would appear on a spectrum monitor.
When you look at the EM spectrum display from zero to several thousand hertz using DigiPan, you’ll see that utility AC energy contains not only the 60-Hz fundamental, but many harmonics. When I saw how much energy exists at the harmonic frequencies in and around my house, my amazement knew no bounds. I had suspected “dirt in the ether,” but not that much! Figure 7-4 shows my own DigiPan display of dirty electricity from 60 Hz to more than 4000 Hz. Each vertical trace represents an EM signal at a specific frequency. If the electricity were “perfectly clean,” you’d see only one bright vertical trace at the extreme left end of the display.
FIGURE 7-4 Dirty electricity at the author’s home as viewed on DigiPan.
Try This!
Place a plug-in type vacuum cleaner near your EM pickup antenna. Switch the appliance on while watching the DigiPan waterfall. When the motor first starts up, do curves suddenly appear on the display, veering to the right and then straightening out as vertical lines? Those contours indicate energy components that increase in frequency as the motor “revs up” to its operating speed and maintain constant frequencies thereafter. When the motor loses power, do the motor’s vertical lines curve back toward the left before they vanish? Those curves indicate falling frequencies as the motor slows down. Try the same tests with a hair dryer, an electric can opener, or any other appliance that plugs into a wall outlet and contains an electric motor. Which types of appliances are the “noisiest”? Which are “quietest”?
If you conduct some serious Internet-based research into this subject, you’ll come across a lot of sites that warn about health hazards posed by dirty electricity. Some sites will give you case histories, horror stories, and wild tales about illnesses, pains, and cancer, and attribute all of the trouble to dirty electricity. How serious is the danger, really? As a former radio-frequency (RF) engineer and antenna specialist, the best answer I can give you is “I don’t know.” I suspect, however, that if the “dirt” in dirty electricity has adverse health effects, the “clean” part, which produces far stronger EM fields, probably does as well, and to a far greater extent. So any efforts to “clean up” electricity, in the hopes of getting rid of its potential ill effects, are probably futile.
Nowadays, most landline-connected telephone sets are of the cordless type. Rare indeed is the old table-top or wall-mounted phone set with a coiled cord between the receiver and the base unit! Cordless phones offer convenience; that’s their main asset. But they also have certain limitations and shortcomings.
The wireless link in a cordless phone set uses a digital encoding system designed to optimize the sound quality, minimize the effects of interference, and make it difficult for people to eavesdrop on your conversations. That hasn’t always been the case.
In the early days of cordless phone technology (the 1980s), the handsets operated at radio frequencies around 1.7 MHz, immediately above the standard AM broadcast band. Power-line interference, poor audio quality, and eavesdropping took place as a general rule! If you had an all-wave or “shortwave” radio receiver, you could tune it to those frequencies and, if you lived in a populated area, hear two or three conversations at any given time! You might even access someone else’s line with your handset, or have someone else take advantage of your line.
If you buy a cordless phone set today, it will probably function at 1.9 gigahertz (GHz), where 1 GHz equals 1000 MHz or 1,000,000,000 Hz. Some of them work at 2.4 GHz or 5.8 GHz. These signal waves are only a few inches long as they travel through the air between the handset and the base unit. Instead of a whip antenna a couple of feet long, the handset has a tiny “stub” antenna or an internal antenna. Instead of conventional AM, which the earliest cordless phones used, the new ones employ a technology called digital spread spectrum (DSS), which makes it almost impossible for anyone to eavesdrop on your conversations or gain access to your phone line without your consent.
Despite all the advanced technology that cordless phones use (or maybe because of it), you can expect some glitches with their operation. Here’s a short list of problems and suggestions for addressing them.
• Bad battery—Weak or dead batteries can pose a problem with cordless phones more than a couple of years old. The rechargeable lithium-ion batteries in the handset normally last three to five years, and then you’ll have to replace them.
• They need utility power—If you experience a power failure, such as might happen during and after a storm, the base unit can’t transfer the signals between the landline and the handset(s), so your cordless phone won’t work unless you have a backup power source such as an emergency generator.
• Poor range—In buildings with a lot of concrete and steel, you shouldn’t expect to get the advertised working range between the base unit and the handset(s). If you place the base unit in a subterranean location such as a cellar, the handsets probably won’t work very well outside the house.
• Whooshing and fading—When you get near the edge of a handset’s useful working range, or if signals happen to conflict with each other as they bounce around among the electrical wires and other metal objects in your house, you’ll hear hissing or whooshing sounds, and might even get completely disconnected without warning. If this problem occurs, move closer to the base unit and stay in one place and position.
• Interference from other devices—As the number of wireless devices in our everyday lives keeps increasing, the risk of them interfering with each other grows as well. It can be difficult to figure out where the source of electromagnetic interference (EMI) lies, but if you happen to bring two conflicting devices within a few inches of each other, you’ll get a good clue.
• Bad audio—Some cordless handsets simply sound terrible. The only thing you can do in a case like that, other than endure the problem, is return the phone system for an exchange to another brand, or for a refund of your money.
• Inadvertent contact with buttons—In the old days, you could hold a phone handset against your ear and shoulder, freeing up both hands. Not now! If you try that with a cordless handset that “bristles with buttons,” chances are good that your chin or cheek will actuate one of them, and you’ll hear tones or beeps that seem to come out of nowhere. You might even accidentally hang up on the person you’re talking with.
Wireless telephone sets operate in a specialized communications system called cellular. Originally, the cellular communications network served mainly traveling business people. Nowadays, most folks regard cell phones as necessities, and most cell-phone sets have extra features, such as text messaging programs, Web browsers, video displays, and built-in cameras.
A cell phone looks like a hybrid between a cordless telephone handset and a “walkie-talkie,” but smaller. Some cell phones have dimensions so tiny that an unsuspecting person might mistake them for packs of chewing gum. A cell-phone unit contains a radio transmitter and receiver combination called a transceiver. Transmission and reception take place on different frequencies so that you can talk and listen at the same time, and easily interrupt the other party, if necessary, a communications capability known as full duplex.
In an ideal cellular network, every phone set always lies within range of at least one base station (also called a repeater), which picks up transmissions from the portable units and retransmits the signals to the telephone network, to the Internet, and to other portable units. A so-called cell encompasses the region of coverage for any particular repeater, also known as a base station.
When a cell phone operates in motion, say while you ride in a car or on a boat, the set can move around in the network, as shown in Fig. 7-5. The dashed curve represents a hypothetical vehicle path. Base stations (dots) transfer access to the cell phone among themselves, a process called handoff. The hexagons show the limits of the transmission/reception range (or cell) for each base station. All the base stations are connected to the regional telephone system, which, in turn, goes to the major telephone networks. Therefore, from a cell phone on a ranch near Bozeman, Montana (for example), you can place calls to, or receive calls from, almost anyone else in the world.
FIGURE 7-5 In an ideal cellular system, a moving cell-phone set (dashed line) always remains within range of at least one base station.
Cellular connections sometimes suffer from connection problems when signals transfer from one repeater to another. A technology called code-division multiple access (CDMA) reduces the prevalence of this problem compared to the early years of cellular technology. In CDMA, the repeater coverage zones might overlap, but signals don’t interfere with each other because every phone set possesses a unique signal code. Rather than abruptly switching from one base-station zone to the next, the signal goes through a region in which it flows through more than one base station at a time. This make-before-break scheme helps to mitigate cell-transfer trouble.
As cell phones grow increasingly sophisticated, they can do more and more things. The earliest cell phones were merely glorified “walkie-talkies.” Today’s smartphones allow you to send and receive text messages, browse the Internet, exchange e-mails, create and view photos and videos, manage bank accounts, compare prices in department stores, pay for items at store checkouts, and much more. Along with the versatility and convenience comes a downside: A lot of things can go wrong with these devices. Let’s look at the most common complaints.
• Connections break up, or completely disconnect, whenever a cell phone set is not well within the range of at least one repeater, or in rare instances where repeaters conflict. Breakup can also take place if the user moves from a location with a “clear shot” to the nearest repeater (such as an open field) to a location with abundant obstructions (such as a valley or a city street among massive buildings).
• You’ll need to keep the battery in good working order and keep it charged, especially if you plan to use the cell phone while traveling. You’ll want it to work in a roadside emergency. It’s easy to forget this detail, so make it a habit! Also, as with most other battery-powered devices, the battery in a cell phone lasts only three to five years. When the battery dies, you’ll have to replace it or upgrade to a new phone.
• Cell phones should not be used while plugged into a charging unit. A few years ago, someone told me a story about a person who got killed when a charging phone exploded in his face! I don’t know if the storyteller was “pulling my leg” or not. Lithium batteries don’t explode very often, but they can blow up, especially if a fully or almost fully charged one gets shorted out.
• Cell phones are somewhat more vulnerable than cordless phones to conflicts with other wireless devices, mainly because cell phones have greater range than cordless handsets do. You’ll do best to avoid using cell phones too close to wireless computer peripherals, smart utility meters, active radio transmitters, and sensitive radio receivers.
• Bad audio quality can occur with cordless phones. You can’t do much about this problem except get a different phone.
• Depending on where you plan to use your phone, you might need a unit that offers exceptionally loud output audio. Make sure, before you buy a phone set, that it produces enough sound, and in a clear enough state, to satisfy you.
• Some older cell phones have a problem with overly sensitive buttons causing unwanted responses, the same sort of thing that can happen with cordless phones. Newer cell phones have few (or no) exposed buttons that can inadvertently contact your face and cause an unintended reaction.
• Some cell phones “obey” your fingertips better than others do. It’s the same sort of variability that you know about if you’ve used tablet computers and electronic book (e-book) readers. Before you buy a cell phone set, try to use it in real time. Can you easily send texts with it? Does it accurately register numbers and characters as you enter them in?
• The display might wash out in sunlight, not be bright enough in darkness, or be otherwise hard to read. If your phone set doesn’t have a display that you like, you should try to exchange it. Ideally, you should check out all the different displays when you’re in the “cell phone store” shopping for a set.
• In the physical sense, some cell phones are a great deal more rugged than others. You can drop some sets onto a concrete floor from shoulder height and they’ll suffer no damage. But few sets can take that kind of abuse, and some are so fragile that you might wonder whether they’re meant for use in weightless environments only.
Fact Or Myth?
• Does extensive use of a cell phone increase the risk of brain cancer?
• Experts disagree on this issue. Some studies seem to show a correlation between cell phone use and brain tumors. However, other studies show no correlation, and a few have come up with a negative correlation, as if cell phone use might actually protect the brain! I don’t know the answer to this mystery.
Warning! Cell phone use is illegal in certain locations and situations. Always make sure that you know when you’re in one of those scenarios, and switch that little thing off. Also, in some parts of the world (and increasingly in the United States), you can’t legally use a cell phone while driving a motor vehicle.
In recent years, tablet devices have grown immensely popular. The simplest ones, called e-book readers, are meant only to display electronic books (e-books) downloaded from the Internet with a computer and then transferred to the tablet with a special cable. Some of these devices can access the Internet by means of Wi-Fi, which employs wireless routers located in homes and businesses with Internet access. Examples of Wi-Fi-equipped e-book readers include Amazon’s Kindle Fire and Barnes and Noble’s Nook Tablet (as of late 2012). The most sophisticated tablet devices comprise small computers with touch screens having diagonal measures of 7 to 12 inches. The flagship example is the Apple iPad. Similar devices are manufactured by Microsoft, Samsung, and others. They can access the Internet in either or both of two ways: by means of Wi-Fi or by means of the cellular network.
The term Wi-Fi comes from the expression “wireless hi-fi.” Since its origins, Wi-Fi has evolved into a technology used almost entirely with personal computers and tablets. Basically, if you connect a wireless router to a regular Internet modem, such as the one you get as part of a cable or satellite installation, you have your own Wi-Fi hotspot. The maximum working range of a wireless router rarely exceeds 100 feet (about 30 meters), and unless you have an ideal installation, you shouldn’t expect it to cover a radius of more than half that.
These days, you’ll find Wi-Fi hotspots in public libraries, hotels, motels, restaurants, bars, and airports. Even a few fast-food places and department stores have them. You can bring your notebook computer or Wi-Fi-equipped tablet device into such a place, obtain the password from one of the employees, and get on the Internet, sometimes with quite respectable download and upload speeds. The quality of a Wi-Fi hotspot connection depends on the quality of the router that the establishment has installed, the speed of their own Internet connection, and the number of other users taking advantage of the hotspot along with you.
When you use any public Wi-Fi hotspot, you should keep in mind the fact that your device, whether it’s a sophisticated notebook computer or a simple e-book reader or anything in between, is effectively networked (connected to) all the other devices using the same hotspot. If one of those other users is somebody who likes to snoop around in other people’s Internet business and has the technical knowledge to do it (or, to use the standard jargon, a hacker), then he or she can not only observe everything you do online, but also take over your computer or corrupt its contents.
Aside from the potential problems you might have with hackers at major hotspots, you might encounter one or more of the following glitches.
• In densely populated areas, some people drive or walk around with portable computers, looking for unsecured wireless routers at residences (hopefully never yours) so they can access the Internet through them. Use a good password with your wireless router. A good password comprises letters and numerals in a random sequence. Never operate any wireless router without a password. Although passwords can’t guarantee protection against roving hackers, they greatly minimize the risk because these nosy folks usually look for easy (unsecured) targets and skip over the more difficult (secured) ones.
• Once in awhile, you’ll find that a device won’t access the wireless router when you have another device online with it already. I have two Wi-Fi-equipped e-book readers. When one of them is online, the other one will usually connect, but not always! The only cure for this trouble, should you ever experience it, is to avoid using both of the incompatible devices with Wi-Fi at the same time.
• If your router is located in a bad spot, surrounded by too many obstructions or other electronics devices, you might get far less than the advertised working range for your home hotspot. Try relocating the router to a room near the center of the house or on an upper floor. Don’t put it in the basement, though.
• In certain locations, other wireless devices’ signals will create so much interference that your router’s signal is drowned out except when you use a tablet or computer within a few feet of it. If you can find the source of the interference and power it down, consider yourself lucky. In most cases you’ll never manage to isolate or identify the source, or even if you do, you won’t be able to do anything about it.
• Once in awhile, a router will lose contact with the modem that serves it, and won’t be able to “figure out” how to identify it and connect with it again. In that case, switch off the router and the modem, wait a couple of minutes, then switch the modem back on until it connects to the Internet, and finally switch the router back on until it connects to the modem. You’ll be able to tell when a modem or router has established its connection by watching the lights on it. A certain pattern of steady and flashing lights will tell you that the device has connected. The exact pattern varies from device to device; you’ll get familiar with yours after you’ve used it for awhile.
The Global Positioning System (GPS) is a network of radiolocation and radionavigation apparatus that operates on a worldwide basis. The system employs numerous satellites, and allows you to determine your location on the earth’s surface, and in some cases, your altitude above the surface as well.
All GPS satellites transmit signals that have wavelengths on the order of a few inches. The signals carry special codes that contain timing information used by the receiving apparatus to make measurements. A GPS receiver determines its location by measuring the distances to three or four satellites (preferably four) by precisely timing the signals as they travel between the satellites and the receiver. The process resembles the triangulation used in old-fashioned navigation, except that with the GPS, it takes place in three-dimensional space rather than on the two-dimensional surface of the earth.
The GPS receiver uses a computer to process the information received from the satellites, which follow circular orbits several thousand miles up. From this information, the GPS unit can give you an indication of your geographical position. For individual users, the accuracy of the positioning readout varies slightly, depending on the relative positions of the satellites with respect to the user’s location. The larger the number of satellites involved, the more accurate the readout.
Because it’s a radio receiver, a GPS unit always requires an antenna. The type of antenna depends on the situation. Figure 7-6 shows two common antenna types found on cars and trucks. The whip design, shown at A, can have a magnetic mount similar to the hardware used with CB and small amateur radio antennas, or it can have a mounting that sticks to a rear window (if that window is near enough to horizontal). The streamlined design, shown at B, is more damage-resistant than the larger type, but doesn’t offer as much sensitivity. More sophisticated antennas are needed for boats and aircraft; they must withstand corrosive environments or extreme airspeeds. Handheld GPS units have antennas built-in, and look like cell phone sets.
FIGURE 7-6 Automotive GPS antennas. At A, whip type; at B, low-profile type.
An increasing number of new automobiles, trucks, and boats have GPS receivers preinstalled. If you are driving in a remote area and you get lost, you can use the GPS system to locate your position. Then, with the aid of a cellular telephone (or an amateur or CB radio, if you’re out of cell phone range), you can call for help and inform authorities of your location. Arguably, every motor vehicle and boat should have preinstalled GPS and cellular communications equipment to enhance the safety of passengers.
Unless you’re a professional electronics technician, you won’t be able to do anything “on your own” about malfunctioning GPS equipment in your vehicle, unless the trouble is caused by a radio transmitter. If that’s the case, you’ll know right away when you press the microphone button and start talking!
Your GPS system shouldn’t have any problems with the signals from CB radios because your CB transmitter outputs only a few watts unless you’re a freebander (one of those folks who flouts the law and uses an amplifier).
If you’re one of the few amateur radio operators who have a high-power transmitter in a motor vehicle or on a boat, you might experience GPS glitches when you transmit on certain frequencies. The only sure-fire cure for this problem is to avoid transmitting with the radio when you want to use your GPS system!
Global Positioning System users complain fairly often about three problems aside from conflicts with mobile radio transmitters. You can sometimes resolve these bugaboos with the help of service technicians. In other cases, you’ll have to get rid of your old system and buy a new one.
1. Incorrect coordinates or location display—Unless the receiver can clearly “see” at least three (and preferably four) satellites, your system might show you an incorrect location. In most cases the discrepancy will be so great as to make itself obvious. For example, you might be driving on a Texas highway, but your system will tell you that you’re in Saskatchewan. A good system will give no reading unless it can get good signals from the necessary number of satellites. Once in awhile, a bad antenna or a defective receiving unit will cause you to see an improbable or ridiculous location display or set of coordinates. A technician can check your receiver and antenna to see if they’re working okay, and can suggest repairs or modifications in case they aren’t.
2. Slow responsiveness—If your GPS unit is “competing” with a lot others in your general area, you’ll observe a long delay in the response, or your system will tell you where you were a few minutes ago, rather than where you are at the moment. There’s not much you can do in a situation like this if you’re in motion, except pull over to the side of the road and wait until your GPS display “catches up.” You can be reasonably sure that it’s “caught up” to you when it, like you, comes to rest! This problem is most likely to occur in and around large cities where GPS and other wireless usage is heavy.
3. “Frozen” display or coordinates—If your GPS receiver loses contact with the network, the coordinates or display will “freeze” or even disappear altogether. If you’re driving along a highway at high speed and you’re used to watching a moving position indicator, and instead you see that it has come to a stop, you’ll know that something has gone wrong. The receiver might have moved out of a line of sight with enough satellites, or you might have a weak receiver or a bad antenna. If you experience the problem often, or when you wouldn’t expect a loss of signal, or if it happens more and more often in the same general location, you should take your system to a technician and have it checked out.
Wireless technology is used in access-control devices, circuits, and networks. Systems range from simple machines, such as card readers or keypad-entry devices, to sophisticated electromechanical networks. Let’s look at three examples of wireless devices that can help you protect your property from unwanted human visitors.
In a knowledge-based security system, authorized people are issued numerical codes. The entrances to your property have locks that disengage when the proper sequence of numbers is punched into a keypad. This keypad can be hard-wired into the system, or it can be housed in a box about the size of a cell phone. It works like a bank automatic-teller machine (ATM) personal identification code.
You might decide that you need only one access code, which you can give to all the people that you want to authorize. Alternatively, you can issue a different code to each authorized person. The term “knowledge-based” arises from the fact that, in order to gain entry to your property, a person must know a specific piece of information (in this case the access code).
One of the main advantages of this type of system is that the codes cannot easily be guessed. The authorized people should memorize their access numbers. The numbers should never be written down in any form that will give away their meaning or purpose. Another asset of knowledge-based security systems is their relatively low cost.
A disadvantage of this scheme is that access codes occasionally leak out. People tend to give secrets away when situations arise that make it expedient to do so (or if it becomes inexpedient not to). Also, codes are sometimes forcibly stolen. Once a code is stolen, anyone who has it can get into the property until that code is invalidated.
A possession-based security system requires authorized people to possess some physical object that unlocks the entry to your property. Magnetic cards are a popular form of possession-based security device. You insert the card into a slot, and a microcomputer reads data encoded on a magnetic strip. This data can be as simple as an access code of the sort you punch on a keypad. Or it might contain many details about you. A so-called smart card can be used for security purposes. So can bank ATM cards, credit cards, and radio-frequency identification (RFID) cards.
A passive transponder provides a wireless form of possession-based security system. It’s a magnetic tag that authorized people can wear or carry. They’re the same little things that department stores employ to deter petty thieves. The transponder can be read from several feet away.
A bar code tag is another form of passive transponder. You’ll see them in stores, where they are used for pricing merchandise and keeping track of inventory. Bar coding allows instant optoelectronic identification of objects. A bar-code tag has parallel bands of various widths. More sophisticated tags have complex patterns of black shapes on a white background. A laser rapidly scans the pattern. The dark regions absorb the laser light, while the white regions reflect the light back to a sensor. The sensor thereby receives a binary data signal unique to the pattern on the tag. This signal can contain considerable information even when the pattern on the tag seems simple.
The main advantage of possession-based security systems is convenience. You need not worry about forgetting a code number. But this advantage is more or less nullified by the fact that little plastic cards can easily get misplaced, and they’re easy to steal. They’re also easy for unscrupulous store employees to “catch and copy.”
A biometric security system gets its name from the fact that it detects and acts on certain biological characteristics of people authorized to enter a property. For example, it might employ a camera along with a pattern-recognition computer program to check a person’s facial contours against information in a gigantic database. The machine might use speech recognition to identify people by breaking down the waveforms of their voices. It might record a hand print, fingerprint, or iris print. It might even employ a combination of all these things. A computer analyzes the data obtained by the sensors, and determines whether or not the person is authorized to enter the premises.
Wherever security requirements are so strict that a biometric security system is deemed necessary, a few brilliant, reckless rogues intent upon defeating it and getting into the premises will doubtless exist, waiting patiently, continuously, and endlessly for an opportunity to strike. A government installation in a hostile country is a prime example of such a property. While the system can be almost impossible to fool, it must also be set up so that a brute-force surprise attack will not likely overcome it. History has shown that this level of security is nigh impossible to attain, and at the same time, ensure that no authorized personnel are falsely arrested or injured as a result of a system error.
For homeowners and small businesses, biometric systems are generally too expensive. But exceptions to this rule do occur. Top-secret archives, priceless works of art, and some scientific experiments (and research personnel) justify the most sophisticated security systems available, no matter what the cost.
In addition to access control, you might want to install a security system that can detect the presence of an intruder who happens to “get past the gate.” Such systems can interconnect with telecommunications networks to alert the police when a security breach occurs, or the fire department in case of a fire.
The simplest device for detecting an unwanted visitor (besides a smart dog) is an electric eye. Narrow beams of visible light shine across all reasonable points of entry, such as doorways and window openings. A photodetector receives energy from each beam. If the photodetector stops receiving its assigned beam, an alarm is actuated. The main problem with this system is that a power failure can trigger the alarm unless the entire system has a backup battery.
The typical electric eye has a light source, usually a laser diode, and a light sensor, such as a photoelectric or photovoltaic (“solar”) cell. These devices are connected into an actuating circuit, as shown in Fig. 7-7. When something interrupts the light beam, the voltage or current passing through, or generated by, the sensor changes dramatically. An electronic circuit detects this voltage or current change. Using amplifiers and switches, even the smallest change can be harnessed to control massive machines.
FIGURE 7-7 Functional diagram of an electric eye.
Many popular intrusion alarm devices make use of infrared (IR) motion-detection transducers. Two or three wide-angle IR pulses are transmitted at regular intervals; these pulses cover most of the room in which the device is installed. A receiving transducer picks up the returned IR energy, normally reflected from the walls, the floor, the ceiling, and the furniture, as shown in Fig. 7-8. The intensity of the received pulses is measured and recorded by a microprocessor. If anything in the room changes position, the intensity of the received energy will vary, and the resulting signal will set off an alarm. These devices consume very little power in regular operation, so batteries can serve as the power source. A typical alarm system of this kind uses six or eight small electrochemical cells, which will operate the device continuously for several months.
FIGURE 7-8 Functional diagram of an infrared (IR) motion detector.
Infrared devices can detect changes in the indoor environment in another way: the direct sensing of IR, often called “radiant heat,” emanating from objects. Humans, and all warm-blooded animals, emit some IR energy. (So does fire, of course.) A simple IR sensor, in conjunction with a microprocessor, can detect rapid or sudden increases in the amount of “radiant heat” present in a particular place. The time threshold can be set so that gradual changes, such as might be caused by the sun warming a room, do not trigger the alarm, while rapid changes, such as a person entering the room, will set it off. The temperature-change (increment) threshold can be set so that a small pet will not actuate the alarm, while a full-grown person will do it.
An IR presence detector, like the IR motion detector, can operate from batteries, and in fact, consumes even less power than the motion detector because no IR pulse generator is required. The main problem with “radiant heat” detectors is that they can be fooled. False alarms pose a significant risk; for example, the sun might suddenly come out on a cloudy day and shine directly on the sensor and trigger the alarm. It’s also possible that a person clad in a winter parka, boots, hood, and face mask, entering from a subzero outdoor environment, might fail to set off the alarm. For this reason, radiant-heat sensors are used more often as fire detectors and alarm actuators than as intrusion detectors.
Motion in a room can be detected by sensing the changes in the relative phase of acoustic waves. An ultrasonic motion detector contains a set of transducers, which resemble loudspeakers that work above the human hearing range. Another set of transducers, which resemble microphones for ultrasound, picks up the reflected acoustic waves, which measure only a fraction of an inch long in the air. If anything in the room changes position, even by a tiny bit (like the diameter of a penny), the relative phase of the waves, as received by the various acoustic pickups, will change. This data goes to a microprocessor, which can trigger an alarm and/or notify the police.
In wireless systems, electromagnetic noise that comes from outside is called external noise. The more sensitive the receiving equipment, and the longer the distance over which it has to work, the more significant this type of noise becomes. If you’re interested in shortwave, CB, or amateur radio, this section will apply especially to you! But as ordinary household wireless devices grow more sophisticated and sensitive, the following terms and concepts will become important to everybody. Put your techie hat on, and let’s forge ahead into the deep realms of Brother Edsel Murphy, the (fictitious) mad scientist who coined Murphy’s Law.
Murphy’s Law
The short, and most commonly invoked, version of Murphy’s Law states that “If something can go wrong, it will.” Some of the more cynical technical people among us might add a corollary that says “If something cannot go wrong, it will.”
Noise from outer space is known as cosmic noise. It occurs all throughout the entire electromagnetic spectrum, from the very-low-frequency (VLF) radio band where waves are kilometers long to the realm of x rays and gamma rays where the waves measure a tiny fraction of a millimeter long. At the lower frequencies, the ionized upper atmosphere of our planet prevents the noise from reaching the surface. At some higher frequencies, the lower atmosphere prevents the noise from reaching us. But at many frequencies, cosmic noise arrives at the surface at full strength.
Cosmic noise can be identified by the fact that it correlates with the plane of the Milky Way, our galaxy. The strongest galactic noise comes from the direction of the constellation Sagittarius (“The Archer”) because this part of the sky lies on a line between our Solar System and the center of the galaxy. Galactic noise was first noticed and identified by Karl Jansky, a physicist working for the Bell Laboratories in the 1930s. Jansky conducted experiments to investigate and quantify the earth’s atmospheric noise at a wavelength of about 15 meters, or a frequency of 20 MHz. He found some radio noise that he couldn’t account for, and then he noticed that its orientation correlated with the location of the Milky Way in the sky. Jansky’s antenna was a simple affair like the ones used by amateur radio operators.
Galactic noise, along with noise from the sun, the planet Jupiter, and a few other celestial objects, contributes to most of the cosmic noise arriving at the surface of the earth. Other galaxies radiate noise, but because those external galaxies lie much farther away from us than the center of our own galaxy does, sophisticated equipment is needed to detect the noise from them.
In 1965, Arno Penzias and Robert Wilson of the Bell Laboratories observed cosmic noise that seemed to come from everywhere. For some time, the noise source remained a mystery. Nowadays, most astronomers believe that the noise originated with the fiery birth of our universe (an event often called the Big Bang), and comes to us “delayed” by billions of years! If they’re right, then when we detect and record this noise, we in effect “hear” the echo of Creation.
The amount of radio noise emitted by the sun is called the solar radio-noise flux, or simply the solar flux. The solar flux varies with frequency. But no matter what the frequency (or wavelength), the level of solar flux increases when an eruption on the sun’s surface, known as a solar flare, occurs. A sudden increase in the solar flux indicates that “shortwave radio” broadcasting and communications conditions (including amateur radio) will deteriorate within a few hours.
The solar flux is commonly monitored at a wavelength of 10.7 centimeters, which corresponds to a frequency of 2800 MHz. At this frequency, which is about 100 times the frequency of everyday CB radios, the earth’s atmosphere has little or no effect on radio waves, so the energy reaches the surface at full strength.
Fact or Myth?
• In recent years, we’ve heard about solar flares and the potential danger that their effects pose to electronic devices. Some people say that these events occur more often during sunspot peaks than at other times. Are they telling the truth?
• Yes, but solar flares can occur at any time.
Electromagnetic noise is generated in the atmosphere of our planet, mostly by lightning discharges in thundershowers. This noise is called sferics. In a radio receiver, sferics cause a faint background hiss or roar, punctuated by bursts of sound we call “static.” Figure 7-9 shows an example of sferics as they might look on the display of a laboratory oscilloscope connected into an AM radio receiver.
FIGURE 7-9 If you connect an oscilloscope into an AM radio receiver and listen to sferics, you’ll see a display that looks something like this.
A gigantic voltage constantly exists between the surface of the earth and the ionosphere. The earth’s surface and the ionosphere behave like concentric, spherical surfaces of a massive capacitor, with the troposphere and stratosphere serving as the insulating material (called a dielectric) that keeps the charges separated. Sometimes this dielectric develops “holes,” or pockets of imperfection, where discharge takes place. Such “holes” are usually associated with thundershowers. There are normally about 700 to 800 such areas on the earth at any given time, concentrated mostly in the tropics. Sand storms, dust storms, and volcanic eruptions also produce some lightning, contributing to the overall sferics level.
Sferics are not confined to our planet! A great deal of radio noise is generated by storms in the atmosphere of the planet Jupiter. Astronomers can “hear” this noise with radio telescopes. Sferics probably also occur on Saturn, and perhaps on Uranus, Neptune, Venus, and Mars as well. In the cases of Venus and Mars, dust storms and volcanic eruptions would likely be the cause of sferics.
Precipitation noise, also called precipitation static, is radio interference caused by electrically charged water droplets or ice crystals as they strike metallic objects, especially antennas. The resulting discharge produces wideband noise that sounds similar to the noise generated by electric motors, fluorescent lights, or other appliances.
Precipitation static is often observed in aircraft flying through clouds containing rain, snow, or sleet. But occasionally, precipitation static occurs in radio communications installations. This is especially likely to happen during snow showers or storms; then the noise is called snow static. Precipitation static can sometimes make radio reception difficult or nearly impossible, especially at low frequencies (long wavelengths).
In a good radio receiver, a noise blanker or noise limiter can reduce the interference caused by precipitation static. A means of facilitating electrical discharge from an antenna, such as an inductor between the antenna and ground, can also help. If the antenna elements have sharp points on the ends, you can blunt them by installing small metal spheres on those ends, although if you do that, you might have to shorten the elements slightly in order to account for loading effects that the spheres produce.
When the voltage on an electrical conductor (such as an antenna or high-voltage transmission line) exceeds a certain level, the air around the conductor begins to ionize. That means the atoms in the air gain or lose electrons, so that they become electrically charged. If the effect becomes significant, it can cause a blue or purple glow called a corona that can be seen at night. This glow commonly occurs at the ends of a radio broadcast or communications transmitting-antenna element at night when the transmitter has high power output. Coronae occur increasingly often as the relative humidity rises because it takes less voltage to ionize moist air than it takes to ionize dry air. A corona produces a strong, sometimes overwhelming hiss or roar in radio receivers.
A corona can occur inside an antenna’s feeder cable just before the dielectric material breaks down and the cable gets ruined. Poorly designed antennas with high-power transmitters can subject a transmission line to that sort of stress. So can nearby thundershowers. A corona is sometimes observed between the plates of capacitors handling large voltages. This effect is more likely to occur with a pointed object, such as the end of a whip antenna, than with a flat or blunt surface. Some antennas have small metal spheres at the ends to minimize the chance of a corona occurring.
Any sudden, high-amplitude voltage pulse will generate an electromagnetic field, and radio receivers will often pick it up. It’s called impulse noise, and it can come from all kinds of household appliances, such as vacuum cleaners, hair dryers, electric blankets, thermostats, and fluorescent-light starters. Impulse noise tends to get worse as the frequency goes down, and can plague AM broadcast receivers to the consternation of their users. Serious interference can occur in “shortwave” radio receivers, but it gets less severe as the frequency rises, and it rarely poses a problem above 30 MHz.
Impulse noise in a radio receiver can be reduced by the use of a good ground system. All the components in the system should be grounded by individual wires to a single point. A noise blanker or noise limiter can also help, if the receiver has one. A “shortwave” or amateur radio receiver should be set for the narrowest response bandwidth consistent with the mode of reception.
Impulse noise can be picked up by high-fidelity (hi-fi) audio systems. The greater the number of external peripherals (such as computers, flash-memory chips, microphones, and speakers) that exist in a hi-fi system, the more susceptible the system becomes to this type of noise. Wireless microphones and wireless headsets are especially vulnerable to its whims. An excellent ground connection is imperative. You might have to shield all speaker leads and other interconnecting conductors, using coaxial cable instead of “open wire.”
Ignition noise is impulse noise generated by the electric arcs in the spark plugs of an internal combustion engine. Many different kinds of devices produce it, including automobiles and trucks, lawn mowers, and gasoline-engine-driven generators. Figure 7-10 shows how ignition noise looks on an oscilloscope connected into the sensitive amplifiers of a radio receiver.
FIGURE 7-10 Here’s what ignition or impulse noise looks like on an oscilloscope display connected into an AM radio receiver.
In a “shortwave” or amateur radio receiver, a noise blanker can often work wonders to get rid of ignition noise problems. The pulses of ignition noise are of very short duration, although their peak intensity can be considerable. Noise blankers are designed to literally switch the receiver off during these short noise pulses.
As you learned earlier in this book, ignition noise often poses a problem for mobile two-way radio operators, especially if communication in the “shortwave” band is contemplated. Ignition noise can be worsened by radiation from the distributor wiring in a truck or automobile. Sometimes, special spark plugs, called resistance plugs, will greatly reduce the amount of ignition noise that an engine produces. A competent automotive technician should know whether or not this option will work for you. An excellent vehicle-chassis ground connection is imperative in any mobile installation.
Ignition noise is not the only source of trouble for the mobile radio operator. Noise can also be generated by the friction of the tires against the pavement, especially in dry, hot weather. High-tension power lines often radiate significant impulse noise. The vehicle’s alternator can cause impulse noise in the form of a whine that changes pitch as the car accelerates and decelerates.
Utility lines, in addition to carrying the 60-Hz AC that they’re meant to transmit, carry other currents. These currents have a broadband nature. They result in an effect called power-line noise, which is a particularly powerful form of dirty electricity. The “rogue currents” usually occur because of electric arcing at some point in the utility grid. The arcing might originate in household appliances; it can take place in faulty or obstructed utility transformers; it can occur in high-tension lines as a coronal discharge into humid air. The currents cause the power line to radiate EM fields like huge radio transmitting antennas!
Power-line noise sounds like a buzz, hiss, or roar when picked up by a radio receiver. Some types of power-line noise can be attenuated by means of a noise blanker. Other types of dirty electric noise defy noise blankers, and the best you can do is hope that an automatic noise limiter (ANL) will give some relief by at least giving the desired signals a “fighting chance” against the noise. Engineers have gone to great lengths, racking their brains to come up with new and innovative ways to deal with dirty electric noise as it becomes more and more of a problem.
FIGURE 7-11 Electromagnetic waves from multiple wireless devices flow around and through us all the time, wherever we go.
Here’s an “electronic odyssey” that will keep you busy for a while. Do a bunch of “tweak freak tests” with wireless devices, two-by-two, and see what interferes with what. You can start with these ideas:
• Wireless headset versus (you fill in the blank)
• Desktop computer versus (you fill in the blank)
• Cell phone set versus (you fill in the blank)
• Wireless tablet computer versus (you fill in the blank)
• Cordless phone set versus (you fill in the blank)
• Compact fluorescent lamp versus (you fill in the blank)
• Light-emitting-diode (LED) lamp versus (you fill in the blank)
• Light dimmer versus (you fill in the blank)
• Television set versus (you fill in the blank)
• FM radio versus (you fill in the blank)
• AM radio versus (you fill in the blank)
• Wireless computer mouse or keyboard versus (you fill in the blank)
• Wireless computer printer versus (you fill in the blank)
• Wireless electric “smart meter” versus (you fill in the blank)
• Garage-door remote box versus (you fill in the blank)
• Appliance remote box versus (you fill in the blank)