Wireless has become a powerful trend in the security world during the past decade. This is true not only for the do-it-yourself market, but also for the professionals who install and service today’s wireless burglar and fire alarm systems.
Wireless’ popularity and salability is primarily based on the fact it requires less muss and fuss to install and results in lower labor costs for security firms that offer it. End users like wireless because its impact on aesthetics is minimal with relatively little cleanup necessary. Also, the time-to-use factor is quite short, which means they are protected sooner than with a hardwired installation.
Company managers also like wireless because a single installation crew can install more systems in a shorter period of time. This means two or more times as many monitored accounts can be put online in the same time it takes for one hardwired system.
This is the first of two articles on wireless technology. This installment features helpful information on the technology, while Part 2 in June will focus on applications and installation.
It is assumed the reader has a basic knowledge of the fundamental elements associated with modern-day burglar and fire alarm systems. Thus, we will not cover issues related to motion detectors, contact switches, keypads and other technologies. What we will cover are the three basic types of wireless technology for radio-based security systems; the three types of radio waves used in wireless applications; and the most common transmission technologies.
Wireless Security Comes in 3 Different Forms
There are three basic kinds of wireless security on the market today: short-range, midrange and long-range.
Short-range wireless security systems consist of a number of device transmitters, along with a main alarm panel equipped with an internal or external radio receiver. Individual transmitters are used to link door and window switches, motion detectors, emergency hold-ups, medial buttons and other devices to the panel’s receiver using radio waves. The practical transmission range for most short-range wireless systems is between 100 and 1,000 feet, while some can attain up to 3,000 feet.
Security firms also use midrange radio systems for data telemetry and to extend a motion detector or contact switch closure over a greater distance than short-range wireless provides. These systems typically offer an open-field transmission range of two to 10 miles, depending on the type of antenna and the environment.
Long-range radio systems are another commonly used wireless technology with an effective open-field transmission range of 10 to 35 miles, sometimes more. Range in this case is dependent on the type of transmitter antenna, its placement within a facility, the terrain between the transmitter and receiving tower, and the outdoor ambient temperature. The manner in which these systems are used relates directly to the monitoring and supervision of remote alarm systems by central station facilities.
Mechanics of Radio Waves Include Frequency, Wavelength
The primary means whereby data of almost any kind is sent from one point to another in a wireless security system involves the use of radio waves.
Radio waves are made up of high-frequency oscillations impressed, or modulated, with data that are made to travel from a transmitter antenna to a receiver at approximately the speed of light, or 186,000 miles per second.
Radio waves are usually referred to in terms of cycles per second, or Hertz (named after Heinrich Hertz, the German physicist who discovered radio waves) and in terms of wavelength, or lambda (measured in meters). These oscillations take the form of sine waves, which we call frequency.
When measured in terms of frequency, or cycles per second, we commonly measure in Hertz. For example, some wireless alarm systems use the frequency of 433 million cycles per second, which is converted to 433MHz (Megahertz).
Each cycle is also referred to in terms of wavelength (see diagram on page 44 of the January issue), or in a mathematical context, lambda. Wavelength is actually a physical measurement as it relates to a single cycle.
Radio waves are also referred to in terms of amplitude, which relates directly to power in watts (W). As a general rule, the higher the wattage, the farther the range of radio wave transmission. Signal amplitude will vary from microwatts to many watts as employed by the various wireless manufacturers in the security market.
How to Work With Wavelengths and Frequencies
When working with radio in any field of endeavor, a fundamental knowledge of frequency and wavelength is required. Understanding the relationship between the two is extremely important when working out problems in the way radio waves propagate through the atmosphere.
For example, when we talk about sine waves and frequency, we must also consider the period of a cycle, or the time (T) between the peak of one cycle and the next. Frequency and time can be expressed in the following mathematical equation:
F = 1/T (F = frequency, T = time)
To solve for frequency when the period of a radio wave, or time, is known, we use the following equations:
F = 1/T
F = 1/100 us
F = 1/100 X 10-6
F = 1 X 106/100
F = .01 X 106
F = 10,000Hz or 10KHz
In order to solve for time (T), we use the same formula but we manipulate it algebraically:
T = 1/F
For example, where frequency is 250Hz, we can solve for T by the following procedure:
T = 1/F
T = 1/250Hz
T = .004
T = 4 X 10-3
T = 4ms
Understanding the relationship between frequency and wavelength is likewise important. Wavelength is the physical distance between the consecutive two trouphs or peaks of a radio wave. As mentioned, the common unit of measurement for this distance is the lambda. The equation we use to determine wavelength when frequency is known is as follows:
Lambda (in meters) = 300/F (in MHz).
Using this formula, we can determine the wavelength of any frequency. For example, when the frequency equals 9500KHz, the mathematical procedure begins with the conversion of this number into MHz, or 9500 KHz = 9500 X 103 = 9.5 X 106. Now we can plug the resultant into our equation:
Lambda = 300/F (in MHz)
Lambda = 300/9.5MHz
Lambda = 31.58 meters
How Radio Waves Propagate Through the Atmosphere
There are three basic types of radio waves to consider when working with wireless equipment in security: ground waves, tropospheric waves and ionospheric waves (see diagram on pages 42 and 43 of January issue).
Ground waves, which include frequencies up to 30MHz, are directly affected by the surface of the Earth and generally used to transmit signals a relatively short distance. There are two components that comprise a ground wave: an earth-guided surface wave and a space wave, which is the result of direct wave and ground reflection. As you will later learn in Part 2, vertically placed (vertical polarized) antennas operate better when specifically communicating through ground waves.
Tropospheric waves, sometimes referred to as direct waves, lend themselves more to frequencies above 30MHz, which include most of the wireless security devices. Direct radio waves travel in straight lines, called “line-of-site” in the radio world.
The most common problem with this mode of operation is the absorption of wave energy by objects in the signal’s line-of-sight path with the receiver. Typical line-of-site operation for long-range wireless can be up to 50 miles or more, but in the practice, 35 miles is more realistic. This is because of objects that radio signals encounter along their path.
Ionospheric waves, also referred to as skywaves, involve the transmission of radio waves above the horizontal plane. This type of wave propagation is
more typical of long-distance communication, called DX in the ham radio community.
Skywaves are actually direct radio waves that strike the ionosphere and bounce off, radiating far beyond line- of-site distances. Commonly called skip, wave propagation is affected by the ambient temperature of the locale, sunlight and a host of other issues. Thus, for all intents and purposes, security dealers do not usually have to consider skywaves when working with wireless security systems.
Common RF Transmission Techniques Using AM and FM
There are several ways to modulate and transmit data at the transmitter and send it to a remote receiver. The most common are amplitude modulation (AM) and frequency modulation (FM).
There’s also spread spectrum (SS), which we will discuss in the next section, and two modulation techniques that include pulse modulation (PM) and pulse positioning modulation (PPM), which we will also talk about later.
AM radio operates by altering the amplitude of the signal in accordance with the information to be conveyed. AM lends itself best to lower frequency use, all the way up through the lower portion of the UHF band. Because lower frequency use is sometimes subject to RF interference (RFI) and spurious signals, manufacturers that use AM have developed a number of techniques that minimize the adverse effects of RFI.
FM radio operates by altering the frequency of the carrier wave in accordance with the data transmitted. FM is less susceptible to the same interference that plagues AM systems and is often a good choice when working in lower frequencies. Some midrange transmission systems on the market operating in the 27MHz band use FM.
Spread-Spectrum Technology Uses Multiple Frequency Wideband
Another method of transmitting data from a remote transmitter to an alarm panel’s receiver is through spread spectrum. The first patented SS system on record was a CW transmission system (Morse code) in Germany in the World War II era.
SS is a wide-band technology that hinges on the transmission of data over multiple radio frequencies in a very short period of time. This method offers a high degree of redundancy because of the many opportunities that exist for the data to reach the receiver.
There are several SS technologies in use for security purposes. Where some employ a proprietary method developed by specific manufacturers, others rely on the licensing of established technologies developed by other technology providers.
The most commonly used SS technology in security is the frequency hopping method. The second type is called direct sequencing.
Frequency hopping works by transmitting on multiple frequencies.
These systems essentially hop from one frequency to another over an established wide band of frequencies. The order in which this occurs is determined by a preprogrammed code sequence. The speed at which the process takes place is a function of the information rate.
The direct sequence SS method is entirely different than frequency hopping. This method employs a high-speed code sequence in conjunction with the data to be sent. The frequency bandwidth is determined directly by the high-speed code sequence, which is used to modulate the carrier.
This method holds the potential of creating extensive binary data words, from 8 to 289 – 1. In addition, code rates are possible from under one bit per second to as many as several hundred megabits per second. This flavor of SS is not widely used in security.
SS systems usually operate within the 928-952MHz range, although the Federal Communications Commission (FCC) says there is some SS taking place in the 890-902MHz frequency range as well (see spectrum chart above).
There are several significant advantages associated with SS that security firms should know about. Its benefits include short signals that are difficult to intercept and decode. SS is also difficult to jam using narrowband radio.
This technology can transmit at relatively high baud rates. Thus, the time it takes to send data over potentially 127 frequencies is only 7 seconds, more or less.
Because of such a short transmission time, through the equation provided by the FCC, transmitter power can potentially reach as high as 1W, compared to the microwatts transmitted by a typical narrowband transmitter using AM or FM. Greater transmission power essentially means more range, therefore, some SS systems have the potential of traveling up to 1 or 11⁄2 miles (although the open-field range stated by manufacturers will usually be less).
Pulse Modulation Is Most Common Transmission Method
The most common modulation method used today is PM and PPM. PM, also referred to as pulse width, involves the use of short and long signal bursts, or pulses, that carry data over a carrier wave. This method is simple and effective using common binary logic. For example, long pulses are taken for binary 1s and short pulses for binary 0s.
These binary (logic) 1s and 0s are then used to represent data words. Data words are the means whereby the status of a device, the condition of a battery, the transmitter’s ID and function, and supervisory data are conveyed to the main alarm system through the attached receiver.
The alternate form of PM is PPM, which involves the transmission of short modulated pulses contained within the first or last half of a given unit of time. Using this technique, when a short data pulse is contained in the first half of the established unit time, it’s a binary 1. When contained in the second half, it’s a binary 0 (see chart on page 46).
The advantage of PPM over PM is realized by the fact that, because of the shorter data pulses, manufacturers can effectively increase peak transmitter power using a special equation mentioned earlier, which is essentially provided by the FCC. This equation determines pulse duration and effective power so Part 15 signals cannot interfere with other allocated radio services operating in adjacent radio channels.
Next on Tap for Part 2: Application and Installation Guidance
In the next edition of this series, we will discuss the practical side of wireless security. Included will be more information on Part 15, the various kinds of short-range transmitters, antenna considerations, and installation advice that can really make a difference in your effort to install a wireless system right the first time.
