LOOK, MA, NO WIRES!
Dec 1, 2000 12:00 PM, Steve Filippini
Working with security system transmittersYou are standing in the entry of a large home with vaulted ceilings and tiled floors. Your job is to run a pair of wires from the front door to the hall closet - 20 feet from the entry. The one condition? The customer does not want to see any exposed wires.
The installer of Christmas Past would have spent a full day pulling up molding and running a cable three or four times the distance just to route it to the location needing protection. Today's installer can use a wireless transmitter and span the distance through radio waves.
Transmitters come in all shapes and sizes. They can be as small as a piece of chalk or as large as a deck of playing cards, and they can be built into most security devices on the market. From door contacts to motion detectors to smoke detectors, these protection sensors make the installer's job a lot easier. But it wasn't always this way; it has taken more than 20 years to achieve the quality found in today's wireless devices.
A BRIEF HISTORYIn the beginning, the only transmitters available were non-supervised and transmitted an AM signal to their matching receiver. Non-supervised transmitters could not report a low battery condition. They could not report whether or not the point of protection they were covering was secured. They were not able to check in several times a day to verify that they were still operational. Still, they were widely accepted when they were introduced to the field.
One of the first companies I worked with that manufactured security transmitters was Linear. When Linear first came out with its wireless product line, the company tuned all of its transmitters and receivers to an AM frequency of 305 MHz. As time moved on, Linear fine-tuned the frequency even further to 303.875 MHz. Because transmitters were relatively new, the FCC limited the frequency that could be used by a low-powered security device. Linear continued improving its product line, and it spawned many imitators.
When Linear transmitters first became available, alarm companies bought them and their single-channel receivers by the thousands. Each receiver had a built-in "Form C" contact relay that changed state when it heard from one of its transmitters. All installers had to do was run a pair of wires from the door/window contact to the transmitter and from the receiver to the alarm panel. From there, they would plug it into an available zone input. As the systems' popularity grew, Linear introduced its 2-, 4- and eventually 8-channel receivers. With their 8-channel model, the same types of transmitters could report a message from several locations throughout the home and still be zoned out for quick identification.
Not to be outdone, Honeywell designed a transmitter that directly interfaced with its home security product line. Honeywell's transmitters used an FM signal that was set at 315 MHz, and they were supervised. The security panel could monitor the strength of the battery and report when it was running low. The panel could also monitor the condition of the point of protection. More important, the security system required that each transmitter check in every eight hours. If a transmitter failed to check in three times within a 24-hour period, the system would display and announce a Trouble Transmitter condition.
Linear's and Honeywell's transmitters carried an Open Field range of roughly 900 feet. ITI also offered a line of security transmitters, which operated at an AM frequency of 318 MHz and quickly gave the competition a run for the money. ITI's systems initially transmitted at a range of 900 feet but have since improved to cover a much larger area. Ademco also offers home security transmitters. They are roughly the same size as the ITI transmitters and operate at a frequency of 345 MHz. Their range is comparable to, if not better than, ITI's.
The introduction of 900 MHz spawned the age of Spread Spectrum technology. The concept behind it was interesting enough: Multiple signals at different frequencies sending the same message made it almost impossible to miss a signal. At 900 MHz, the previous problems regarding building materials deflecting signals were no longer an issue. The only drawback was that the signal did not carry the same range as the competition.
TRANSMITTER TIPSNow there are some installers who claim they can hardwire a point of protection in the same time it takes to mount and program a transmitter. I believe that to be true, and I offer this advice: If you can hardwire a point of protection, then by all means do so. Hardwired points are usually maintenance-free and more reliable. Wireless points can be affected by atmospheric conditions and poor installation practices. By following some simple installation guidelines, you can give your system a fair chance at operating to its fullest potential.
- Avoid mounting transmitters on metal flashing near door and window frames. The metal may bounce signals back into themselves and cancel their messages.
- Avoid mounting transmitters on lathe and plaster walls. These walls use chicken wire and can cause signal reflection.
- Do not mount transmitters near large mirrors or metal sidings.
- Avoid mounting your receivers in bedroom closets that have large sliding mirrors as doors.
- Mount your receivers as high as you can in the home. Locate them in areas that are temperature-controlled.
- Mount your transmitters at least 3 feet off the floor, especially on ground-level points of protection. The earth and foundation have metal properties that will bounce signals back and cause cancellation.
Finally, the easiest step and by far the most overlooked by customers and field technicians alike: Check the transmitter batteries on a regular basis. If the transmitter has a stated battery life of one year, keep a record and see that they are replaced on schedule. I used to carry an indelible marker and would write the date I installed the battery right on the battery itself. For transmitters that are used continuously every day - those at front doors and garage doors, and motion detectors in high-traffic areas - I make it a point to replace their batteries in half the stated time frame. As for smoke detectors, if there were ever a battery that needed to be fresh and up-to-date, this is the one. Removing the battery to stop the annoying "dit" sound puts lives at risk.
Early transmitters used 9-volt carbon batteries and usually lasted a year. Alkaline batteries were more expensive and lasted only a few months longer. Kodak introduced a 9-volt Lithium battery that was supposed to last up to three years. Early battery designs and storage practices dramatically reduced the life of the Kodak Lithiums, but the company has since improved its design and now offers a wide variety to consumers. These days, the battery of choice is the 3-volt Lithium, and several manufacturers offer them; they are rated to last up to five years. That is good news since they cost a few dollars apiece and finding them in bulk can prove to be challenging.
Technology has a way of improving when given enough time. I can't wait for the next product line to come out. Until then, I hope this column keeps the cobwebs clear and your installations stress-free.
The receiver is designed to hear any and all RF signals that fall within the preset scanning range. For example, if a receiver is set to scan a range of 310 to 320 MHz, anything that transmits in that range will be heard by the receiver. You would be surprised at how many non-security devices happen to transmit in that range. Garage door openers are notorious noise generators. There is a misconception that they only affect other RF devices when the button is pressed, but garage door opener receivers have been known to saturate their surroundings with an RF signal, thereby stopping any desired signals from getting through.
A wireless product that was popular many years ago was called the "Rabbit." The Rabbit plugged into the back of a VCR or cable box and transmitted TV signals to a matching receiver. This eliminated the need to run coaxial cable from one TV to another. But the Rabbit also stopped any FM transmitter from reaching security system control panels.
Not all producers of RF interference come from the home. I used to work near the El Toro Marine base in California. Every time the Blue Angels were in town, we were busy chasing down false medical alarms from our wireless systems. Air Force One caused garage doors to open and close on their own whenever it flew overhead on its way to the marine base. I watched an electronic engineer wallpaper the closet that housed the receiver with aluminum foil in an attempt to block out foreign RF signals. All he succeeded in doing was stopping all RF messages (alarm transmitters included) from communicating with the panel.
One of the biggest problems facing alarm systems using transmitters is a condition called "Multipath." Multipath occurs when multiple bursts of a signal spread out and reflect off objects with metallic properties. Two problems can result from this. The first is the distinct possibility the signal may bounce back into itself and cancel out. The second is that the signal decreases in intensity each time it bounces off an object or surface. You cannot eliminate multiple signal paths, but you can take steps to avoid them. See my "Transmitter Tips."