Wireless signals are used in almost every installation Security Solution NW installs. From wireless door/window sensors and motion detectors to cellular and AES radios in our fire and alarm communicators. None of these wireless systems behave in the same manner because they are different frequencies and bandwidths. This article will focus on the effect walls and floors have on wireless security sensors, keypads, repeaters, and RF receivers, but to some degree, the material here applies to all radio waves.
When laying out the locations of wireless devices in a security system, take the time to do a site survey of the buildings construction and the contents within the building.
How many walls or floors will a signal need to pass-through, and at what angle?
What is in the building now or maybe in the future?
Where are your possible locations for wireless receivers and devices?
Doors and Windows are fixed locations, so your options are limited, but motion detectors are highly flexible. Do not get fixated on specific locations for your keypads (wireless receiver) or motion detectors if you have any concerns on wireless reception in your installation.
Radio waves do pass through floors and walls; otherwise, we would all be using our cell phones outside. But, then again, sometimes you have to go outside to use your cell phone. Cell phone frequencies are not the same as wireless security devices, but the concept is the same. The material used to construct the walls and floors can have a HUGE impact on your wireless system.
Angle of Attack
When dealing with walls, the best approach is to reduce the angle of attack. A 12" cinder block wall at a straight-on approach means your wireless signal passes through 12" of cinder block, concrete (to fill the cavities) and associated rebar. When your angle of attack changes, the distance through the wall gets longer. The image below illustrates a 45° angle of attack.
The second component in the angle of attack is the perspective of the materials within the wall. Rebar, electrical wires, metal conduit, metal and wood studs all affect the propagation of wireless signals. At a straight-on angle, the rebar in the image below may not seem to impose a wireless signal and it may not.
When you change the angle of attack, the perspective horizontal separation of the rebar is reduced as seen below.
You can see this effect if you hold a hand up with fingers spread apart, palm facing you. Slowly rotate your hand, and the perceived distance between your fingers reduces.
This same principle applies to studs, conduit, and other items placed vertically within a wall or horizontally within a floor. Floors are just walls on their side. If your signals have to pass up or down through a floor, keep in mind the vertical angle of attack. Commercial floors may be constructed with corrugated steel, Gyp-Crete, or steel beams. The angle in which the signals pass from the wireless sensor to the receiver must be considered in your security system layout.
Buildings are useless unless we fill them full of stuff. That stuff affects wireless signals. Take the time to survey the building and consider the stuff that is present and what may be added later. A warehouse will be filled with stuff. The kitchen cupboards will be filled with stuff and if that stuff is a pantry full of canned goods, you may have problems. When radio wave energy passes through a conductive material, a portion is converted to heat. The more conductive the material, the greater the conversion and thereby absorption of the radio waves.
All materials attenuate or scatter radio waves to some degree. The more dense or thicker the material, the greater the effect.
A repeater has no idea which direction it receives signals. The repeater receives signals and repeats them. It receives from all directions and transmits in all directions. The image below shows a home with a large stone fireplace between the RF receiver and a wireless sensor. In reality, there would also be more walls and stuff between the two, but you get the idea. If your Go-No-Go test fails or you experience inconsistent operation, select a location where both the sensor and receiver have optimal pathways.
The fireplace creates signal shadows. Select a location out of all shadows.
When installing repeaters to overcome signal loss due to a wall or floor, select locations at the lowest angle of attack possible. The image below shows a building with a concrete wall.
With the repeater located at a right angle to the RF receiver, the maximum signal level is received by the RF receiver.
Selecting The Best RF Receiver For The Site.
Often you will have little to no choice on the RF receiver you use, or where to locate it. A Lyric is an all-in-one system, leaving little flexibility for location options. A Vista 20P can have one RF receiver, but it can use an RF keypad or standalone RF receiver.
If you have more than one keypad in the system you can select which keypad is the RF keypad. Be careful when swapping keypads to solve a wireless reception issue that you do not create another issue. Keyfobs are a wireless device and need to operate as needed from where the customer expects them to work. Keyfobs have no feedback to inform the user that the alarm system has been successfully armed, or disarmed when they leave. TEST ALL KEYFOBS using a Go-No-Go test from where the customer will use them. Discover the reliable range of the keyfobs and inform the customer.
If the keyfob flashes RED when you push a button, the fob is in High-Security Encrypted Mode and has a reduced range. Read the manual on how to change the keyfob to standard, unencrypted mode.
If you are using a Vista 128 or 256 model, you can use two RF receivers. This gives you significantly more design choices in challenging wireless installations. Before considering using a 128 or 256, remember they cannot use SBUS zone expanders which are commonly used in larger installations. You may have to swap out zone expanders for V-Plex style zone expanders. So be careful when considering a panel change.
In most installations, you will not have a problem with wireless security devices. Over time you will learn from experience what installations may pose issues for your wireless system. The best approach is to pay attention to the building and the stuff in the building. If you have concerns, test the components BEFORE you install them. Often it may only take relocating an RF keypad, or adding a dedicated RF receiver to the job to solve a signal problem.