The following tech tip describes the basics of wireless communications and how sensor placement effects how far apart you can place sensors from the gateway.
Monnit WIT™ wireless sensors incorporate a 900MHz/868MHz bi-directional chipset designed to transmit data (and notifications) from the WIT sensor to the MonnitLINK wireless gateway. Being able to create a reliable link between the two points is key to creating and maintaining a functioning wireless sensor network (WSN).
The environment between the WIT sensor(s) and the MonnitLINK™ gateway greatly increases or decreases the range in which your sensors can “talk” to each other. As most sensors deployed in a commercial, industrial or home setting will have obstructions between the sensor and the gateway, Monnit will only focus on optimizing non-line-of-sight (N-LOS) in this tech tip. (Should you achieve line-of-sight (LOS) between the gateway and sensors, you should expect up to 1,000 ft communication distance.)
Physical obstructions limit the range the WIT sensors can successfully communicate to the MonnitLINK gateway. Simple obstructions such as vegetation, vehicles, doors, office machinery and furniture to complex structures inside of a building such as walls of various materials (wood, drywall, concrete, steel), ventilation duct work and building wiring can all reduce the range the sensors can be deployed from the gateway. It is in your best interest to conduct a site survey of the area prior to deployment to find the optimal communication path for the sensors.
In additional to physical obstructions, other RF interference (often referred to as “noise”) in the same bandwidth can decrease the range of a wireless sensor deployment. Fortunately, Monnit has selected the 900MHz frequency – a license-free ISM band frequency – for its sensor network, reducing interference often found in other commonly used frequencies.
Placement of Sensors
For optimal communication the higher the sensor and/or gateway can be placed for the application the stronger the RF link becomes and the greater distance the sensors can be placed from the gateway. If at all possible, do not place your sensors on or near ground level, as it affects the Fresnel zone of the antenna. (For more information about Fresnel zones and antennas, see this related article.)
For sensors that are to be located several feet from the area to be sensed (such as water on the floor or the temperature of a cooler), make sure your sensors have the appropriate lead lengths in place.
Conducting a Site Survey
Before deploying your WIT sensors, conduct a site survey. Most Monnit wireless sensor kits include a Placement Sensor. The WIT Placement Sensor is a button sensor set-up to communicate with the gateway when the button is pushed. To evaluate if a location is good for placement of a sensor, follow the steps below:
- Plug-in the MonnitLINK USB gateway to your PC if not already in place.
- Once the sensor list has loaded, click the Placement Sensor button once and wait for one green flash from the LED. Once the LED has flashed, you will see the button push logged into the “History” of the Monnit Sphere software.
- Proceed to the location where you would like to place a sensor and push the button on the Placement Sensor. If you receive a green flash confirmation, you have good RF communication. If you receive a red flash, the sensor could not establish a link with the gateway. Position the Placement Sensor about one foot away from your last spot and repeat the process. Once a green flash is received, attach your sensor to that location with one of the included Monnit mounting options.
For additional information about sensor placement or to have further questions answered, please contact a Monnit Sales Representative at 801-561-5555 (8-5 pm MST). We will be happy to assist you.