How to choose the antenna gain when configuring the Vanguard TCD60 wireless gas detector:

Often we are asked by our customers if the wireless gas detector can be installed at longer distances from a mesh network than we recommend. A typical transmitter, like the Vanguard detector, running at 2.4 GHz will have a 2 dBi gain antenna. With a 2 dBi gain antenna, the signal transmitted by the device will be fairly close to omni-directional in the horizontal plane providing reliable performance over reasonable distances. Our customers may also have a tank farm several hundred feet from the closest gateway or mesh network. Two solutions employed typically is the use of repeaters to bridge the distance or higher gain antennas. The use of repeaters can be expensive and not necessary at times. Using a higher gain antenna may be a great choice, but not as straightforward as it may seem. The trade-off is around directionality of a signal and not understanding how gain relates to transmission distance. Correct antenna selection and proper placement of devices can make all the difference in communication performance.

What determines gain in an antenna?

The gain of an antenna is a combination of its efficiency and its directivity. The directivity is how powerful the signal is compared to a ‘perfect antenna’ which would spread the power in all directions equally. A ‘perfect antenna’ is assumed to be an ideal, single point source with equal radiation in all directions, which is not really possible. When you see antenna gain specifications they are usually stated in dBi (dB isotropic) comparing the antenna to the ‘perfect antenna’. The antenna efficiency is just the efficiency at which it converts it’s input signal into radiation and vice versa.

How does gain impact beam spread and what impact is this on installation?

It’s important to note that gain and beam spread are mutually exclusive. An antenna cannot have high gain in all directions, so when a higher gain antenna is selected, the beam spread will be limited. For instance, when moving from a 2dB antenna to a 6dB antenna, the beam width will change from approximately 80 degrees in the vertical plane to approximately 40 degrees. So if there is a large elevation distance between the devices, you could actually see poorer performance with a high gain antenna. During design and installation, it is important to take into consideration this trade-off.

How does gain impact distance?

In a perfect world, 6dB (4x) more gain would provide 3dB (2x) more range. This is due to the fact that the surface area of a sphere is related to the square of it’s radius. So, changing a 2dB antenna to a 4.3dB antenna could yield about 28% more range. Changing from the 2dB antenna to the 6 dB antenna could yield about 58% longer range!

Of course we do not live in a perfect world and this is seldom the case. In general RF radiation can be expected to do exactly what you don’t want it to do. There are some steps you can take to get more reliable performance when placing antennas however:

1. Clear the immediate area of the antenna. Obstacles close to the antenna will provide much more interference than those far away, as they might reflect energy and change the radiation pattern of the antenna. In the worst case, a conductive object could couple with the antenna enough to change it’s impedance, thus reflecting the power back into the radio. Basically, the antenna will only perform as specified if there’s about ½ meter of free space around it. This including the area behind the antenna range.
2. Raise the antenna and clear the range. Often times raising the antenna above obstacles can be more effective than increasing its gain. As with our Vanguard TCD60 wireless gas detector, antennas are often removable and can be extended with commonly available cables.
3. Choose the right antenna for the job. In general, a higher gain omnidirectional antenna is preferable in outdoor environments with fewer obstacles and little to no elevation gain. When indoors, a signal can be reflected off several obstacles causing the line of site signal to be the weaker signal. In these environments, it is sometimes preferable to have a lower gain omnidirectional antenna which is less directive, especially if there is a significant elevation difference between sensors.