Tradeoffs of different antenna types
We recently had a case where our customer needed to place a detector high on a tank and the nearest gateway was about 150 ft below the walkway.
There were not enough wireless instruments to relay the signal and the antenna was fixed to the detector. The device would not sync with the gateway until it was tilted at 45 degrees. The gateway was fixed and wired. This made moving it an expensive proposition. If the detector was moved closer, it would no longer be monitoring the correct location of interest. Adding a repeater could also be costly.
So the question is why? Why was it possible to simply tilt the device rather than requiring the devices to be pushed closer?
It all comes down to understanding how antennas work. There are several types of antenna styles on the market serving different needs. The type used in this case was a 2 dB dipole antenna. The important thing to realize on antennas like these, the signal is not equal in all directions. The radiating signal does not look like a ball moving out in all directions. However, it looks more like a donut (or bagel if you are from New York City), with the antenna in the hole of the doughy snack as shown in Figure 1.
Figure 1: Signal propagation for dipole antenna
When seeing this, it becomes clearer why the end user was able to get the detector to sync with the gateway by simply rotating the device 45 degrees. This is simulated in Figure 2 below. In the normal vertical position, the radiating signal will miss the gateway and not sync. When the detector is rotated slightly, the signal will be seen by the gateway and start operating normally.
Figure 2: Impact of rotating the detector
We know from the customer that they intend to add more instrumentation to the facility. We worked with them to understand how often they believe they may run into this situation and discussed more effective ways to address it. Still, this case is a good example to demonstrate how dipole antennas operate.
As mentioned before, there are several operations with antennas to address specific needs.
For instance, uni-directional antennas, like Yagi antennas, are designed to concentrate a signal in one direction to push far greater distances than possible with a standard omnidirectional antenna as shown above. Yagi antennas are a subset of arrays that can deploy multiple dipole antennas, to have them operate like one larger one. The tradeoff is these can’t be used in mesh networks typically, as it is not omnidirectional and able to pick up and relay a transmission from another device.
Figure 3: Typical Yagi antenna
The key is understanding the application of the antenna and selecting accordingly.