There are many factors that go into deploying a radio frequency (RF) solution. One that is often overlooked is the environment of the chosen application. Whether you are looking to connect across 10 meters in a crowded hall or 10 kilometers outdoors, the environment plays a significant role in the maximum range that can be achieved.
In ideal conditions, the inverse square law is the only factor affecting range. In the real world, range can also be degraded by other factors:
- Obstacles such as walls, trees, and hills can cause significant signal loss.
- Water in the air (humidity) can absorb RF energy.
- Metal objects can reflect radio waves and cause the waves to destructively (and sometimes constructively) interfere with themselves — this is called multipath.
If you wanted to provide a wireless communication system for a large outdoor facility, such as a ranch, you would need to consider all of these factors. For instance, a ranch located in Arizona is managed by “Rancher Dave” and he wants to ensure that adequate water is available for his cattle to drink at each of the thirty various stock watering facilities across the ranch covering 75,000 acres. He needs a way to monitor stock water tank levels and be alerted in the absence of water. However, Rancher Dave wants a solution that has low initial and ongoing costs. In addition, he wants to use Solar Power for these remote sites so the solution has to incorporate low power consumption. He needs a solution that is reliable under even the worst outdoor conditions while meeting the challenge of accommodating the rough terrain with large cliffs and various RF obstacles
This article will follow Rancher Dave through his path to finding the right RF solution. It provides a high-level overview of the factors that can affect RF range, including hardware selection, environmental factors, frequency ranges, and proper implementation.
Choosing the Right Radio
In order to cover the ranch that spans over 75,000 acres, Rancher Dave must maximize his RF range to transmit data from each water tank. He requires a radio solution that would cover a great distance while simple enough to transmit small amounts of data. After some research, he discovers that radio waves at lower frequencies broadcast further than radio waves at higher frequencies. For example, a 900 MHz radio will transmit more than twice as far in free space as a 2.4 GHz radio when both radios use the same modulation and output power. In addition, the longer wavelength of 900 MHz radios versus 2.4 GHz means that a 900 MHz signal will transmit through obstacles such as walls, trees, and hills to a greater degree than a 2.4 GHz signal.
While lower frequencies provide better range for a given output power and receiver sensitivity, other considerations may require the use of higher frequencies, such as 2.4 GHz. These considerations are:
- Need for a smaller antenna.
- Need for more bandwidth.
- Need for a worldwide frequency band for use in multiple countries.
- Line-of-sight considerations over long distances.
Since Rancher Dave requires an RF solution that transmits over a great distance but only requires small amounts of data sent periodically to alert of low water levels, he decides to go with the 900 MHz radio as it best fits his specific application. This solution also has a range of up to 20 miles (32 km) line of sight which would allow him to setup a network that would cover the large ranch.
Selecting the Right Antenna
Longer wavelengths also require greater area to transmit and receive resulting in increased antenna size and length at lower frequencies. Now Rancher Dave needs to select the most appropriate antenna for his application.
Antennas come in a variety of physical packages and radiation patterns and a detailed study of each antenna’s datasheet will be necessary to identify the best antenna. Antennas reduce the signal strength in some directions and increase the signal strength in others, providing gain. Omnidirectional antennas radiate out perpendicular to the direction of the antenna in a donut pattern, as shown in Figure 1.
However, directional antennas not only provide better gain; they also help reduce the amount of interference received at the antenna by producing an overall signal loss from directions where the antenna does not point. If there is a known interferer in proximity, placing the antenna such that there is a loss from that direction can help alleviate interference. After carefully considering his options, Rancher Dave decides to go with a directional antenna in order to reach maximum range and avoid signal loss.
Often, to place an antenna in the best location for transmission, a cable will be required to connect the transceiver to the antenna. Cables can be a huge loss for the signal strength and care should be taken to choose the right cable type and length. A poorly chosen cable can more than offset any gains which would be received by placing the antenna in an optimal location. In general, you get what you pay for with RF cables, so read the specifications carefully and choose the one which fits your application the best. Cables with less loss are often more expensive; they also tend to be less flexible and may not work in a specific application.
After selecting the radio transceivers to account for the largest maximum pass loss, and after selecting the appropriate antenna for his ranch, only one more thing was left to do to achieve his maximum RF range: put the antenna as high as possible. A higher antenna can help get you above any possible interferers like trees and buildings. It can also help you reach your true RF line-of-sight by getting at least 60% clearance in the Fresnel zone. In addition, the curvature of the earth also comes into play. For a 2.4 GHz transmission path to transmit 5 miles, you would need antennas at 9.6 m (31 ft). For 900 MHz at 20 miles (32 km), you would need antennas of at least 46 m (152 ft) to achieve a good signal.
In many practical settings, your transceivers may function with a lower antenna height, but the higher the better. There is also a trade-off between the antenna height and the amount of RF cable needed to span the transceiver to the antenna. It is possible a lower antenna height will work better because there is less loss in the cable.
When configuring the height of your antenna, make sure you check with local regulations about how high an antenna can be. Some local and federal agencies regulate the height of antennas, so be aware of the regulations in your area.
Rancher Dave was able to set up a working network to easily monitor water levels across his ranch and ensure that his cattle always had fresh drinking water. There are always many factors that should be taken into consideration when selecting the right solution and even small obstructions can significantly degrade the signal. This application is one example in selecting the right radio and antenna for those looking to achieve the absolute maximum range possible in a challenging RF environment. The information in this article should be used to give you an idea if the distances you are trying to achieve are possible, but only a real-world test will tell you for certain.