3 Factors that Limit Range in RF Applications
Environmental factors can have a drastic effect on range, making it one of the key aspects to understand when deploying a radio frequency (RF) solution. Whether the goal is 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. The following factors can limit range:
- Antenna and Cable Selection
- Antenna Height
Range as a Function of Frequency
Radio waves at lower frequencies propagate 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 (333 mm vs. 125 mm) means that a 900 MHz signal will propagate through typical construction walls to a greater degree than a 2.4 GHz signal. Longer wavelengths also require greater area to transmit and receive resulting in increased antenna size and/or length at lower frequencies.
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:
- The need for a smaller antenna
- The need for more bandwidth
- The need for a worldwide frequency band for use in multiple countries
- Line-of-sight considerations over long distances
Radio waves emanating from an antenna will spread out slightly, such that what would be considered line of sight for an RF system is more than just the visual line of sight.
The amount of clearance required is higher for lower frequencies than it is for higher frequencies. As an example, at 8 km (5 miles) a 2.4 GHz radio needs 9.6 m (31 feet) to reach 60% clearance from the Fresnel zone, where a 900 MHz radio would need 15.2 m (50 feet). To achieve the best range possible, the 900 MHz antenna needs to be almost 60% higher.
Antenna and Cable Selection
Once you have chosen transceivers for the appropriate frequency and the best transmit power and receiver sensitivity, you need to match the transceivers to an appropriate antenna, possibly connecting the two through an RF cable. Antennas come in a variety of physical packages and radiation patterns; a detailed study of each antenna’s datasheet will be necessary to identify the best antenna.
Figure 1: Toroid (http://en.wikipedia.org/wiki/File:Elem-doub-rad-pat-pers.jpg)
All antennas are passive devices. An ideal isotropic antenna (which is only theoretical) would radiate the signal out in all directions with no gain (0 dBm). In reality, 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 donut (or flattened torus) pattern, as shown in Figure 1.
Examples of omnidirectional antennas include dipole and monopole antennas. A dipole antenna consists of two metal conductors in line with each other. Traditional “rabbit ears”, such as television antennas and small whip antennas, are common examples. Monopole antennas have a single conductive line and are mounted over a ground plane. The ground plane plays a critical role in the quality of the transmission. For lower frequencies a larger ground plane is necessary; in these cases, the earth is often used. Examples of monopole antennas include whip antennas and mast radiators, such as the ones sometimes used in AM broadcast towers.
By redirecting some of the energy of the signal, the antenna can provide gain to the overall signal strength; a dipole antenna could gain between 1 and 5 dBm. More directional antennas (such as Yagi antennas) can provide even greater gains, on the order of 6 dBm to 15 dBm, by providing a very narrow transmission beam. Yagi antennas consist of multiple elements used to focus the transmission beam and produce larger gain. Figure 3 shows a radiation pattern from a 900 MHz Yagi antenna with 13 dBi of gain.
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. Due to the specific directional nature of the Yagi and other directional antennas, they are limited to applications where the antenna can be pointed at the destination, such as in point-to-point networks. Additionally, too much gain on an antenna can cause it to violate local regulatory restrictions for radiated output power. Refer to the user manual on the transceiver or with a local regulatory body for emissions rules.
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, but tend to be less flexible and may not work in a specific installation.
After selecting your radio transceivers to account for the largest maximum path loss, and after selecting the appropriate antenna, you then need to do only one thing to get the maximum RF range from your equipment: put the antenna as high as possible. A higher antenna does two main things. First, it can help get you above any possible interferers like cars, people, trees, and buildings. Second, it can help get your true RF line-of-sight by getting you at least 60% clearance in the Fresnel zone.
Finally, don’t forget about the curvature of the earth. At eight kilometres (5 miles) the Earth’s height at midpoint is .95m, (3.12 ft), not accounting for hills and other terrain features. At 32 km (20 miles) the height at midpoint is 15.2 m (50 ft). For a 2.4 GHz transmission path to go 5 miles, you would need antennas at 9.6 m (31 ft). For 900 MHz at 32 km, you need antennas at least 46 m (152 ft.) to achieve a good signal of at least 60% of the Fresnel zone. 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.
The information in this post highlights some of the factors that should be considered when attempting to achieve the optimal range for your RF application. Download the Understanding Range for RF Devices white paper for complete technical information such as how range numbers are calculated.
For a real world example on how to achieve wireless range in complex RF environments, check out Know Your Device’s Environment: A Guide for Understanding Range in RF Devices.