Now that we know everything can be connected, enabling a product for the Internet of Things is commonly a design requirement. Cutting the cord can be an ambitious undertaking, but many manufacturers have created highly integrated RF modules to smooth the wireless learning curve for engineers with limited RF experience. Still, you may find your product dropping packets and falling off the network even when you have diligently designed the system to the module’s specifications. What’s going wrong?
How to Improve Wireless Signal Range
It is tempting to think in terms of the range of a technology, because that is how protocols are advertised. LoRa (long range) is being popularized by big names like Semtech and Microchip and is proclaimed to have a range of over 20km. Wi-Fi modules will claim ranges of up to 500 feet – why, then, do we struggle to get a good signal from our Wi-Fi router in a room 100 feet away?
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The asterisk on all these range values is that they are measured as line of sight, or LOS. This means that the distance was measured by a device that could literally see the emitter through nothing but open air. In just about every real life application, there are obstacles. Radio waves only move in a straight line, so a signal will never go around an obstacle, only through. The difficulty with which it moves through a material lowers the power of, or attenuates, the signal. Signal strength and attenuation are both measured in decibel-milliwatts, commonly written as dBm. dBm is a ratio between power emitted and 1mW that uses the log dB term to express the entire reasonable spectrum of emissions on a single scale. For example, a standard 1kW microwave oven can be represented as 60dBm while a typical Wi-Fi signal of 1nW is represented as -60dBm. Even though those two values are wildly different in actual power, using dBm lets us think of them on the same scale. A Bluetooth II module like Panasonic’s ENW-8937A3FK emits signals at up to 4dBm but can receive signals attenuated down as far as -88dBm.
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Practically, the expected range of a module depends heavily on the environment. Cooking Hacks’ LoRa shield claims 21km in LOS range, but that number goes down to 2km in an urban setting.
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This 10x loss in range is not unusual because unfortunately everything that can get in the way, does. In buildings we have concrete walls filled with rebar, wires, plumbing, and other materials. In workspaces and homes we have humans and animals that are mostly water – another effective attenuator. Though it can be a painful concession, many IoT designs should only budget for 10% of the advertised range of their module. There’s a reason we deal in logs….
Antenna Signal Strength
While the environment is a huge factor in poor wireless performance, obstacles can only attenuate a signal that is actually present. The maximum output power of an RF IC is usually specified with a certain antenna and at the highest possible broadcast power software setting. If you do not intend to use the same antenna or need to reduce emission power to conserve battery life, your results will be different.
Types of Antennas
Whip Antennas
There are three main types of antenna. The large rods sticking out of most routers and cars are called whip antennas. These are typically sized according to the wavelength of the frequency they are trying to capture and both feel and radiate power in all directions normal to their axis. This makes them perfect for capturing FM waves while driving or emitting Wi-Fi, because you have no way of knowing exactly where to direct your emitter or receiver. The position of a radio tower changes as your car moves, and a device expecting a Wi-Fi signal can be in any direction of the router. This tends to be the most expensive option, especially if the antenna needs to tolerate extreme environments and requires a larger total solution size.
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Trace Antennas
The most economical antenna option is to create a copper pattern on a PCB. This can be a viable option for products operating at relatively high frequencies (2.4 GHz and above) because the length of quarter-wave antenna can likely be achieved using a spiral pattern on even a small PCB. This works best for situations where there is more information available about the relative positions of transmitters and receivers in the system, as the trace antenna does not have as wide of a radiation pattern as a whip. There are several downsides to this solution, most notably a lack of tune-ability. Once the board is printed the antenna is permanent, and no changes can be made without a costly board spin. Unless you have an experienced RF engineer on your team, the first attempt will likely not be perfect and could benefit from a few tweaks. This solution also may not even end up saving money, as the trace antenna cannot have other components too close on either side and may require a board so much larger that it would be just as expensive to use a chip antenna on a smaller board.
Chip Antennas
Chip antennas are small devices that can be soldered down onto a PCB like most other components and require much less work than designing a trace antenna. These antennas are much more tolerant of close component neighbors and can typically be mounted in any direction. This makes them more adaptable than the trace antennas because they can listen/radiate at a plane normal to the PCB if desired. They do not have the same gain capabilities of whip antenna, but their performance can be enhanced by using a few external discrete components like inductors and capacitors in a practice known as matching. Best of all, if you decide you need a slightly different peak after testing the final product, a part change is drastically easier and cheaper than changing the entire board.
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Electromagnetic Interference
Wi-Fi is a relatively old wireless communication method and is arguably among the least elegant. It just broadcasts its information in all directions as loudly as possible, without any consideration for the environment. Because Wi-Fi applications tend to require high bandwidths, this tends to be a lot of information. If you live in an apartment building where every neighbor uses their own 2.4GHz router, you may have signal issues even from only a few dozen feet away from your router. More recent 2.4GHz protocols like ANT carefully monitor the existing signal noise and wait for a time to slip their data out on a relatively quiet channel, but even they can struggle when there is just so much happening. We separate relevant data from random data in software so as not to load every webpage browsed in the area on every device, but the airspace is still incredibly crowded. 2.4GHz is a very common frequency, but it may not just be your wireless devices causing interference.
Have you ever had trouble focusing because of a high-pitched noise? You know it isn’t a message directed at you and it isn’t even in the frequency range you normally associate with useful information, but it is still distracting. Wireless devices have the same problem – appliances like microwaves and refrigerators, lights, poorly shielded power or data cables, and even LCD screens emit frequencies that can interfere with signals.
Is There an Answer?
Sadly no, there is no magic bullet that can easily and cost effectively solve all these issues. Even an IoT product that tests will in the lab may suffer from signal integrity issues in a home environment. Still, there are a few key places to check if you are having performance issues. Design for 10% of the advertised range of a protocol, even if it means using more power and cutting your emissions certification a little closer.
Testing Wireless Signal Strength
Use a chip antenna at least in the first version of a product, and use a trace antenna as a possible cost-down opportunity rather than a part of the product you intend to use to disrupt the market. Test in as many crowded environments as possible, and make sure you are not causing any interference of your own with poorly shielded power supplies or noisy on-board components. If it is an option given other design needs, use a lower-bandwidth 2.4GHz protocol that has built in tools to keep the network up or explore lower/higher frequencies like the popular 5GHz Wi-Fi band or 900MHz protocols. Predictions about the number of connected devices in coming years are only becoming more staggering, so using a non-standard frequency is no guarantee that you will not face these same congestion issues a few years down the road – therefore, designing for a congested space will never be a waste of time.
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