3 RF Behaviors to Know as a Wireless Engineer
Radio frequencies can be a challenging subject, especially to those who aren't accustomed to it or have changed jobs or roles; More networking roles than ever require a basic or intermediate understanding of RF. One of the first steps in understanding RF as a whole is the basic behaviors so you know what to expect — and the units or descriptions so that you can speak about RF in the right context.
What are RF Behaviors?
RF is confusing initially for many people. We're used to the behaviors of light, air or water. RF doesn't exactly behave like them, but it does have qualities of each. RF is nominally a wave, but it's a bit more complicated than that. If you could discretely isolate a single RF wave, it would behave more like a ray or light than anything else. Most of the motile properties of RF come in here, and let's conceptualize them as a ray for now.
What is Absorption?
RF energy, and all other types of energy have the property of absorption or rather the objects they strike do. Different frequencies have different rates of absorption in differing objects as well. Absorption is the property of the energy from the wave being taken in and dispersed within the object. In particular the energy isn't just taken in, it's changed to a different sort of energy in the object itself. The important thing to remember is absorption doesn't have to be complete, but absorbed signals do not continue.
For IoT, a good frequency range to talk about is 2.4Ghz. You might be shocked to learn you've likely one of the highest-energy emitters of 2.4Ghz radiation in your home right now. Don't panic, it's your microwave. Your microwave emits 2.4Ghz radiation at a much, much higher power than your WiFi AP does. The 2.4Ghz range of frequency is absorbed excellently by water, meaning it changes the energy from electromagnetic energy to heat energy very efficiently. This is how it heats your food. The same happens outside when the sun makes your skin warm. It's the solar electromagnetic rays being absorbed and changed to heat.
What are Reflection and Refraction?
Continuing on with the light analogy, RF has a pair of properties called reflection and refraction. This is the same meaning as in casual English. The energy bounces off of the item, instead of being absorbed as above. This property has an inverse relation to the previous property, in that if something is reflected it cannot be absorbed, and the converse is true as well. When you look in a mirror, the light is being reflected into your eyes, light that bounces off of your body and is absorbed into your eyes. Notable here is the difference between reflections and refractions. This difference has to deal with the angle the light bounces off of a given object.
Think of bouncing a laser off of a mirror. In that the laser comes back off of the mirror at exactly the opposite angle that it came at it. In numerical terms, if you send a laser at a mirror at a 90 degree angle, you'll receive a reflection at a 270 degree angle — it's always 180 degrees more or less than the angle of incidence.
Refraction, on the other hand, is when the signal, or light bounces off of the reflecting medium at an angle that is more or less than 180 degrees. Objects viewed in water show this phenomenon well. You're seeing refraction when an object in water appears to be broken, smaller or larger, or at an odd angle when you know it to be straight. In reality, light is moving at a different speed and angle through two different mediums to produce this effect, or through inconsistencies in a single medium. This is why things seem to waver when viewed in water.
In RF, metal in environments will likely present your greatest risk of reflection. Beware of large metal devices and windows when considering reflection. Windows vary depending on glass and coating in what degree it will reflect and what will pass through.
What are Scattering and Diffraction?
Scattering is when the signal is a larger wavelength than the discrete pieces that make up the medium it's bouncing off of. It results in the signal being bounced at odd angles, and in directions that are somewhat unpredictable. These can be both reflections and diffractions. Think of a disco ball here, if it was a single discrete object, how a straight beam of light scatters off of the ball and goes in several directions. A refraction does not change the individually transmitted waves themselves, but only alters the direction.
A diffraction may change the signal itself however. Diffraction causes the path of the light or signal to change, and bends the path that it takes. This is roughly demonstrated by fun-house mirrors that make you seem taller or shorter, skinnier or wider. What's happening here is that the light bouncing off of your body's path is being altered, so that a small portion of your body is represented as a much larger one, the light that would take up that square inch being instead spread to cover a square foot once it arrives at your eyes.
How Does This Apply to RF?
Each of these properties are fairly common in the wild as a design engineer — for WiFi, IoT or even cellular. When you're designing, if you treat a mirror, or office windows as you would a wall, you'll be pretty shocked when you do your validation survey. RF is a wave, which means it will spread out to cover more ground, but the same principle still applies. This is one reason that best practice is not to place an omnidirectional antenna next to metal. Metal is highly reflective and will send the energy right back into your antenna causing a whole host of issues. When designing, take heed of the materials surrounding your APs and your clients.
For example, a common use of IoT devices is to report statistics of devices. Consider a freezer. Without knowledge of the materials, you might be able to place a sensor and the accompanying AP closest to the source of the information inside the freezer. Freezers are usually made of metal though, and as discussed are likely RF impermeable and will result in your signal bouncing off of the walls through reflection instead of continuing to the next AP or gateway. The same applies to pipes in a refinery but with refraction. This can lead to hotspots and dead spots in your coverage.
Likewise a common source of absorption is water. Many industrial spaces will have tanks, storage vessels, even drinking water that will nearly entirely absorb and neutralize your signal. That doesn't mean that all absorption is bad. A quick truism for design is "Signal where you want it, not where you don't." It sounds silly, but RF in places where it isn't explicitly desired is both a security risk and makes your network more difficult to manage.
Knowing the expected behavior of your network and RF in general is essential to both design and troubleshooting. It behooves any designer or admin to understand them in context in your network in particular before you begin troubleshooting; This is especially true in complex environments and when entering a new environment. If you're using a software to aid your design or troubleshooting, be sure to double-check what the software does and does not simulate. Not all software will model all of the properties well, as it's very computationally expensive, and this can lead to some surprises when you go back to validate the network after implementation. Avoid these surprises at all costs, by knowing what to expect from your RF.