As an RF signal travels through the air and other different mediums, it can move and behave in different manners. RF propagation behaviors include absorption, reflection, scattering, refraction, diffraction, loss, free space path loss, multipath, attenuation, and gain.
Now that you have learned about some of the various characteristics of an RF signal, it is important to have an understanding of the way an RF signal behaves as it moves away from an antenna. Eelectromagnetic waves can move through a perfect vacuum or pass through materials or other media.
The way in which the RF waves move—known as wave propagation—can vary drastically depending on the materials in the signal’s path. Drywall will have a much different effect on an RF signal than metal. What happens to an RF signal between two locations is a direct result of how the signal propagates.
When we use the term propagate, try to envision an RF signal broadening or spreading as it travels farther away from the antenna. An excellent analogy is shown in Figure 1, which depicts an earthquake.
Note the concentric seismic rings that propagate away from the epicenter of the earthquake. RF waves behave in much the same fashion. The manner in which a wireless signal moves is often referred to as propagation behavior.
The most common RF behavior is absorption. If the signal does not bounce off an object, move around an object, or pass through an object, then 100 percent absorption has occurred. As pictured in Figure 2, most materials will absorb some amount of an RF signal to varying degrees.
Brick and concrete walls will absorb a signal significantly, whereas drywall will absorb a signal to a lesser degree. Water is another example of a medium that can absorb a signal to a large extent. Absorption can be a leading cause of attenuation.
One of the most important RF propagation behaviors to be aware of is reflection. When a wave hits a smooth object that is larger than the wave itself, depending upon the media, the wave may bounce in another direction.
This behavior is categorized as reflection. An analogous situation could be a child bouncing a ball off a sidewalk and the ball changing direction. Figure 3 depicts a laser beam pointed at a single small mirror.
Depending on the angle of the mirror, the laser beam bounces or reflects off into a different direction. RF signals can reflect in the same manner depending on the objects or materials the signals encounter.
There are two major types of reflections: sky wave reflection and microwave reflection. Sky wave reflection can occur in frequencies below 1 GHz where the signal has a very large wavelength.
The signal bounces off the surface of the charged particles of the ionosphere in the earth’s atmosphere. This is why you can be in Charlotte, North Carolina, and listen to WLS-AM in Chicago on a clear night.
Microwave signals, however, exist between 1 GHz and 300 GHz. Because they are higher-frequency signals, they have much smaller wavelengths, thus the term microwave. Microwaves can bounce off of smaller objects like a metal door.
Microwave reflection is what we are concerned about in wireless LAN environments. In an outdoor environment, microwaves can reflect off of large objects and smooth surfaces such as buildings, roads, bodies of water, and even the earth’s surface.
In an indoor environment, microwaves reflect off of smooth surfaces such as doors, walls, and file cabinets. Anything made of metal will absolutely cause reflection. Other materials such as glass and concrete may cause reflection as well.
Did you know that the color of the sky is blue because the wavelength of light is smaller than the molecules of the atmosphere? This blue sky phenomenon is known as Rayleigh scattering.
The shorter blue wavelength light is absorbed by the gases in the atmosphere and radiated in all directions. This is another example of an RF propagation behavior called scattering, sometimes called scatter.
Scattering can most easily be described as multiple reflections. These multiple reflections occur when the electromagnetic signal’s wavelength is larger than pieces of whatever medium the signal is passing through.
Scattering can happen in two different ways. The first type of scatter is on a smaller level and has a lesser effect on the signal quality and strength. This type of scatter may manifest itself when the RF signal moves through a substance and the individual electromagnetic waves are reflected off the minute particles within the medium.
Smog in our atmosphere and sandstorms in the desert can cause this type of scattering. The second type of scattering occurs when an RF signal encounters some type of uneven surface and is reflected into multiple directions.
Chain link fences, tree foliage, and rocky terrain commonly cause this type of scattering. When striking the uneven surface, the main signal dissipates into multiple reflected signals, which can cause substantial signal downgrade and may even cause a loss of the received signal. Figure 4 shows a flashlight being shined against a disco mirror ball.
Note how the main signal beam is completely displaced into multiple reflected beams with less amplitude and into many different directions.
In addition to RF signals being absorbed or bounced (via reflection or scattering), if certain conditions exist, an RF signal can be bent in a behavior known as refraction. A straightforward definition of refraction is the bending of an RF signal as it passes through a medium with a different density, thus causing the direction of the wave to change.
RF refraction most commonly occurs as a result of atmospheric conditions. The three most common causes of refraction are water vapor, changes in air temperature, and changes in air pressure.
In an outdoor environment, RF signals typically refract slightly back down toward the earth’s surface. However, changes in the atmosphere may cause the signal to bend away from the earth.
In long-distance outdoor wireless bridge links, refraction can be an issue. An RF signal may also refract through certain types of glass and other materials that are found in an indoor environment. Figure 5 show several examples of refraction.
Not to be confused with refraction, another RF propagation behavior exists that also bends the signal; it’s called diffraction. Diffraction is the bending of an RF signal around an object (whereas refraction, as you recall, is the bending of a signal as it passes through a medium).
Diffraction is the bending and the spreading of an RF signal when it encounters an obstruction. The conditions that must be met for diffraction to occur depend entirely on the shape, size, and material of the obstructing object as well as the exact characteristics of the RF signal, such as polarization, phase, and amplitude.
Typically, diffraction is caused by some sort of partial blockage of the RF signal, such as a small hill or a building that sits between a transmitting radio and a receiver. The waves that encounter the obstruction slow down in speed, which causes them to bend around the object.
The waves that did not encounter the object maintain their original speed and do not bend. The analogy depicted in Figure 6 is a rock sitting in the middle of a river. Most of the current maintains the original flow; however, some of the current that encounters the rock will reflect off the rock and some will diffract around the rock.
Sitting directly behind the obstruction is the receiver radio that is now in an area known as the RF shadow. Depending upon the change in direction and velocity of the diffracted signals, the area of the RF shadow can become a dead zone of coverage or still possibly receive degraded signals.
Loss, also known as attenuation, is best described as the decrease of amplitude or signal strength. A signal may lose strength while on a wire or in the air. On the wired portion of the communications (RF cable), the AC electrical signal will lose strength due to the electrical impedance of coaxial cabling and other components such as connectors.
Attenuation is typically not desired, however, on rare occasions an RF engineer may even add a hardware attenuator device on the wired side of an RF system to introduce attenuation to remain compliant with power regulations.
Once the RF signal is radiated into the air via the antenna, the signal will attenuate due to absorption, distance, and the negative effects of multipath. You already know that as an RF signal passes through different mediums, the signal can be absorbed into the medium, which in turn causes a loss of amplitude.
Different materials typically yield different attenuation results. As discussed earlier, water is a major source of absorption as well as dense materials such as cinder blocks, all of which lead to attenuation.
Both loss and gain can be gauged in a relative measurement of change in power called decibels (dB). List below shows the different attenuation values for several materials.
- Foundation Wall : –15 dB
- Brick, Concrete, Concrete Blocks : –15 dB
- Elevator or metal obstacle : –10 dB
- Metal Rack : –6 dB
- Drywall or Sheetrock : –3 dB
- Non-tinted Glass Windows or Door : –3 dB
- Wood Door : –3 dB
- Cubicle Wall : –2 dB
It is important to understand that an RF signal will also lose amplitude merely as a function of distance in what is known as free space path loss. Also, reflection propagation behaviors can produce the negative effects of multipath and as a result cause attenuation in signal strength.
Free Space Path Loss
Due to the laws of physics, an electromagnetic signal will attenuate as it travels despite the lack of attenuation caused by obstructions, absorption, reflections, diffractions, and so on. Free space path loss is the loss of signal strength caused by the natural broadening of the waves, often referred to beam divergence.
RF signal energy spreads over larger areas as the signal travels farther away from an antenna, and as a result, the strength of the signal attenuates. One way to illustrate free space path loss is to use a balloon analogy.
Before a balloon is filled with helium, it remains small but with a dense rubber thickness. After the balloon is inflated and has grown and spread in size, the rubber becomes very thin.
RF signals will lose strength in much the same manner. Luckily, this loss in signal strength is logarithmic and not linear, thus the amplitude does not decrease as much in a second segment of equal length as it decreases in the first segment.
A 2.4 GHz signal will change in power by about 80 dB after 100 meters but will only lessen another 6 dB in the next 100 meters. Here are the formulas to calculate free space path loss:
LP = 36.6 + (20log10F) + (20log10D)
LP = path loss in dB
F = frequency in MHz
D = distance in miles between antennas
LP = 32.4 + (20log10F) + (20log10D)
LP = path loss in dB
F = frequency in MHz
D = distance in kilometers between antennas
An even simpler way to estimate free space path loss is called the 6dB rule (remember for now that decibels are a measure of gain or loss). The 6dB rule states that doubling the distance will result in a loss of amplitude of 6 dB. Table below shows estimated path loss and confirms the 6dB rule.
|Distance [km]||2.4 GHz||5 GHz|
Also notice that the 5 GHz signal attenuates more than the 2.4 GHz signal. It should be noted that higher frequency signals attenuate faster because of the shorter wavelength.
Multipath is a propagation phenomenon that results in two or more paths of a signal arriving at a receiving antenna at the same time or within nanoseconds of each other. Due to the natural broadening of the waves, the propagation behaviors of reflection, scattering, diffraction, and refraction will occur.
A signal may reflect off an object or scatter, refract, or diffract. In an indoor environment, reflected signals and echoes can be caused by walls, desks, floors, file cabinets, and numerous other obstructions.
In an outdoor environment, it could be a flat road, large body of water, building, or atmospheric conditions. Therefore we have signals bouncing and bending in many different directions.
The principal signal will still travel to the receiving antenna, but many of the bouncing and bent signals may also find their way to the receiving antenna. In other words, “multiple paths” of the RF signal arrive at the receiver, as seen in Figure 7.
It usually takes a little bit longer for the reflected signals to arrive at the receiving antenna because they must travel a longer distance than the principal signal. The time differential between these signals can be measured in millionths of a second (nanoseconds).
The time differential between these multiple paths is known as the delay spread. You will learn later in the book that certain spread spectrum technologies are more tolerant than others of delay spread. So what exactly happens when mutipath presents itself?
In television signal transmissions, multipath causes a ghost effect with a faded duplicate image to the right of the main image. With RF signals, the effects of multipath can be either constructive or destructive. Quite often they are very destructive.
Due to the differences in phase of the multiple paths, the combined signal will often attenuate, amplify, or become corrupted. These effects are sometimes called Rayleigh fading named after British physicist Lord Rayleigh.
The four results of multipath are as follows:
- Downfade This is decreased signal strength. When the multiple RF signal paths arrive at the receiver at the same time and are out of phase with the primary wave, the result is a decrease in signal strength (amplitude). Phase differences of between 121 and 179 degrees will cause downfade.
- Upfade This is increased signal strength. When the multiple RF signal paths arrive at the receiver at the same time and are in phase or partially out of phase with the primary wave, the result is an increase in signal strength (amplitude).
Smaller phase differences of between 0 and 120 degrees will cause upfade. Please understand, however, that the final received signal can never be stronger than the original transmitted signal due to free space path loss.
- Nulling This is signal cancellation. When the multiple RF signal paths arrive at the receiver at the same time and are 180 degrees out of phase with the primary wave, the result can be a complete cancellation of the RF signal.
- Data Corruption Intersymbol interference can cause data corruption. Because of the difference in time between the primary signal and the reflected signals known as the delay spread, along with the fact that there may be multiple reflected signals, the receiver can have problems demodulating the RF signal’s information.
The delay spread time differential can cause bits to overlap with each other and the end result is corrupted data, as seen in Figure 8. This type of multipath interference is often known as intersymbol interference (ISI).
Gain, also known as amplification , can best be described as the increase of amplitude or signal strength. The two types of gain are known as active gain and passive gain. A signal’s amplitude can be boosted by the use of external devices.
Active gain is usually caused by the use of an amplifier on the wire that connects the transceiver to the antenna. The amplifier is usually bidirectional, meaning that it increases the AC voltage both inbound and outbound. Active gain devices require the use of an external power source.
Passive gain is accomplished by focusing the RF signal with the use of an antenna. Antennas are passive devices that do not require an external power source. Instead, the internal workings of an antenna focus the signal more powerfully in one direction than another.
Despite the usual negative effects of multipath, it should be reiterated that when multiple RF signals arrive at the receiver at the same time and are in phase or partially out of phase with the primary wave, the result can be an increase or gain in amplitude.
There are two very different tools that can be used to measure the amplitude of a signal at even given point. The first, a frequency domain tool, can be used to measure amplitude in a finite frequency spectrum. The frequency domain tool used by WLAN engineers is also called a spectrum analyzer.
The second tool, a time domain tool, can be used to measure how a signal’s amplitude changes over time. The conventional name for a time domain tool is an oscilloscope. Figure 9 shows how both these tools can be used to measure amplitude.