Lesson 2

1. Lesson 2 Radio Frequency Fundamentals Dr. Tahseen Al-Doori

2. Objectives • Define a Radio Frequency Signal • Define and Describe the Following RF Characteristics • Polarity • Wavelength • Frequency • Amplitude • Phase Dr. Tahseen Al-Doori

3. Objectives (Cont.) • Define and Describe the Following RF Behaviors • Wave Propagation • Absorption • Reflection • Scattering • Refraction • Diffraction • Loss (Attenuation) • Free Space Path Loss • Multipath • Gain (Amplification) Dr. Tahseen Al-Doori

4. To properly design, deploy, and administer an 802.11 wireless network, in addition to understanding the OSI model and basic networking concepts, you must broaden your understanding of many other networking technologies. • For instance, when administering an Ethernet network, you typically need a comprehension of TCP/IP, bridging, switching, and routing. The skills to manage an Ethernet network will also aid you as a Wi-Fi administer because most 802.11 wireless networks act as “portals” into wired networks. The IEEE only defines the 802.11 technologies at the Physical layer and the MAC sublayer of the Data-Link layer. Dr. Tahseen Al-Doori

5. In order to fully understand the 802.11 technology, it is necessary to have a clear concept of how wireless works at the first layer of the OSI model, and at the heart of the Physical layer is radio frequency (RF) communications. Dr. Tahseen Al-Doori

6. if you have a good grasp of the RF characteristics and behaviors, your skills as a wireless network administrator will be ahead of the curve. • Why does a wireless network perform differently in an auditorium full of people than it does inside an empty auditorium? Why does the performance of a wireless LAN seem to degrade in a storage area with metal racks? Why does the range of a 5 GHz radio transmitter seem shorter than the range of a 2.4 GHz radio card? These are the type of questions that can be answered with some basic knowledge of how RF signals work and perform. Dr. Tahseen Al-Doori

7. What Is an RF (Radio Frequency) Signal? • An RF signal radiates in a continuous pattern that is governed by certain properties such as wavelength, frequency, amplitude, phase, and polarity. Additionally, electromagnetic signals can travel through mediums of different materials or travel in a perfect vacuum. • When an RF signal travels through a vacuum, it moves at the speed of light, which is approximately 300,000,000 meters per second, or 186,000 miles per second. • RF signals travel using a variety or combination of movement behaviors. These movement behaviors are referred to as propagation behaviors. We will discuss some of these propagation behaviors later, including absorption, reflection, scattering, refraction, diffraction, amplification, and attenuation. Dr. Tahseen Al-Doori

8. Identifying Radio Frequency Characteristics In every RF signal exists characteristic that are defined by the laws of physics: • Polarity • Wavelength • Frequency • Amplitude • Phase We will look at each of these in more detail in the following sections. Dr. Tahseen Al-Doori

9. Polarity • When the movement of the electron flow changes direction in an antenna, electromagnetic waves that change and move away from the antenna are also produced. • The waves consist of two component fields: the electrical (E-field) and the H-field, which is magnetic. Dr. Tahseen Al-Doori

10. Think of a wave as a physical disturbance that transfers energy back and forth between these two fields. These fields are at right angles to each other, and the transfer of energy between these fields is known as oscillation. • Polarization is the vertical or horizontal positioning of an antenna. The orientation of the antenna affects the polarity of the signal. The electric field always resides parallel in the same orientation (plane) of the antenna element. As shown in Figure 1, the parallel plane is called the E-plane and the plane that is perpendicular to the antenna element is known as the H-plane. Dr. Tahseen Al-Doori

11. Fig. 1Polarity, E-Plane and H-plane Dr. Tahseen Al-Doori

12. Wavelength • A wavelength is the distance between the two successive crests (peaks) or two successive troughs (valleys) of a wave pattern. In simpler words, a wavelength is the distance that a single cycle of an RF signal actually travels. Dr. Tahseen Al-Doori

13. It is very important to understand the following statement: • The higher the frequency, the less distance the propagated wave will travel. AM radio stations operate at much lower frequencies than wireless LAN radios. For instance, WSB-AM in Atlanta broadcasts at 750 KHz and has a wavelength of 1,312 feet, or 400 meters. That is quite a distance for one single cycle of an RF signal to travel. • In contrast, some radio navigation satellites operate at a very high frequency, near 252 GHz, and a single cycle of the satellite’s signal has a wavelength of less than .05 inches, or 1.2 millimeters. Figure 2 displays a comparison of these two extremely different types of RF signals. Dr. Tahseen Al-Doori

14. Fig. 2750 KHz wavelength and 252 GHz wavelength Dr. Tahseen Al-Doori

15. The majority of wireless LAN (WLAN) radio cards operate in either the 2.4 GHz frequency range or the 5 GHz range. In Figure 3, you see a comparison of a single cycle of the two different frequency WLAN radio cards. Fig.3 Dr. Tahseen Al-Doori

16. As you can see by these illustrations, the wavelengths of the different frequency signals are different because, although each signal only cycles one time, the waves travel dissimilar distances. Wavelength=C/Freq. Where C=3x10^8 • Because the wavelength property is shorter in the 5 GHz frequency range, Wi-Fi equipment using 5 GHz radio cards will have shorter range and coverage area than Wi-Fi equipment using 2.4 GHz radio cards. Dr. Tahseen Al-Doori

17. Frequency • Frequency is the number of times a specified event occurs within a specified time interval. • The measurement unit for frequency is Hz. Dr. Tahseen Al-Doori

18. Amplitude • In Fig. 4, you can see that (λ) represents wavelength and (y) represents the amplitude. • The first signal’s crests and troughs have more magnitude, thus it has more amplitude. • The second signal’s crests and troughs have decreased, and therefore the signal has less amplitude. Dr. Tahseen Al-Doori

19. Amplitude Dr. Tahseen Al-Doori

20. Note that although the signal strength (amplitude) is different, the frequency of the signal remains constant. • A variety of factors can cause an RF signal to lose amplitude, otherwise known as attenuation, which we will discuss later in this chapter in the section “Loss (Attenuation).” Dr. Tahseen Al-Doori

21. Phase • Phase is not a property of just one RF signal but instead involves the relationship between two or more signals that share the same frequency. The phase involves the relationship between the position of the amplitude crests and troughs of two waveforms. • Phase can be measured in distance, time, or degrees. If the peaks of two signals with the same frequency are in exact alignment at the same time, they are said to be in phase. Dr. Tahseen Al-Doori

22. What is important to understand is the effect that phase has on amplitude when radio cards receive multiple signals. • Signals that have 0 (zero) degrees phase separation (in phase) actually combine their amplitude, which results in a received signal of much greater signal strength, or twice the amplitude. • If two RF signals are 180 degrees out of phase (the peak of one signal is in exact alignment with the trough of the second signal), they cancel each other out and the effective received signal strength is null. • Depending on the amount of phase separation of two signals, the received signal strength may be either cumulative or diminished. Dr. Tahseen Al-Doori

23. Identifying RF Behaviors • 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, multi-path, attenuation, and gain. Dr. Tahseen Al-Doori

24. Wave Propagation • 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. • 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. Dr. Tahseen Al-Doori

25. 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 5, 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. Dr. Tahseen Al-Doori

26. Earth quick As a WLAN engineer, it is important to have an understanding of RF propagation behaviors for making sure that access points are deployed in the proper location, for making sure the proper type of antenna is chosen, and for monitoring the health of the wireless network. Dr. Tahseen Al-Doori

27. Absorption • 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. Dr. Tahseen Al-Doori

28. Scenario • Mr. Sabir performs a wireless site survey at a campus lecture hall. He determined how many access points are required and their proper placement so that he will have the necessary RF coverage. Ten days later, Professor Banks gives a heavily attended lecture on business economics. During this lecture, the signal strength and quality of the wireless LAN was less than desirable. What happened? Dr. Tahseen Al-Doori

29. Reflection • 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. Dr. Tahseen Al-Doori

30. 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 Dubai, UAE, and listen to Iran Station on a clear night. Dr. Tahseen Al-Doori

31. 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. Dr. Tahseen Al-Doori

32. Reflection can be the cause of serious performance problems in a wireless LAN. • As a wave radiates from an antenna, it broadens and disperses. If portions of this wave are reflected, new wave fronts will appear from the reflection points. If these multiple waves all reach the receiver, the multiple reflected signals cause an effect called multipath. • Multipath can degrade the strength and quality of the received signal or even cause data corruption or cancelled signals. Dr. Tahseen Al-Doori

33. Although reflection and multipath can be your number one enemy, new antenna technologies such as Multiple Input Multiple Output (MIMO) may become commonplace in the future to actually take advantage of reflected signals. • MIMO is very much WiMax technology. Dr. Tahseen Al-Doori

34. Scattering • 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. Dr. Tahseen Al-Doori

35. 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. Dr. Tahseen Al-Doori

36. 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. Dr. Tahseen Al-Doori

37. Scattering Dr. Tahseen Al-Doori

38. Refraction • 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. Dr. Tahseen Al-Doori

39. The three most common causes of refraction are: • water vapor, changes in air temperature, and changes in air pressure. Dr. Tahseen Al-Doori

40. Diffraction • 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). Dr. Tahseen Al-Doori

41. 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. Dr. Tahseen Al-Doori

42. 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. • Example is a rock in a river. Dr. Tahseen Al-Doori

43. Loss (Attenuation) • Loss, also known as attenuation, is best described as the decrease of amplitude or signal strength. • Try the EMANIM software to view Attenuation effect of materials due to absorption. Dr. Tahseen Al-Doori

44. Both loss and gain can be gauged in a relative measurement of change in power called decibels (dB), which will be discussed extensively in Lesson 3. • 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. Dr. Tahseen Al-Doori

45. 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. • Freespace 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 Dr. Tahseen Al-Doori

46. 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. Dr. Tahseen Al-Doori

47. 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 Dr. Tahseen Al-Doori

48. The dB calculations will be covered in the next subject which is “RF components and measurements and mathematics.” Dr. Tahseen Al-Doori

49. Multipath • 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. Dr. Tahseen Al-Doori

50. Multipath Dr. Tahseen Al-Doori