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Chapter 4

Chapter 4. Propagation, Antennas and Feed Lines. Chapter 4 Propagation, Antennas & Feed Lines. Today’s agenda How radio signals travel from place to place Basic concepts of antennas How feed lines are constructed and used What SWR is and what it means to you

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Chapter 4

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  1. Chapter 4 Propagation, Antennas and Feed Lines

  2. Chapter 4 Propagation, Antennas & Feed Lines • Today’s agenda • How radio signals travel from place to place • Basic concepts of antennas • How feed lines are constructed and used • What SWR is and what it means to you • Practical antenna system construction Technician -- 1 July 2010 - 30 June 2014

  3. Chapter 4 Propagation Radio waves travel in many ways and are affected by several factors. Like light waves, radio waves are affected by the phenomena of reflection, refraction, diffraction, absorption, polarization and scattering. Radio propagation is also affected by the daily changes of water vapor in the troposphere and ionization in the upper atmosphere, due to the sun. Technician -- 1 July 2010 - 30 June 2014

  4. Chapter 4 Propagation Fortunately, you don’t have to know or understand most of the stuff on the previous slide to get your Tech license. Generally speaking, radio waves travel in a straight line unless they are reflected or diffracted along the way. The strength of a radio wave decreases as it travels from the transmitting antenna and eventually becomes too weak to be received. The distance over which a radio transmission can be received is called “range”. Technician -- 1 July 2010 - 30 June 2014

  5. Chapter 4 Propagation The curvature of the Earth sets an effective range limit for many signals, creating a “radio horizon”. “Line-of-sight” propagation occurs between transmitting and receiving antennas are within direct sight of each other. Most propagation at VHF and higher frequencies is “line-of- sight”. You can increase the range of line-of-sight propagation by increasing the height of the antenna or your transmitter’s power. Technician -- 1 July 2010 - 30 June 2014

  6. Chapter 4 Propagation Radio waves at HF and lower frequencies can also travel along the surface of the earth as “ground wave” propagation. Radio waves can be reflected by buildings, hills and even weather-related activity. Radio waves can be “diffracted” (bent or spread) as they travel past the sharp edges of a building or other object. This is called “knife-edge” propagation. Technician -- 1 July 2010 - 30 June 2014

  7. Chapter 4 Propagation When VHF/UHF signals are diffracted around a solid object, some signals appear behind the object as they are bent in different ways. The result is “shadow zones” where signals can be found. Technician -- 1 July 2010 - 30 June 2014

  8. Chapter 4 Propagation Diffraction also makes the Earth seem less curved to VHF and UHF radio waves than light waves. This allows VHF and UHF signals to travel somewhat farther than the visual “line-of-sight” horizon. Certain radio waves can penetrate solid objects. UHF signals are more effective in propagating into and out of buildings in urban areas. Technician -- 1 July 2010 - 30 June 2014

  9. Chapter 4 Propagation Radio signals can arrive at a receiver after taking different paths from the transmitter. When this happens they can be out of phase and interfere with each other or even cancel each other completely. This phenomenon is known as “multipath” and can cause a signal to become weak and distorted. Simply moving your antenna just a few feet may avoid the location where the cancellation is occurring. Technician -- 1 July 2010 - 30 June 2014

  10. Chapter 4 Propagation Multipath propagation of signals from distant stations results in irregular fading even when reception is generally good. VHF or UHF signals from mobile stations moving through an area where multipath is present produce a characteristic rapidly changing signal strength known as “mobile flutter” or “picket fencing”. Variations in signal strength from multipath can also cause digital data signals to be received with higher error rates, especially at VHF and UHF. Technician -- 1 July 2010 - 30 June 2014

  11. Chapter 4 Propagation Propagation at or above VHF is sometimes “assisted” by atmospheric phenomenon such as weather fronts or temperature inversions. This phenomenon is called “tropospheric” propagation or just plain “tropo”. Layers in the air form structures called “ducts” that can allow even microwave signals to be heard over long distances. Tropo is regularly use by radio amateurs to make normally impossible long distance contacts at VHF and UHF. It is not uncommon for these contacts to exceed over 300 miles. Technician -- 1 July 2010 - 30 June 2014

  12. Chapter 4 Propagation Radio signals are also conducted by conductive things in the sky. For example: Airplanes can reflect 2-meter and 70-centimeter signals over hundreds of miles. About 30 miles above the surface of the Earth begins an area the extends to about 270 miles above the Earth. This area is called the “ionosphere” and it plays a critical role in amateur radio communications. The ionosphere consists of four layers: D, E, F1 and F2. The D-layer is closest to Earth and the F1 and F2 combine at night to form the F-layer. Technician -- 1 July 2010 - 30 June 2014

  13. Chapter 4 Propagation Depending on the time of day and the intensity of solar radiation, these layers (E, F1 and F2) can diffract or absorb (D and E) radio waves. Radio waves at HF (and sometimes VHF) can be completely bent back toward the Earth by diffraction in the E and F layers of the ionosphere as if they were reflected. This is called “skywave” propagation or “skip”. Technician -- 1 July 2010 - 30 June 2014

  14. Chapter 4 Propagation Since the Earth’s surface is also conductive, it too can reflect radio waves. And yes, this means a radio wave can be reflected between the ionosphere and ground multiple times. Each reflection from the ionosphere is called a “hop” and allows radio waves to be received hundreds or even thousands of miles away. This is the most common way for hams to make long-distance contacts on the HF bands. Technician -- 1 July 2010 - 30 June 2014

  15. Chapter 4 Propagation When sky-wave propagation on an amateur band is possible between two points, the band is said to be “open”. If not, then the band is “closed”. Because the Sun plays a huge role in HF propagation, that means that propagation may not be supported in all directions all the time. The changing seasons and time of day and the frequency are all important factors that affect propagation as well. Technician -- 1 July 2010 - 30 June 2014

  16. Chapter 4 Propagation Higher frequencies are bent less than lower frequencies. At VHF and higher frequencies, the waves usually pass through the ionosphere and are lost to space. VHF and UHF signals from beyond the “radio horizon” are rarely heard without being relayed by a repeater. The highest frequency that can be reflected back to Earth is the “maximum useable frequency” or “MUF”. The lowest frequency that can travel between those points without being absorbed is the “lowest useable frequency” or “LUF”. Technician -- 1 July 2010 - 30 June 2014

  17. Chapter 4 Propagation The MUF rises as the sun illuminates the ionosphere. The upper HF bands such as 10 meters (28-29 MHz) are more likely to be open during the day. VHF and UHF fans also experience ionospheric propagation, especially during the peak of the 11-year sunspot cycle when VHF signals can be bent back to Earth. Under these circumstances long-distance communications are possible. Technician -- 1 July 2010 - 30 June 2014

  18. Chapter 4 Propagation At all points in the solar cycle, portions of the E-layer can become sufficiently ionized to reflect VHF and UHF signals back to Earth. This is called “sporadic-E” or “ES” propagation and it is most common during early summer and mid-winter months on 10, 6 and 2 meters. Technician -- 1 July 2010 - 30 June 2014

  19. Chapter 4 Propagation Looking for a challenge? Try bouncing your signal off the Aurora Borealis or “Northern Lights”. It’s a challenge because the aurora is constantly changing and the reflected signals change strength quickly and they are often distorted. Instructor: Click here for audio G6DER KI6IJ Technician -- 1 July 2010 - 30 June 2014

  20. Chapter 4 Propagation Here’s another challenge: Bouncing signals off the tail of a meteor. It called “Meteor scatter” Best band is 6 meters Contacts with stations 1200 to 1500 miles away are possible. CQ Italy 4 X-ray Charlie Charlie Instructor: Click here for audio Technician -- 1 July 2010 - 30 June 2014

  21. Chapter 4 Antenna Fundamentals • There are three important rules about antennas… • There is no perfect antenna (except mine) • Any antenna is better than no antenna • Higher is better • Any electrical conductor can act as an antenna for radio signal. You do not have to purchase an antenna – why not build one instead? They’re not that complicated. Technician -- 1 July 2010 - 30 June 2014

  22. Chapter 4 Antenna Fundamentals • A “feed line” is used to deliver radio signals to or from the antenna. • The connection of the antenna and the feed line is called the “feed point” of the antenna. • The ratio of the radio frequency voltage to current at the feed point is the antenna’s “feed point impedance”. • The conducting portions of an antenna are called elements. • “I have a 15 element tri-bander on 10, 15 and 20-meters” Technician -- 1 July 2010 - 30 June 2014

  23. Chapter 4 Antenna Fundamentals An antenna with more than one element is called an “array”. The element connected to the feed line is called the “driven element”. If all elements are connected to a feed line that’s a “driven array”. Elements not directly connected to the feed line but influence the performance of the antenna, are called “parasitic elements”. Technician -- 1 July 2010 - 30 June 2014

  24. Chapter 4 Antenna Fundamentals RF current in the antenna element creates radio waves that travel away from the antenna. The antenna wave consists of electrical and magnetic energy as a result of electrons moving back and forth in the antenna. The wave is a combination of an electric and a magnetic field, just like those in a capacitor and an inductor, but spreading out into space like ripples on the surface of water. Technician -- 1 July 2010 - 30 June 2014

  25. Chapter 4 Antenna Fundamentals • Because a radio wave is made up of electric and magnetic fields, it is called an “electromagnetic wave”. • The wave’s electric and magnetic fields oscillate at the same frequency as the RF current in the antenna. • “Polarization” refers to the direction in which the electric field of a radio wave is oriented. • “Horizontally polarized” antennas radiate a radio wave whose electric field is oriented horizontally (parallel with Earth). Technician -- 1 July 2010 - 30 June 2014

  26. Chapter 4 Antenna Fundamentals • “Vertically polarized” antennas radiate a radio wave perpendicular to the surface of the Earth. • When the electric field of the radio wave and the antenna element have the same polarization, the maximum amount of signal is created in the antenna. Hold that handheld radio upright! Technician -- 1 July 2010 - 30 June 2014

  27. Chapter 4 Antenna Fundamentals • Why else do we care about polarization? • When the polarization is mismatched, the received signal can be dramatically reduced by as much as 100 times! • When polarization is mismatched, less current is created in the antenna. • By the way, as radio waves travel through the ionosphere, their polarization can change randomly so it could be any polarization when it hits your antenna. Good luck! Technician -- 1 July 2010 - 30 June 2014

  28. Chapter 4 Antenna Fundamentals Signal Strength • Antennas, propagation and electronic circuits change signal strengths by many factors of ten. • Radio signals vary in strength. • At the input to a receiver signals are often measured in one ten-billionth of a watt! • When they come out of the transmitter they’re often measured in kilowatts! Technician -- 1 July 2010 - 30 June 2014

  29. Chapter 4 Antenna Fundamentals Signal Strength • So how do we level the playing field so to speak, with regard to signal strength? • We use very precise linear tool called the “Dessy Bell” • We wave a piece of paper with the French noun “d’Bee” written on it • We shout, “Give me S-units or give me Dessie Belle” • We introduce the concept of the decibel (dB) at this point. Technician -- 1 July 2010 - 30 June 2014

  30. Chapter 4 Antenna Fundamentals Signal Strength • The “decibel” measures the ratio of two quantities as a power of 10. The formula for computing decibels is: • dB = 10 log (power ratio) • and • dB = 20 log (voltage ratio) • Positive values of dB mean the ratio is greater than 1 • Negative values of dB mean the ratio is less than 1 Technician -- 1 July 2010 - 30 June 2014

  31. Chapter 4 Antenna Fundamentals Signal Strength • You have a choice • a. Memorize the answers to the three questions about decibels in the question pool – only one of which could appear in you exam. • Use the calculator that will be provided at the exam session. • Today, we’ll use the calculator on the laptop. In theory. Technician -- 1 July 2010 - 30 June 2014

  32. Chapter 4 Antenna Fundamentals Signal Strength • What’s the approximate amount of change in decibels of a power increase from 5 watts to 10 watts? • 10 Log (10/5) = ? • 10 Log (2) = ? • Log 2 = 0.30102999 (type “2”; press “Log”; multiply by 10) • 10 x 0.30102999 = 3.0102999 or 3 dB Technician -- 1 July 2010 - 30 June 2014

  33. Chapter 4 Antenna Fundamentals Signal Strength • What’s the approximate amount of change in decibels of a power decrease from 12 watts to 3 watts? • 10 Log (12/3) = ? • 10 Log (4) = ? • Log 4 = 0.60205999 (type “4”; press “Log”; multiply by 10) • 10 x 0.60205999 = 6.0205999 or 6 dB Technician -- 1 July 2010 - 30 June 2014

  34. Chapter 4 Antenna Fundamentals Signal Strength • What’s the approximate amount of change in decibels of a power decrease from 200 watts to 20 watts? • 10 Log (200/20) = ? • 10 Log (10) = ? • Log 10 = 1 (type “1”; press “Log”; multiply by 10) • 10 x 1 = 10 dB Technician -- 1 July 2010 - 30 June 2014

  35. Chapter 4 Antenna Fundamentals Here’s another pesky concept: Antenna Gain The concentration of radio signals in a specific direction is called “gain”. Antenna gain increases signal strength in a specified direction when compared to a reference antenna. An antenna can create gain by radiating radio waves that add together in the preferred direction and cancel in others. Gain only focuses power - it does not create power. Technician -- 1 July 2010 - 30 June 2014

  36. Chapter 4 Antenna Fundamentals An “isotropic” antenna has no gain because it radiates equally in every possible direction. In reality, isotropic antennas do not exist. They are used as imaginary references. An “omnidirectional” antenna radiates a signal equally in every horizontal direction. An antenna with gain in a specific direction is called a “beam” or “directional” antenna. Technician -- 1 July 2010 - 30 June 2014

  37. Chapter 4 Antenna Fundamentals This is an “azimuthal pattern” of an antenna’s transmission pattern while looking from above. There are two primary lobes and two secondary lobes with “nulls” between each. Technician -- 1 July 2010 - 30 June 2014

  38. Chapter 4 Antenna Fundamentals A second “azimuthal pattern” of an antenna’s transmission pattern while looking from above. There are two primary lobes and four very minor secondary lobes with “nulls” between each. Technician -- 1 July 2010 - 30 June 2014

  39. Chapter 4 Antenna Fundamentals Looking at the side view we can see how the antenna radiates upwards. There are four lobes going forwards and 3 minor lobes going to the back. This is an example of a directional radiation pattern. Technician -- 1 July 2010 - 30 June 2014

  40. Chapter 4 Antenna Fundamentals Here is an example of how the height of the antenna affects the radiation patterns of the antenna. Technician -- 1 July 2010 - 30 June 2014

  41. Chapter 4 Feed Lines and SWR Feed lines are what connect an antenna to a radio. We also use feed lines to transfer RF signals from one piece of equipment to another. For example: From a radio to a tuner or to an amplifier. Feed lines consist of two conductors separated by an insulating material. Technician -- 1 July 2010 - 30 June 2014

  42. Chapter 4 Feed Lines and SWR Feed lines use special materials and construction methods to minimize the loss of power which is dissipated as heat and to prevent signals from leaking in or out. When a loss occurs and there is always some loss, we call it “feed line loss”. Feed line loss increases with frequency for all types of feed lines. Technician -- 1 July 2010 - 30 June 2014

  43. Chapter 4 Feed Lines and SWR The most popular feed line used by hams to connect radios and antennas is “coaxial cable” or “coax” for short. It’s easy to use and requires very few considerations for installation. Technician -- 1 July 2010 - 30 June 2014

  44. Chapter 4 Feed Lines and SWR • Coax consists of the following: • A wire center conductor • An insulator or “dialectric” • A tubular shield of braided wire or foil • A rubber/plastic sheath called the “jacket” Technician -- 1 July 2010 - 30 June 2014

  45. Chapter 4 Feed Lines and SWR Coax comes in a variety of types and sizes for various applications. Type most commonly used Technician -- 1 July 2010 - 30 June 2014

  46. Chapter 4 Feed Lines and SWR Coax comes in a variety of types and sizes for various applications. One special type of coax is called “hard line” because its shield is made from a semi-flexible solid tube of aluminum or copper. This severely restricts the amount of bending the cable can do. The advantage to this type of coax is that it has the lowest loss of any type of coaxial feed line. Technician -- 1 July 2010 - 30 June 2014

  47. Chapter 4 Feed Lines and SWR Feed lines have a “characteristic impedance” which is a measurement of how energy is carried by the feed line and is abbreviated as Z0. The dimensions of feed line conductors, the spacing between them, and the insulating material determine a feed line’s characteristic impedance. Most coaxial cable used in ham radio has a characteristic impedance of 50 ohms. Technician -- 1 July 2010 - 30 June 2014

  48. Chapter 4 Feed Lines and SWR Here is yet another pesky concept. The power carried by the feed line is transferred to your antenna when the impedance of the antenna and the feed line are identical or “matched”. If the impedances don’t match some of the power is “reflected” by the antenna. Power going toward the antenna is “forward power” and power reflected by the antenna is called “reflected power”. Technician -- 1 July 2010 - 30 June 2014

  49. Chapter 4 Feed Lines and SWR The greater the difference between the antenna and feed line impedances, more power is reflected. Reflected and forward power going in opposite directions create a stationary wave-like interference in the feed line called a “standing wave”. The ratio of the maximum value to the minimum value of the interference is call the “standing wave ratio”. It is usually measured at the point where the feed line connects to the transmitter. Technician -- 1 July 2010 - 30 June 2014

  50. Chapter 4 Feed Lines and SWR SWR in an antenna system is an indicator of how well the antenna and feed line impedances are matched. When there is no reflected power the SWR is 1:1 which is called a “perfect match”. A the amount of reflected power increases so does the SWR. SWR is always greater than or equal to 1:1. SWR greater than 1:1 is called an “impedance mismatch” or “mismatch”. Technician -- 1 July 2010 - 30 June 2014

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