Chapter 4 Propagation, Antennas and Feed Lines

2. Chapter 4 Propagation, Antennas and Feed Lines

3. Propagation Radio Waves. • Radio Waves travel in straight lines. • Except: • Reflection. • Bouncing off reflective surface. • Refraction. • Gradual bending while traveling through atmosphere. • Diffraction. • Bending around edge of solid object.

4. Propagation Radio Waves. • Line-of-sight. • Radio horizon. • Distance at which radio signals are blocked by curvature of the earth. • Slightly greater than optical horizon. • Refraction increases radio horizon by about 15%.

5. Propagation Radio Waves. • Diffraction.

6. Propagation Radio Waves. • Multi-Path. • Radio waves reflected off of many objects arrive at receive antenna at different times. • Radio waves can take several different paths through the ionosphere and arrive at receive antenna at different times.

7. Propagation Radio Waves. • Multi-Path. • If the transmitter and/or receiver are moving multi-path can cause rapid signal fading known as “picket fencing”. • Multi-path can cause error rates to increase on data signals.

8. Propagation Radio Waves. • Multi-Path.

9. Propagation Radio Waves. • VHF & UHF radio waves are affected by obstructions in the path. • Buildings can block radio waves. • Radio waves can pass through openings in solid objects such as buildings. • Longest dimension of opening at least 1/2λ. • Because of their shorter wavelength, UHF signals can pass through buildings better than VHF signals.

10. Propagation Radio Waves. • VHF & UHF radio waves are affected by obstructions in the path. • Foliage. • Absorbs radio waves and decreases signal strength. • The higher the frequency, the higher the absorption.

11. Propagation Radio Waves. • VHF & UHF radio waves are affected by obstructions in the path. • Rain, fog. • Absorb radio waves and decrease signal strength of UHF and microwave signals. • Little effect on VHF and lower frequencies.

12. Propagation Radio Waves. • Tropospheric Ducting. • Radio waves can travel for long distances along boundaries of different temperature air layers. • Propagation of 300 miles or more on VHF or UHF.

13. T3A01 -- What should you do if another operator reports that your station’s 2 meter signals were strong just a moment ago, but now they are weak or distorted? • Change the batteries in your radio to a different type • Turn on the CTCSS tone • Ask the other operator to adjust his squelch control • Try moving a few feet or changing the direction of your antenna if possible, as reflections may be causing multi-path distortion

14. T3A02 -- Why might the range of VHF and UHF signals be greater in the winter? • Less ionospheric absorption • Less absorption by vegetation • Less solar activity • Less tropospheric absorption

15. T3A06 -- What term is commonly used to describe the rapid fluttering sound sometimes heard from mobile stations that are moving while transmitting? • Flip-flopping • Picket fencing • Frequency shifting • Pulsing

16. T3A08 -- Which of the following is a likely cause of irregular fading of signals received by ionospheric reflection? • Frequency shift due to Faraday rotation • Interference from thunderstorms • Random combining of signals arriving via different paths • Intermodulation distortion

17. T3A10 -- What may occur if data signals arrive via multiple paths? • Transmission rates can be increased by a factor equal to the number of separate paths observed • Transmission rates must be decreased by a factor equal to the number of separate paths observed • No significant changes will occur if the signals are transmitted using FM • Error rates are likely to increase

18. T3A12 -- How might fog and light rain affect radio range on 10 meters and 6 meters? • Fog and rain absorb these wavelength bands • Fog and light rain will have little effect on these bands • Fog and rain will deflect these signals • For and rain will increase radio range

19. T3A13 -- What weather condition would decrease range at microwave frequencies? • High winds • Low barometric pressure • Precipitation • Colder temperatures

20. T3C05 -- Which of the following effects might cause radio signals to be heard despite obstructions between the transmitting and receiving stations? • Knife-edge diffraction • Faraday rotation • Quantum tunneling • Doppler shift

21. T3C06 -- What mode is responsible for allowing over-the-horizon VHF and UHF communications to ranges of approximately 300 miles on a regular basis? • Tropospheric scatter • D-layer refraction • F2-layer refraction • Faraday rotation

22. T3C08 -- What causes tropospheric ducting? • Discharges of lightning during electrical storms • Sunspots and solar flares • Updrafts from hurricanes and tornadoes • Temperature inversions in the atmosphere

23. T3C11 -- Why do VHF and UHF radio signals usually travel somewhat farther than the visual line of sight distance between two stations? • Radio signals move somewhat faster than the speed of light • Radio waves are not blocked by dust particles • The Earth seems less curved to radio waves than to light • Radio waves are blocked by dust particles

24. Propagation The Ionosphere. • The upper layers of the atmosphere are ionized by UV radiation from the sun. • 30 to 260 miles above the surface.

25. Propagation The Ionosphere. • The ionosphere is divided into layers or regions. • Each layer has unique characteristics.

26. Propagation The Ionosphere. • Some radio frequency ranges (HF & lower VHF frequencies) will be reflected off of the ionosphere & return to earth. • Called “skip”. • Distances well beyond the range of line-of-sight. • Several hundred to several thousand miles. • Maximum of about 2500 miles for a single hop. • Can have multiple hops.

27. Propagation The Ionosphere. • The higher the amount of ionization, the better radio waves are reflected off of the ionosphere.

28. Propagation The Ionosphere. • Amount of ionization varies with time of day. • Sunrise to sunset  higher ionization level. • Amount of ionization varies with sunspot activity. • More sunspots  higher ionization level. • Larger sunspots  higher ionization level. • Number & size of sunspots varies over an 11-year cycle. • Currently in minimum between Cycle 24 & Cycle 25.

29. Propagation The Ionosphere. • Skip is not really reflection (bouncing) but rather refraction (bending). • The shorter the wavelength (higher frequency), the less the signal is refracted (bent). • At some frequency, the wave is no longer bent enough to return to earth. • Critical frequency. • Skip normally occurs in the F-layer (F1 & F2). • Can occur in the E-layer.

30. Propagation The Ionosphere. • The highest frequency that can be used to communicate between 2 points is called the Maximum Useable Frequency (MUF). • The lowest frequency that can be used to communicate between 2 points is called the Lowest Useable Frequency (LUF). • MUF & LUF vary depending on amount of ionization of the ionosphere.

31. Propagation The Ionosphere.

32. Propagation The Ionosphere. • E-Layer Propagation. • Sporadic-E. • Any time during solar cycle. • Early summer & mid-winter • 10m, 6m, & 2m.

33. Propagation The Ionosphere. • E-Layer Propagation. • Aurora. • Rapid signal strength changes. • Sounds fluttery or distorted. • Primarily 6m. • Meteor scatter. • Primarily 6m.

34. Propagation The Ionosphere. • The lowers regions of the ionosphere absorb radio waves. • Primarily D-layer. • Some absorption in E-layer. • The longer the wavelength (lower frequency), the more absorption.

35. T3A11 -- Which part of the atmosphere enables the propagation of radio signals around the world? • The stratosphere • The troposphere • The ionosphere • The magnetosphere

36. T3C01 -- Why are direct (not via a repeater) UHF signals rarely heard from stations outside your local coverage area? • They are too weak to go very far • FCC regulations prohibit them from going more than 50 miles • UHF signals are usually not reflected by the ionosphere • UHF signals are absorbed by the ionospheric D layer

37. T3C02 -- Which of the following is an advantage of HF vs VHF and higher frequencies? • HF antennas are generally smaller • HF accommodates wider bandwidth signals • Long distance ionospheric propagation is far more common on HF • There is less atmospheric interference (static) on HF

38. T3C03 -- What is a characteristic of VHF signals received via auroral reflection? • Signals from distances of 10,000 or more miles are common • The signals exhibit rapid fluctuations of strength and often sound distorted • These types of signals occur only during winter nighttime hours • These types of signals are generally strongest when your antenna is aimed west

39. T3C04 -- Which of the following propagation types is most commonly associated with occasional strong over-the-horizon signals on the 10, 6, and 2 meter bands? • Backscatter • Sporadic E • D layer absorption • Gray-line propagation

40. T3C07 -- What band is best suited for communicating via meteor scatter? • 10 meter band • 6 meter band • 2 meter band • 70 centimeter band

41. T3C09 -- What is generally the best time for long-distance 10 meter band propagation via the F layer? • From dawn to shortly after sunset during periods of high sunspot activity • From shortly after sunset to dawn during periods of high sunspot activity • From dawn to shortly after sunset during periods of low sunspot activity • From shortly after sunset to dawn during periods of low sunspot activity

42. T3C10 -- Which of the following bands may provide long distance communications during the peak of the sunspot cycle? • 6 or 10 meter bands • 23 centimeter band • 70 centimeter or 1.25 meter bands • All of these choices are correct

43. Antenna and Radio Wave Basics Antennas. • Converts an RF electrical signal into an electromagnetic wave (radio wave) or vice versa. • Any electrical conductor can act as an antenna. • Some sizes & configurations work better than others.

44. Antenna and Radio Wave Basics Antennas. • Feed point. • Place where the feed-line is connected to antenna. • Feed Point Impedance. • Ratio of RF voltage to RF current at the feed point. • If impedance is pure resistance (no reactance) then antenna is said to be resonant.

45. Antenna and Radio Wave Basics Antenna Elements. • Conductive parts of an antenna. • Driven element • Element that feed-line is connected to. • Parasitic element(s) • Element(s) not directly connected to feed-line. • Driven array • More than one driven element.

46. Antenna and Radio Wave Basics Polarization. • An electromagnetic wave consists of an electric wave & a magnetic wave at right angles to each other. • Polarization is the orientation of the electric wave with respect to the earth.

47. Antenna and Radio Wave Basics Polarization. • If electric wave is horizontal (parallel to the ground), then wave is said to be horizontally polarized. • If electric wave is vertical (perpendicular to the ground), then wave is said to be vertically polarized.

48. Antenna and Radio Wave Basics Polarization.

49. Antenna and Radio Wave Basics Polarization. • The direction of the electric field is the same as the direction of the antenna element. • Loop antennas & circular polarization are exceptions. • If polarizations are not matched, reduced signal strength results. • If polarization of radio wave is precisely 90° from that of the antenna, NO signal will be received. • Especially important on VHF, UHF, & up.

50. Antenna and Radio Wave Basics Polarization. • Polarization of sky wave signals (skip) is random & continuously changing. • Elliptically polarized. • Any polarization antenna may be used.