Technician License Course Chapter 4

Transcription

1 Technician License Course Chapter 4 Propagation, Basic Antennas, Feed lines & SWR K0NK 26 Jan 18

2 The Antenna System Antenna: Facilitates the sending of your signal to some distant station. Feed line: Connects your station to the antenna. Test and matching equipment: Allows you to monitor/modify antenna performance.

3 Basic Station and Antenna System Antenna Transceiver Feed line

4 Line-of-Sight Propagation Preview

5 Propagation Review Line-of-Sight is blocked

6 Let s Get Clever Add a repeater However we are still limited to line-of-sight

7 HF Propagation Let the ionosphere reflect the radio waves Skip

8 Ionospheric Skip Hambands HF (High Frequency) VHF / UHF Line of-sight Pg 1-22 Pg

9 Antenna Words of Wisdom The antenna is often considered to be the most important component of your station: It is your antenna that launches your transmitted signal off into space It your antenna that captures the received signals from space 9

10 Radio Communication Radio Signal Propagation Transmitter Receiver Modulation Demodulation

11 The Important Bands HF 3 to 30 MHz High Frequency 80, 60, 40, 30, 20, 17, 15, 12 & 10 m bands VHF 30 to 300 MHz Very High Frequency 6, 2 & 1.25 m bands. UHF 300 to 3000 MHz Ultra High Frequency 70 cm

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13 VHF UHF Line of Sight Line-of-Sight Typical distances HT HT: ½ to 2 mi Mobile Mobile: 5 15 mi Base Base: mi

14 VHF UHF Limited Range Line-of-Sight is blocked

15 Radio Horizon 4-1 The distance at which radio signals are blocked by the curvature of the earth. Line-of-Sight Earth

16 Stretching the Radio Horizon The earth seems less curved to radio waves than to light. Minor bending (refraction) of the radio wave 4-1 Earth

17 Repeater Communications Repeaters extends the range However we are still limited to line-of-sight Typical distances HT Repeater: 5-10 mi Mobile Repeater: mi Base Repeater: mi

18 VHF UHF Reflection Water tower or building reflection

19 Operating from Inside Buildings 4-1 & 4-2 Higher frequencies have shorter wavelengths Shorter wavelengths, like UHF (440 MHz), may be better able to propagate into and out of metal buildings than VHF (144 MHz). Opening that are ½ wavelength or greater tend to let radio signals through.

20 Multi-Path 4-2 Yields Picket-fencing (2 m) Rapid flutter as you drive along Or irregular fading Random combining of signals

21 Multi-Path 4-2 In addition to picket fencing in voice signals It can degrade data communications Higher error rate Sometimes moving a few feet can improve communications

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23 Knife Edge 4-2 The signal is refracted

24 Topographic Scatter Can regularly allow for over the horizon VHF and UHF communications for miles Requires high transmitter power & big antennas Troposphere < 12 mi Earth

25 Temperature Inversion 4-2 Causes Troposphere Ducting VHF 2 meter ( MHz) Temperature Inversion

26 Sporadic E 4-4 Sporadic E can cause the occasional reception of strong VHF signals from long distances Sporadic E cloud Earth

27 Auroral Reflections (Northern Lights) 4-4 Aurora can reflect VHF signals The signals exhibit rapid flutter & distortion Solar energy Charged particles Earth s magnetic field in polar regions

28 Meteor Scatter meter (50-54 MHz) is the best band for Meteor Scatter Earth

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30 Ionospheric Skip Hambands HF (High Frequency) VHF / UHF Line of-sight Pg 1-22 Pg

31 The Ionosphere 4-3 A region from 30 to 260 miles above the surface of the Earth Atmosphere thin enough for atoms to be ionized by solar ultraviolet radiation Ions are electrically conductive 2014 Technician License Course

32 Ionospheric Levels 4-3 Because of varying density, the ionosphere forms layers with different amounts of ionization Ionization varies with solar illumination (hour to hour) and intensity of solar radiation Higher ionization refracts or bends radio waves more strongly 2014 Technician License Course

33 Sunspot Cycle 4-3 The level of ionization depends on the intensity of radiation from the Sun. Radiation from the Sun varies with the number of sunspots on the Sun s surface. High number of sunspots results in high levels of ionizing radiation emitted from the Sun. Sunspot activity follows an 11-year cycle Technician License Course

34 The Ionosphere An RF Mirror Reflection depends on frequency and angle of incidence. Too high a frequency or angle and the waves are lost to space. Skip is very rare on the 144 MHz and higher UHF bands. Each reflection is called a hop 4-3 & Technician License Course

35 The Ionosphere An RF Mirror 4-4 Sky-wave or skip propagation is responsible for most over-the-horizon propagation on HF and low VHF (10 and 6 meters) during peaks of the sunspot cycle. During periods of high solar activity, 10 meters can support communications between dawn and shortly after sunset. Each ground-to-sky-to-ground trip is called a hop Technician License Course

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37 Basic Station and Antenna System Antenna Transceiver Feed line

38 The Radio Wave Pg 4-6 Electromagnetic wave: Radio waves are electromagnetic waves, which is a combination of an electric field and a magnetic field. The electric wave carries the signal from the transmitter to the receiver

39 Electromagnetic Waves 4-6 Radio wave leave an antenna much like ripples from a stone thrown into a pond

40 A Look at Polarization Pg 4-6 Polarization: The direction of the electric field relative to the surface of the earth. Same as the physical direction of the antenna Vertical Horizontal Circular A vertically polarized antenna has an electric field perpendicular to the surface of the earth.

41 A Look at Polarization Pg 4-6 For effective VHF and UHF communications, both antennas in a communication link need to have the same polarization or the signal strength will be greatly reduced If the signal is reflected off the ionosphere, the polarization is randomized (scrambled) Both vertical and horizontal antennas are effective on the HF bands when using skip propagation.

42 The Antenna (Some Vocabulary) Omni-directional radiates in all directions. Directional beam focuses radiation in specific directions. Gain apparent increase in power in a particular direction because energy is focused in that direction. Measured in decibels (db)

43 Antenna Radiation Patterns Pg 4-7 Radiation patterns are a way of visualizing antenna performance. The further the line is away from the center of the graph, the stronger the signal at that point.

44 Antenna Radiation Patterns Pg 4-7

45 What is a db? A decibel (db) is used to express an increase or decrease in an electrical or electromagnetic signal

46 Feedlines have Loss Antennas (can) have Gain Pg 4-7 Gain and loss are measured in decibels db Decibel Change in Power Observed Effect 1 db Only 20% Hardly perceivable 3 db Factor of 2 Just noticeable 6 db Factor of 4 Significant 10 db Factor of 10 Very significant 20 db Factor of 100 Extremely significant

47 Remember Ohm s Law?

48 Remember Ohm s Law?

49 Characteristics of Radio Equipment Transceiver Feedline Test Equipment Antennas Size, Weight, Color, Characteristic Impedance

50 Impedance AC Resistance What is Impedance? Impedance is somewhat similar to resistance. Measured in Ohms; Symbol: Ω Affects AC waveforms (radio frequencies) Characteristic Impedance. Radios Feedlines Antennas all have characteristic impedances

51 50 Ohm Characteristic Impedance Pg Transceiver 50 Antenna Transceiver 50 Coax (Feedline) We strive to have all of the impedances be the same.

52 Antenna System Efficiency For efficient transfer of energy from the transmitter to the feed line and from the feed line to the antenna, the various impedances need to match. When there is mismatch of impedances, things may still work, but not as effectively as they could.

53 Feed Line types (Transmission Line) The purpose of the feed line is to get energy from your station to the antenna. Basic feed line types. Coaxial cable (coax). Open-wire, ladder line, parallel conductor. Each has a characteristic impedance, each has its unique application. Pg 4-8

54 Coaxial Cable Pg 4-8 & 4-9 Coax is the most common feed line. Easy to use & install. Matches impedance of modern radio equipment (50 ohms). Some loss of signal, the amount of loss depends on the coax quality (cost). Loss creates heat.

55 Some Coax Facts Pg 4-8 A the frequency is increased, the loss will increase in a coax cable A hardline coax will have the lowest loss (and is mostly used at VHF and UHF).

56 Open-Wire/Ladder Line Pg 4-8 Not common today except in special applications. Difficult to use. Need an antenna tuner to make impedance match but this allows a lot of flexibility. Theoretically has very low loss.

57 Test and Matching Equipment Proper impedance matching is important enough to deserve some simple test equipment as you develop your station repertoire. Basic test equipment: SWR meter. Matching equipment: Antenna tuner.

58 Adding a SWR Meter 50 Transceiver Non-50 Antenna Xcvr SWR Meter 50 SWR Meter 50 Coax (Feedline)

59 SWR Meters Measure SWR directly by sensing power flow in the line Usually installed at the transmitter 2014 Technician License Course

60 Standing Wave Ratio (SWR) Pg 4-10 If the antenna and feed line impedances are not perfectly matched, some RF energy is not radiated into space and is returned (reflected) back to the source. Something has to happen to this reflected energy generally converted into heat or unwanted radio energy (bad news). SWR is a measure of how well the load (antenna) is matched to the transmission line.

61 Standing Wave Ratio (SWR) SWR % of Power Being Reflected Back What it Means to Operation 1 : 1 0% Perfect match / Very good operation 1.5 : 1 4% Good match / Good operation 2 : 1 11% Fair match / Reduced performance 3 : 1 25% Poor match / Poor performance 4 : 1 or higher 38% or higher Bad match / Bad performance

62 What happens with high SWR? 50 Transceiver 200 Antenna 38% Xcvr SWR Meter 50 SWR Meter 50 Coax (Feedline)

63 Operating with High SWRs can get EXPENSIVE

64 Making SWR Measurements Pg 4-10 It is common to make SWR measurements at several frequencies in the band of interest. A plot helps us visualize the SWR. The lowest SWR point is generally called the resonant frequency.

65 Nothing is Perfect Pg 4-10 Although the goal is to get 100% of your radio energy radiated into space, that is virtually impossible. What is an acceptable level of loss (reflected power or SWR?) 1:1 is perfect. 2:1 should be the max you should accept (as a general rule). Modern radios will start lowering transmitter output power automatically when SWR is above 2:1. 3:1 is when you need to do something to reduce SWR. 4:1 or higher indicates a serious impedance mismatch

66 Some SWR Facts Pg 4-10 An antenna with a low SWR will allow for more efficient transfer of power from the transmitter to the antenna Low SWR reduces losses between the transmitter and the antenna Bad connections can cause erratic SWR