Technicians Exam-Antennas

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rledwith
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222570
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Technicians Exam-Antennas
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2013-06-13 12:18:48
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rdl tech exam t9
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Amateur Radio Technicians Exam - T9 Antennas
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  1. T9A01 What is a beam antenna?
    A. An antenna built from aluminum I-beams
    B. An omnidirectional antenna invented by Clarence Beam
    C. An antenna that concentrates signals in one direction
    D. An antenna that reverses the phase of received signals
    • C. An antenna that concentrates signals in one direction
    • The term “beam” is used in the same sense as a flashlight beam. In fact, the reflector of a flashlight acts to concentrate the light bulb’s “signal” in one direction, just as a radio frequency beam antenna does. The beam doesn’t create more power, only focuses it. Beams can be used to increase signal level at a distant station or to reject interference or noise. [Ham Radio License Manual, page 4-14]
  2. T9A02 Which of the following is true regarding vertical antennas?
    A. The magnetic field is perpendicular to the Earth
    B. The electric field is perpendicular to the Earth
    C. The phase is inverted
    D. The phase is reversed
    • B. The electric field is perpendicular to the Earth
    • The vertical antenna has a pair of reasons for the name. First, the antenna is, in fact, vertically oriented! Second, the vertical orientation of the antenna element means that the antenna is vertically polarized — the electric field component of its radiated waves is vertical or perpendicular to the surface of the Earth. [Ham Radio License Manual, page 4-6]
  3. T9A03 Which of the following describes a simple dipole mounted so the conductor is parallel to theEarth’s surface?
    A. A ground wave antenna
    B. A horizontally polarized antenna
    C. A rhombic antenna
    D. A vertically polarized antenna
    • B. A horizontally polarized antenna
    • Like the vertical, a dipole is called a horizontal antenna for two reasons. The antenna element is oriented horizontally and the radiated radio wave is horizontally polarized — its electric field is oriented parallel to the Earth’s surface. [Ham Radio License Manual, page 4-11]
  4. T9A04 What is a disadvantage of the “rubber duck” antenna supplied with most handheld radiotransceivers?
    A. It does not transmit or receive as effectively as a full-sized antenna
    B. It transmits a circularly polarized signal
    C. If the rubber end cap is lost it will unravel very quickly
    D. All of these choices are correct
    • A. It does not transmit or receive as effectively as a full-sized antenna
    • When you buy a new VHF or UHF hand-held transceiver, it will usually have a flexible rubber-coated antenna commonly called a “rubber duck.” This antenna is inexpensive, small, lightweight and difficult to break. On the other hand, its performance is not nearly as good as a full-sized antenna, such as a mobile antenna or a telescoping whip. [Ham Radio License Manual, page 4-13]
  5. T9A05 How would you change a dipole antenna to make it resonant on a higher frequency?
    A. Shorten it
    B. Lengthen it
    C. Insert coils in series with radiating wires
    D. Add capacity hats to the ends of the radiating wires
    A. Shorten it
    (this multiple choice question has been scrambled)
  6. T9A06 What type of antennas are the quad, Yagi, and dish?
    A. Non-resonant antennas
    B. Loop antennas
    C. Directional antennas
    D. Isotropic antennas
    • C. Directional antennas
    • All of these antennas focus the radiated energy toward one direction. The quad and Yagi work by having groups of elements work together to reinforce radiated energy in the desired direction. The dish antenna works like a flashlight, reflecting energy so that a great deal of it is focused in one direction. [Ham Radio License Manual, page 4-15]
  7. T9A07 What is a good reason not to use a “rubber duck” antenna inside your car?
    A. Signals can be significantly weaker than when it is outside of the vehicle
    B. It might cause your radio to overheat
    C. The SWR might decrease, decreasing the signal strength
    D. All of these choices are correct
    • A. Signals can be significantly weaker than when you are outside of the vehicle
    • The vehicle’s metal roof and doors act like shields, trapping the radio waves inside. Some of the signal gets out through the windows (unless they’re tinted by a thin metal coating), but it’s much weaker than if radiated by an external antenna. [Ham Radio License Manual, page 4-13]
  8. T9A08 What is the approximate length, in inches, of a quarter-wavelength vertical antenna for 146 MHz?
    A. 112
    B. 50
    C. 19
    D. 12
    • C. 19
    • Start with the formula for estimating the length of a half-wavelength dipole: Length (in feet) = 468 / frequency (in MHz) = 468 / 146 = 3.2 feet. Convert to inches by multiplying by 12 = 38.5. Because this is a quarter-wavelength antenna, divide the result by two to get about 19 inches. [Ham Radio License Manual, page 4-11]
  9. T9A09 What is the approximate length, in inches, of a 6 meter 1/2-wavelength wire dipole antenna?
    A. 6
    B. 50
    C. 112
    D. 236
    • C. 112
    • Since you already know the half-wavelength (3 meters), convert meters to feet by multiplying by 3.1 = 9.3 feet. Convert to inches by multiplying by 12 = 112 inches. [Ham Radio License Manual, page 4-11]
  10. T9A10 In which direction is the radiation strongest from a half-wave dipole antenna in free space?
    A. Equally in all directions
    B. Off the ends of the antenna
    C. Broadside to the antenna
    D. In the direction of the feedline
    • C. Broadside to the antenna
    • A dipole radiates strongest broadside to the axis of the dipole and weakest off the ends. The radiation pattern for a dipole isolated in space looks like a donut as seen in the figure below. Figure T9-1 shows both two- and three-dimensional patterns. The two-dimensional pattern shows what the three-dimensional pattern would look like if cut through the axis of the dipole. [Ham Radio License Manual, page 4-11]
  11. T9A11 What is meant by the gain of an antenna?
    A. The additional power that is added to the transmitter power
    B. The additional power that is lost in the antenna when transmitting on a higher frequency
    C. The increase in signal strength in a specified direction when compared to a reference antenna
    D. The increase in impedance on receive or transmit compared to a reference antenna
    • C. The increase in signal strength in a specified direction when compared to a reference antenna
    • The concentration of radio signals in a specific direction is called gain. (Antenna gain should not be confused with the gain of a transistor.) Antenna gain is a measure of how much signal strength is increased in a specified direction when compared to the signal from a reference antenna in the same direction. Gain aids communication in the preferred direction by increasing transmitted and received signal strengths. [Ham Radio License Manual, page 4-6]
  12. T9B01 Why is it important to have a low SWR in an antenna system that uses coaxial cable feedline?
    A. To reduce television interference
    B. To allow the efficient transfer of power and reduce losses
    C. To prolong antenna life
    D. All of these choices are correct
    • B. To allow the efficient transfer of power and reduce losses
    • Power reflected from a mismatched antenna bounces back and forth along the feed line. A bit more is transferred to the antenna each time but with each pass through the feed line, some power is lost as heat. Low SWR reduces losses in the feed line because less power is reflected from the antenna. As SWR increases, more power is reflected and more power is lost. Since coaxial feed line has higher losses than open-wire line, low SWR is more important if coaxial feed line is used. [Ham Radio License Manual, page 4-10]
  13. T9B02 What is the impedance of the most commonly used coaxial cable in typical amateur radioinstallations?
    A. 8 ohms
    B. 50 ohms
    C. 600 ohms
    D. 12 ohms
    • B. 50 ohms
    • Most coaxial cable used in Amateur Radio has a characteristic impedance (Z0) of 50 ohms. Coaxial cables used for video and cable television have a Z0 of 75 ohms. Open-wire or twin-lead feed lines have a Z0 of 300 to 450 ohms. [Ham Radio License Manual, page 4-9]
  14. T9B03 Why is coaxial cable used more often than any other feedline for amateur radio antenna systems?
    A. It is easy to use and requires few special installation considerations
    B. It has less loss than any other type of feedline
    C. It can handle more power than any other type of feedline
    D. It is less expensive than any other types of feedline
    • A. It is easy to use and requires few special installation considerations
    • Coaxial cable or coax is easy to work with because it is a single cable. All of the energy being conducted by the feed line is completely contained within the cable, so the cable can be run alongside or even within metallic trays, conduits and towers. It can be coiled up and placed next to other cables without effect. While it may have more loss than open-wire line and weigh a bit more per foot of length, coaxial cable’s ease of use make it a practical choice for most installations. [Ham Radio License Manual, page 4-8]
  15. T9B04 What does an antenna tuner do?
    A. It matches the antenna system impedance to the transceiver’s output impedance
    B. It helps a receiver automatically tune in weak stations
    C. It allows an antenna to be used on both transmit and receive
    D. It automatically selects the proper antenna for the frequency band being used
    • A. It matches the antenna system impedance to the transceiver’s output impedance
    • If the SWR at the end of the feed line is too high for the radio to operate properly, devices called impedance matchers or transmatches or antenna tuners are connected at the output of the transmitter as shown in Figure T9-2. An antenna tuner is adjusted until the SWR measured at the transmitter output is acceptably close to 1:1. This means the antenna system’s impedance has been matched to that of the transmitter output. An antenna tuner doesn’t really tune the antenna, it just adjusts the impedance at the input to the antenna system. Think of the antenna tuner as an electrical gearbox that lets the engine (the transmitter) run at the speed it likes no matter how fast the tires are turning (feed point impedance). [Ham Radio License Manual, page 4-18]
  16. T9B05 What generally happens as the frequency of a signal passing through coaxial cable is increased?
    A. The apparent SWR increases
    B. The reflected power increases
    C. The characteristic impedance increases
    D. The loss increases
    • D. The loss increases
    • Feed lines used at radio frequencies use special materials and construction methods to minimize power being dissipated as heat by feed line loss and to avoid signals leaking in or out. Feed line loss increases with frequency for all types of feed lines. [Ham Radio License Manual, page 4-8]
  17. T9B06 Which of the following connectors is most suitable for frequencies above 400 MHz?
    A. A UHF (PL-259/SO-239) connector
    B. A Type N connector
    C. An RS-213 connector
    D. A DB-23 connector
    • B. A Type N connector
    • Which connector to use depends on the frequency of the signals being used. The UHF series of connectors are the most widely-used for HF equipment. (UHF does not stand for “ultra-high frequency” in this case.) Above 400 MHz, the Type N connectors are used. You’ll find both UHF and N connectors on 6, 2 and 1.25 meter equipment. [Ham Radio License Manual, page 4-16]
  18. T9B07 Which of the following is true of PL-259 type coax connectors?
    A. They are good for UHF frequencies
    B. They are water tight
    C. They are commonly used at HF frequencies
    D. They are a bayonet type connector
    • C. They are commonly used at HF frequencies
    • Which connector to use depends on the frequency of the signals being used. The UHF series of connectors are the most widely-used for HF equipment. (UHF does not stand for “ultra-high frequency” in this case.) Above 400 MHz, the Type N connectors are used. You’ll find both UHF and N connectors on 6, 2 and 1.25 meter equipment. [Ham Radio License Manual, page 4-16]
  19. T9B08 Why should coax connectors exposed to the weather be sealed against water intrusion?
    A. To prevent an increase in feedline loss
    B. To prevent interference to telephones
    C. To keep the jacket from becoming loose
    D. All of these choices are correct
    • A. To prevent an increase in feedline loss
    • Coax connectors exposed to the weather must be carefully waterproofed because water in coaxial cable degrades the effectiveness of the braided shield and dramatically increases losses. (See question T7C09.) [Ham Radio License Manual, page 4-17] *T7C09 - Coaxial cables must be protected. The performance of coaxial cable depends on the integrity of its outer coating, the jacket. Nicks, cuts and scrapes can all breach the jacket allowing moisture contamination, the most common cause of coaxial cable failure. Prolonged exposure to the ultraviolet (UV) in sunlight will also cause the plastic in the jacket to degrade, causing small cracks that allow water into the cable. To protect the cable against UV damage the jacket usually contains a pigment that absorbs and blocks the UV. Capillary action of the strands making up the braided outer shield can also draw water into coaxial cable. Once in the cable, the water causes both corrosion and heat losses in the shield. Water can also get into coaxial cable through an improperly sealed connection. [Ham Radio License Manual, page 4-16]
  20. T9B09 What might cause erratic changes in SWR readings?
    A. The transmitter is being modulated
    B. A loose connection in an antenna or a feedline
    C. The transmitter is being over-modulated
    D. Interference from other stations is distorting your signal
    • B. A loose connection in an antenna or a feedline
    • Remember that SWR is caused by a mismatch between feed line impedance and load or antenna impedance. If there is a loose connection where the load is attached (at the antenna feed point), then the erratic connection acts like an erratically changing load impedance. The result is an erratic change in SWR. [Ham Radio License Manual, page 4-10]
  21. T9B10 What electrical difference exists between the smaller RG-58 and larger RG-8 coaxial cables?
    A. There is no significant difference between the two types
    B. RG-58 cable has less loss at a given frequency
    C. RG-8 cable has less loss at a given frequency
    D. RG-58 cable can handle higher power levels
    • C. RG-8 cable has less loss at a given frequency
    • An important characteristic of coax is feed line loss. Loss is specified in dB per 100 feet of cable at a certain frequency. The following table gives cable loss at 30 MHz (close to the 10 meter band) and at 150 MHz (close to the 2 meter band). In general, if made from the same materials a larger diameter cable such as RG-8 will have less loss than a small cable such as RG-58. [Ham Radio License Manual, page 4-16]  
  22. T9B11 Which of the following types of feedline has the lowest loss at VHF and UHF?
    A. 50-ohm flexible coax
    B. Multi-conductor unbalanced cable
    C. Air-insulated hard line
    D. 75-ohm flexible coax
    • C. Air-insulated hard line
    • A special type of air-insulated coaxial feed line is called hard line because its shield is made from a semi-flexible solid tube of aluminum or copper. This limits the amount of bending the cable can do, but hard line has the lowest loss of any type of coaxial feed line. [Ham Radio License Manual, page 4-8]

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