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General - Antennas and feed lines
Antennas and feed lines groups the G9 section questions for focused review and easier weak-point tracking.
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Read the condition in the stem before comparing key terms in the options.
Study rules, calculations, and equipment questions as separate patterns.
After this topic, return to the full pool for mixed practice.
Topic Study Guide
This topic covers the core concepts of antennas and feed lines, including characteristic impedance, SWR calculations, physical properties of dipole and Yagi antennas, and applications of specialized antennas like NVIS and Beverage.
High-Yield Checks
- SWR and impedance matching: Master simple SWR ratio calculations and understand that feed line loss artificially lowers the SWR measured at the input.
- Antenna length calculations: Memorize the formulas in feet for half-wave dipoles (468/f) and quarter-wave verticals (234/f).
- Yagi antenna characteristics: The driven element is a half-wavelength, dBi gain is 2.15 dB higher than dBd, and vertical stacking narrows the elevation lobe.
- Specialized antenna applications: NVIS requires low-mounted horizontal dipoles, Beverage antennas are receive-only, and log-periodic elements vary logarithmically.
Trap Patterns
- Confusing dBi and dBd: Mistakenly thinking dBd is higher; the isotropic radiator (dBi) is the lowest baseline, so dBi is always 2.15 higher for the same antenna.
- Misunderstanding line loss effect on SWR: Intuitively assuming high loss worsens SWR, when it actually absorbs reflected power, making the SWR look better at the transmitter.
- Using the wrong length formula: Habitually using 300/f for meters instead of applying 468/f or 234/f when the question requires length in feet.
- NVIS height misconception: Assuming "higher is always better," forgetting that NVIS must be close to the ground (1/10 to 1/4 wavelength) for near-vertical reflection.
Review Steps
- Memorize the empirical formulas for antenna length (468/f and 234/f) and practice calculating with specific frequencies.
- Categorize feed line characteristics: parallel lines (spacing and radius determine impedance), window line (450 ohms), and coax (attenuation increases with frequency).
- Visualize antenna radiation patterns: the figure-eight of a dipole, the in-plane omnidirectionality of a halo, and how stacking alters lobes.
- Summarize impedance matching techniques: such as the hairpin match (shorted stub) and adjusting ground-plane radials (sloping downward for 50 ohms).
Quick Checks
- What are the formulas to calculate the length of a half-wave dipole and a quarter-wave vertical antenna?
- How does the attenuation of a coaxial cable change as frequency increases?
- Does high feed line loss cause the measured SWR at the input to appear higher or lower?
- Is the primary purpose of a Beverage antenna for transmitting or receiving?
Questions in This Topic
Choose Practice ModeG9A01Which of the following factors determine the characteristic impedance of a parallel conductor feed line?G9A02What is the relationship between high standing wave ratio (SWR) and transmission line loss?G9A03What is the nominal characteristic impedance of “window line” transmission line?G9A04What causes reflected power at an antenna’s feed point?G9A05How does the attenuation of coaxial cable change with increasing frequency?G9A06In what units is RF feed line loss usually expressed?G9A07What must be done to prevent standing waves on a feed line connected to an antenna?G9A08If the SWR on an antenna feed line is 5:1, and a matching network at the transmitter end of the feed line is adjusted to present a 1:1 SWR to the transmitter, what is the resulting SWR on the feed line?G9A09What standing wave ratio results from connecting a 50-ohm feed line to a 200-ohm resistive load?G9A10What standing wave ratio results from connecting a 50-ohm feed line to a 10-ohm resistive load?G9A11What is the effect of transmission line loss on SWR measured at the input to the line?G9B01What is a characteristic of a random-wire HF antenna connected directly to the transmitter?G9B02Which of the following is a common way to adjust the feed point impedance of an elevated quarter-wave ground-plane vertical antenna to be approximately 50 ohms?G9B03Which of the following best describes the radiation pattern of a quarter-wave ground-plane vertical antenna?G9B04What is the radiation pattern of a dipole antenna in free space in a plane containing the conductor?G9B05How does antenna height affect the azimuthal radiation pattern of a horizontal dipole HF antenna at elevation angles higher than about 45 degrees?G9B06Where should the radial wires of a ground-mounted vertical antenna system be placed?G9B07How does the feed point impedance of a horizontal 1/2 wave dipole antenna change as the antenna height is reduced to 1/10 wavelength above ground?G9B08How does the feed point impedance of a 1/2 wave dipole change as the feed point is moved from the center toward the ends?G9B09Which of the following is an advantage of using a horizontally polarized as compared to a vertically polarized HF antenna?G9B10What is the approximate length for a 1/2 wave dipole antenna cut for 14.250 MHz?G9B11What is the approximate length for a 1/2 wave dipole antenna cut for 3.550 MHz?G9B12What is the approximate length for a 1/4 wave monopole antenna cut for 28.5 MHz?G9C01Which of the following would increase the bandwidth of a Yagi antenna?G9C02What is the approximate length of the driven element of a Yagi antenna?G9C03How do the lengths of a three-element Yagi reflector and director compare to that of the driven element?G9C04How does antenna gain in dBi compare to gain stated in dBd for the same antenna?G9C05What is the primary effect of increasing boom length and adding directors to a Yagi antenna?G9C07What does “front-to-back ratio” mean in reference to a Yagi antenna?G9C08What is meant by the “main lobe” of a directive antenna?G9C09In free space, how does the gain of two three-element, horizontally polarized Yagi antennas spaced vertically 1/2 wavelength apart typically compare to the gain of a single three-element Yagi?G9C10Which of the following can be adjusted to optimize forward gain, front-to-back ratio, or SWR bandwidth of a Yagi antenna?G9C11What is a beta or hairpin match?G9C12Which of the following is a characteristic of using a gamma match with a Yagi antenna?G9D01Which of the following antenna types will be most effective as a near vertical incidence skywave (NVIS) antenna for short-skip communications on 40 meters during the day?G9D02What is the feed point impedance of an end-fed half-wave antenna?G9D03In which direction is the maximum radiation from a VHF/UHF “halo” antenna?G9D04What is the primary function of antenna traps?G9D05What is an advantage of vertically stacking horizontally polarized Yagi antennas?G9D06Which of the following is an advantage of a log-periodic antenna?G9D07Which of the following describes a log-periodic antenna?G9D08How does a “screwdriver” mobile antenna adjust its feed point impedance?G9D09What is the primary use of a Beverage antenna?G9D10In which direction or directions does an electrically small loop (less than 1/10 wavelength in circumference) have nulls in its radiation pattern?G9D11Which of the following is a disadvantage of multiband antennas?G9D12What is the common name of a dipole with a single central support?