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Lesson 8: Antennas and Feed Lines

Lesson 8 of 10, syllabus topic T9. The exam asks 2 questions from this topic, one from each of its 2 groups.

Antennas and feed lines rely on wavelength from the propagation lesson and on SWR from the practical circuits lesson.

What you will learn

  • T9A Antennas: vertical and horizontal polarization, concept of antenna gain, definition and types of beam antennas, antenna loading, common portable and mobile antennas, relationships between resonant length and frequency, dipole pattern
  • T9B Feed lines: types, attenuation vs frequency, selecting; SWR concepts; Antenna tuners (couplers); RF Connectors: selecting, weather protection

1. Read

  1. Antennas and feed lines: The half-wave dipoleHelps with T9A
  2. Antennas and feed lines: Quarter-wave verticalsHelps with T9A
  3. Antennas and feed lines: Gain and directivityHelps with T9A
  4. Antennas and feed lines: Yagis and other beamsHelps with T9A
  5. Antennas and feed lines: Loading and shortened antennasHelps with T9A
  6. Radio waves and the spectrum: PolarizationHelps with T9A
  7. Radio waves and the spectrum: Wavelength sets antenna sizeHelps with T9A
  8. Antennas and feed lines: Coax and other feed linesHelps with T9B
  9. Antennas and feed lines: SWR: what it does and does not tell youHelps with T9B
  10. Antennas and feed lines: Antenna tunersHelps with T9B
  11. Antennas and feed lines: Connectors and weatherproofingHelps with T9B
  12. Connectors and cables: The common RF connectorsHelps with T9B
  13. Connectors and cables: Weatherproofing outdoor connectionsHelps with T9B
  14. Connectors and cables: Choosing coaxHelps with T9B
  15. All 23 T9 questions with their answers and explanationsEvery group, with the rule or source behind each answer

2. Explore

  • Antenna length

    Compare a half-wave dipole for 14.0 MHz with one for 14.3 MHz. The higher frequency needs the shorter wire, which is why shortening a dipole raises its resonant frequency.

    This is exam question T9A05.

  • SWR and feed line loss

    Enter a 50 ohm load on 50 ohm line, then a 100 ohm load. The SWR goes from 1 to 1 to 2 to 1.

3. Practice

Practice one group at a time. A group is ready when you have answered every one of its questions and learned at least 80 percent of them; a question counts as learned when your last three answers to it were correct. To review quickly, go through this lesson’s questions as flashcards.

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