ARRL

Chpt 4 - Propagation, Antennas & Feedlines

Chpt 4 - Propagation, Antennas & Feedlines

No piece of equipment has as great an effect on the performance of a radio station, whether hand-held or home-based, as the antenna.  Experimenting with antennas has been a favorite of hams from the very beginning, contributing greatly to the development of antennas for all radio services.  To choose and use an antenna effectively, it’s important to understand some basics of propagation – how radio waves get from one place to another.  For these reasons, knowledge of antennas and propagation is very important for amateurs. 

  • Miscellaneous Antenna and Feed Line Topics

    Here are two useful print references on antennas:
    Simple and Fun Antennas for Hams – an introduction to antennas and feed lines
    The ARRL Antenna Book – the amateur’s comprehensive antenna and propagation reference

  • General Resources for Propagation (pages 4-1 through 4-4)

    The ARRL’s Technical Information Service Web page on Propagation is a great resource for introductory to advanced articles on many types of propagation.  The Web page also has an extensive list of links to other propagation Web sites.

    The difference between “diffraction” (what happens to a wave at a sharp corner or edge) and “refraction” (what happens to a wave when it encounters a change in media) is explained on the “Physics Classroom” Web page on “Reflection, Refraction, and Diffraction”.

  • General Resources for Antennas (pages 4-5 through 4-8 and 4-15)

    Since antennas are so fundamental to Amateur Radio, the ARRL literature is full of information about antennas, such as the ARRL Technical Information Service’s “How Antennas Work” Web page.  A complete series of antenna tutorials is found in the Antenna-Theory Web site’s Fundamentals area.

  • Feed Lines and SWR (pages 4-8 through 4-18)

    The two types of feed lines used by amateurs are Coaxial Cable and Parallel-Conductor.  How well the feed lines are matched to the antenna or load is measured by SWR.  This interesting SWR simulator/calculator allows you to vary the antenna or load impedance to see the effect on power transfer and how the radio wave is reflected in the feed line.  Try the article “Do You Need An Antenna Tuner?” for more information about these common ham shack accessories.   There are also articles on transmission lines and SWR in the ARRL’s Technical Information Service.

    The on-line Times-Microwave Cable Calculator allows you to figure the cable’s signal loss at any frequency and for any length of cable and was used to generate Table 4-1.

    Page 4-16: Common UHF connector part numbers are the PL-259 (the plug that is installed on a coaxial cable) and the SO-239 (the receptacle installed on equipment).  The PL-258 is a double-receptacle adaptor, popularly known as a “barrel connector”, that allows cables with PL-259 connectors to be connected together.

  • Tutorial on Decibels and Power Ratios

    This tutorial provides assistance with the concepts behind the decibel (dB).

    (The calculations for the questions on the Technician exam involving decibels are worked out below.
    Learn More

  • Technician Exam Decibel Problems (Subelement Topic T5B)

    The questions ask only for the amount of change in decibels (dB), so use the absolute value of all calculations as your answer.

    T5B09  - What is the approximate amount of change, measured in decibels (dB), of a power increase from 5 watts to 10 watts?

    • dB = 10 log (final power / initial power)
    • dB = 10 log (10 watts / 5 watts) = 10 log (2) = 10 (0.3) = 3 dB

    T5B10 - What is the approximate amount of change, measured in decibels (dB), of a power decrease from 12 watts to 3 watts?

    • dB = 10 log (final power / initial power)
    • dB = 10 log (3 watts / 12 watts) = 10 log (0.25) = 10 (-.6) = -6 dB (the magnitude of the change is 6 dB)

    (NOTE: In the Technician Q&A (5th edition, 1st printing) the result of the calculation is mistakenly shown as -9 dB.)

    T5B11 - What is the approximate amount of change, measured in decibels (dB), of a power increase from 20 watts to 200 watts?

    • dB = 10 log (final power / initial power)
    • dB = 10 log (200 watts / 20 watts) = 10 log (10) = 10 (1) = 10 dB

  • Tutorial on Soldering

    The ARRL’s tutorial “The Art of Soldering” can be found in the Technical Information Service.