text: Communications
  Section VI. ANTENNAS
  This section discusses several concepts to help communications personnel select the best antenna.
  WAVELENGTH AND FREQUENCY
  6-65. A wavelength is the distance that an electromagnetic wave travels to complete one cycle at a
  particular frequency (Figure 6-6). In radio communication, the length of an antenna relates directly to the
  frequency's wavelength. This relationship is important to know when building antennae for a specific
  frequency or frequency range.
  Figure 6-6. Measurement of a wavelength.
  RESONANCE
  6-66. Antennas are classified as either resonant or nonresonant, depending on their design. Both are
  commonly used on tactical circuits. However, if you can get a clear signal with a resonant antenna, that
  should be your first antenna choice rather than a nonresonant or standing-wave-ratio antenna.
  RESONANT ANTENNAS
  6-67. A resonant antenna matches the wavelength of one particular frequency.
  • Advantage is efficiency; most radio signals sent to a resonant antenna radiate successfully.
  • Disadvantage is lack of flexibility; a separate antenna must be built for each frequency used.
  NONRESONANT ANTENNAS
  6-68. These antennas match a range of frequencies.
  • Advantage: This kind of antenna works with more than one frequency.
  • Disadvantage: A nonresonant antenna, as the name implies, reduces resonance, which weakens
  the signal. The more frequencies the antenna resonates, the lower the resonance quality, which
  in turn reduces the efficiency of the signal.
  STANDING WAVE RATIO
  6-69. Signal energy resonates, or causes energy waves in a certain pattern on an antenna. These waves
  are measured and compared to the standard wave to determine if an antenna resonates at a particular
  frequency. Although a 1-to-1 ratio to a standing wave (standing wave ratio) is ideal, 1.1-to-1 ration is about
  the best ratio obtainable. When building wire antennas, the operator should adjust the length of the antenna
  until he obtains the lowest possible standing wave ratio. A 3-to-1 standing wave ratio is acceptable. Check
  the operator’s manual for the particular radio in use to determine the maximum standing wave ratio that the
  radio can tolerate. Some radios automatically lower the power output of the transmitter if the standing
  wave ratio is too high.