Single wire radiation and omnidirectional antenna characteristics

A single wire radiates when its charges are accelerated, i.e. when it carries a time-varying (AC) current: l·dI/dt = l·q·dv/dt. A static charge or a charge moving at uniform speed on a straight infinite wire does not radiate; bends, ends and oscillating current produce radiation. Field lines formed between the oscillating charges detach every half cycle and travel out as EM waves.

Pattern and polarization of a vertical wire (dipole)

  E-plane (vertical)         H-plane (horizontal)
        z
        |                        .---.
   .-.  |  .-.                 /       \
  (   ) | (   )               |   (o)   |
   '-'  |  '-'                 \       /
        |                        '---'
  figure-of-eight            circle (omni)

• E ∝ sinθ: maximum broadside (θ = 90°), null along the wire.
• E-field lies along the wire, so a vertical wire gives vertical (linear) polarization; a horizontal wire gives horizontal polarization.

Characteristics of an omnidirectional antenna

• Radiates uniformly in all directions of one plane (usually the horizontal/azimuth plane).
• Directional in the orthogonal (elevation) plane; 3-D pattern is a doughnut.
• Has nulls along the axis of the antenna.
• Moderate gain, e.g. λ/2 dipole 2.15 dBi, λ/4 monopole 5.15 dBi.
• Usually vertically polarized.
• Examples: vertical dipole, monopole/whip, discone, normal-mode helix.
• Used in broadcasting, mobile phones, Wi-Fi access points, base stations where users are in all directions.