Standing vs travelling wave; V antenna

Standing wave vs travelling wave antenna

A standing wave (resonant) antenna has its far end open, so the wave reflects and forms standing waves; a travelling wave (non-resonant) antenna is terminated in its characteristic impedance, so the wave travels only forward.

V antenna: construction

                 leg 1, length l (2λ-8λ)
           /‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ (R)
  open-  <   apex angle 2θₘ      ===> beam
  wire     \___________________________ (R)
  feeder         leg 2      (R only in non-resonant V)

• Two long horizontal wires forming a V, fed at the apex by a 600 Ω balanced line in antiphase.
• Apex angle = 2θₘ, θₘ = main-lobe angle of a long wire, cosθₘ ≈ 1 − 0.371λ/l (e.g. l = 4λ → θₘ ≈ 25°).

Working principle

Each leg is a long wire radiating its main lobe at θₘ to the wire. By placing the legs at ±θₘ to the bisector, one main lobe from each leg points along the bisector, where they add in phase; elsewhere they partly cancel. With open ends the V carries standing waves and is bidirectional; with terminated ends it carries travelling waves and is unidirectional.

Characteristics

• HF band, horizontal polarization, gain ≈ 3 dB more than a single long wire (≈ 10–15 dBi for long legs).
• Wide bandwidth; input impedance ≈ 500–800 Ω.
• Simple (three poles), but needs large area and has side lobes.
• Used for HF point-to-point communication.