Standing vs travelling wave; Rhombic antenna

Standing wave vs travelling wave antenna

A standing wave (resonant) antenna is open at its ends, so the current reflects and forms a standing wave; a travelling wave (non-resonant) antenna is terminated in its characteristic impedance, so the current travels only one way with no reflection.

Rhombic antenna

Construction: four straight wires (legs), each of length L = 2λ to 8λ, are arranged as a rhombus (diamond) and held horizontally on four poles at a height H above ground. One acute-angled end is fed by a balanced line; the other acute end is terminated by a non-inductive resistor of about 600–800 Ω, equal to the characteristic impedance of the antenna.

               B
             /   \
   feed    /       \     R ≈ 800 Ω
  --- A <           > C ---/\/\---
           \       /      (load)
             \   /
               D
   Main beam: along A -> C (major axis)
   φ = tilt angle (half of obtuse angle at B, D)

Working principle

• Because of the matched termination, each leg carries a travelling wave. A long travelling-wave wire radiates a conical main lobe tilted at a small angle to the wire, pointing toward the load.
• The rhombus angles are chosen so that the main lobes of all four legs line up along the major axis (A to C). Lobes of the other directions cancel in pairs.
• The ground reflection (image) adds the direct wave in phase at the wanted elevation angle Δ when H is chosen properly.
• Result: a unidirectional, horizontally polarised beam toward the terminated end. Backward energy is absorbed in the resistor.

Characteristics

• Non-resonant, so input impedance (≈ 700–800 Ω) and pattern stay steady over a wide band (about 2:1 frequency range).
• High gain (roughly 10–15 dB) and directive, used for HF (3–30 MHz) point-to-point sky-wave links.
• Simple to build and adjust.
• Drawbacks: needs a large land area; about half the power is lost in the terminating resistor (low efficiency); many side lobes.
• Design rules (maximum-output design): H = λ/(4 sin Δ), φ = 90° − Δ, L = 0.371λ / sin² Δ.