Rhombic antenna working and design

A rhombic antenna is a non-resonant (travelling-wave) antenna made of four long horizontal wires arranged as a rhombus, fed at one acute corner and terminated in a matched resistor at the other.

               B
             /   \       L = leg length
   feed    /       \
  ===== A <    →    > C --R≈800Ω-- gnd
           \       /
             \   /       height H above ground
               D
  φ = tilt angle = half of the obtuse angle at B, D

Working principle

• The terminating resistor (≈ 600–800 Ω) absorbs the wave, so each leg carries a travelling wave and has a unidirectional conical lobe at an angle θ to the wire, pointing toward the load.
• The tilt angle is chosen so that one lobe of each of the four legs points along the major axis A→C. These four lobes add in phase; the other lobes cancel in pairs.
• The ground image adds to the direct wave at the wanted elevation angle Δ when the height is right.
• The result is a unidirectional, horizontally polarised beam toward the terminated end, tilted up at angle Δ for sky-wave links.

Design (for elevation angle Δ)

The standard design for maximum field intensity at angle Δ:

1. Height above ground (image in phase): H = λ/(4 sin Δ)
2. Tilt angle: φ = 90° − Δ
3. Leg length: L = 0.371λ / sin² Δ

The alignment design keeps H and φ the same but uses L = 0.5λ/sin² Δ, so that the main lobe of each leg points exactly at Δ (gives slightly less field but a cleaner pattern).

Example: f = 15 MHz (λ = 20 m), Δ = 20° (sin Δ = 0.342): H = 20/(4 × 0.342) = 14.6 m, φ = 70°, L = 0.371 × 20 / 0.117 = 63.4 m.

Characteristics

• Wide bandwidth (about 2:1); impedance stays near 700–800 Ω.
• High gain (≈ 10–15 dB), simple construction, easy to build.
• Used for HF (3–30 MHz) long-distance point-to-point and sky-wave communication.
• Drawbacks: very large area; about half the power is wasted in the termination; large side lobes.