Rhombic antenna construction and working

A rhombic antenna is a non-resonant (travelling-wave) HF wire antenna made of four long wires arranged as a horizontal rhombus (diamond), fed at one acute corner and terminated in a resistor at the opposite corner. It gives a unidirectional, high-gain beam for long-distance sky-wave links.

Construction

                  pole
                 /    \
         leg 1  /      \  leg 3
               /   φ    \
 feeder ==> A <    ->    > B --[ R ≈ 600-800 Ω ]--
(balanced)     \   main /        termination
         leg 2  \ beam /  leg 4
                 \    /
                  pole
  each leg length l = 2λ to 8λ, height H above ground

• Four straight wires, each several wavelengths long, supported on four poles at height H (≈ 0.25λ to 1λ).
• φ = tilt angle: the angle between each leg and the short diagonal, i.e. half of the obtuse angle at the side corners (so the half-angle at the feed corner is 90° − φ).
• Fed at corner A by a balanced 600 Ω open-wire line.
• Corner B is terminated in a non-inductive resistor equal to the characteristic impedance (≈ 600–800 Ω).

Working principle

1. Because the far end is matched, the current travels from A to B as a travelling wave; there is no reflected wave, so no standing waves.
2. A long travelling-wave wire radiates a conical main lobe at an angle θₘ to the wire, given by cosθₘ ≈ 1 − 0.371λ/l.
3. The tilt angle φ is chosen so that one main lobe of each of the four legs points in the same forward direction (along A → B). These four lobes add in phase.
4. Lobes pointing in other directions largely cancel; the backward wave is absorbed by the termination, so the pattern is unidirectional.
5. The ground acts as a reflector; height H sets the elevation angle Δ of the beam.

Alignment design for maximum field at elevation Δ:

H = λ / (4 sinΔ)
φ = 90° − Δ
l = λ / (2 sin²Δ)

Characteristics

Advantages and disadvantages

• Simple, cheap, wide bandwidth, high gain, easy matching to an open-wire line.
• Needs large land area; much power wasted in the termination; significant side lobes; beam direction fixed.

Applications: long-distance HF point-to-point sky-wave communication and broadcast reception.