Rhombic antenna advantages and applications

A rhombic antenna is a non-resonant (travelling-wave) antenna formed by four long wires arranged as a horizontal rhombus, fed at one acute corner and terminated at the opposite corner by a resistor equal to its characteristic impedance (≈ 600–800 Ω).

            pole
           /    \
     l    /      \   l
         /   φ    \
 TX ==> <          > --/\/\/-- R (≈ 800 Ω)
         \        /     ===> beam
     l    \      /   l
           \    /
            pole       height H above ground

Working principle

1. The matched termination absorbs the wave arriving at the far end, so a travelling wave flows from feed to load with no reflection.
2. Each long leg (several λ) radiates a main lobe at angle θₘ to the wire, cosθₘ ≈ 1 − 0.371λ/l.
3. The tilt angle φ is chosen so that one main lobe of each of the four legs lies along the long diagonal in the forward direction, where all four add in phase.
4. Other lobes cancel partly; backward radiation is absorbed in the resistor, so the beam is unidirectional.
5. Ground reflection (height H) lifts the beam to the desired elevation angle Δ:

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

Characteristics

• HF band (3–30 MHz); horizontal polarization.
• Unidirectional beam along the major axis; gain ≈ 10–17 dBi.
• Non-resonant: almost constant input impedance (≈ 600–800 Ω) over a wide band (2:1 or more).
• Efficiency ≈ 50–75 %, as part of the power is dissipated in the terminating resistor.

Advantages

1. Very wide bandwidth; pattern and impedance change little with frequency.
2. High gain and directivity with a simple structure.
3. Simple, cheap construction with wires and four poles.
4. Easy to match to an open-wire line; insensitive to small dimensional errors.
5. Unidirectional pattern reduces interference from the back.

Disadvantages

• Requires very large land area; significant side lobes; power lost in termination; beam fixed.

Applications

• Long-distance HF sky-wave point-to-point communication (telegraph, telephone links).
• HF broadcast and reception stations, military and diplomatic HF links.