--- title: "Rhombic antenna advantages and applications" subject: "Propagation and Antenna" chapter: 3 key: "semester_6/propagation_and_antenna:rhombic-advantages-applications" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/rhombic-advantages-applications site: BCT Engineering --- # Rhombic antenna advantages and applications Propagation and Antenna · Chapter 3: Antenna Classification - Asked in: 2079 Chaitra, 7 marks - Video: [Rhombic Antenna (Basics, Structure, Types, Operation, Designing, Radiation & Array) Explained](https://www.youtube.com/watch?v=ub-CA0H5YX8) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/rhombic-advantages-applications - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3 ## Question Explain the working principle, characteristics, advantage of a rhombic antenna and its applications. ## Short answer Rhombic antenna is a terminated travelling-wave wire rhombus; each long leg radiates, and tilt angle aligns forward lobes for a unidirectional beam. Characteristics: HF, horizontal polarization, gain 10–17 dBi, constant input impedance ~600–800 Ω over 2:1 bandwidth. Advantages: very wide bandwidth, simple construction. Applications: long-distance HF sky-wave links, broadcasting, military. ## Answer 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). ```text 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: ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the purpose of the terminating resistor in a rhombic antenna? A. It supplies DC power to the poles B. It increases standing waves for gain C. It tunes the antenna to a single frequency D. It absorbs the arriving wave so only a travelling wave exists, with no reflection Answer: D. Matched termination removes reflections, giving travelling-wave, unidirectional operation. (Key point: Termination role) 2. How does the rhombic produce a unidirectional beam along its long diagonal? A. The tilt angle is chosen so one main lobe of each of four legs points along the diagonal and adds in phase B. All four legs radiate only backwards C. Standing waves cancel the front lobe D. The ground absorbs the forward lobe Answer: A. Proper tilt angle aligns one lobe of each leg forward, where they add; backward radiation is absorbed in the resistor. (Key point: Lobe addition) 3. Why does a rhombic have a very wide bandwidth? A. It is resonant at one length B. It is non-resonant (travelling-wave), so impedance and pattern change little with frequency C. It uses a coil to tune D. Its legs are exactly lambda/2 long Answer: B. Being non-resonant, its input impedance (~600-800 Ω) stays nearly constant over a wide band. (Key point: Bandwidth property) 4. Which is a disadvantage of the rhombic, and a typical use? A. Narrow bandwidth; used for FM broadcast B. Needs no ground; used for satellite DTH C. Large land area and power lost in termination; used for HF sky-wave point-to-point links D. Bidirectional pattern; used for car radios Answer: C. Rhombics need big areas and waste power in the load (efficiency 50-75%) but suit HF sky-wave links. 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