--- title: "Rhombic antenna construction and working" subject: "Propagation and Antenna" chapter: 3 key: "semester_6/propagation_and_antenna:rhombic-antenna" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/rhombic-antenna site: BCT Engineering --- # Rhombic antenna construction and working Propagation and Antenna · Chapter 3: Antenna Classification - Asked in: 2078 Chaitra, 8 marks; 2076 Bhadra, 3 marks; 2073 Bhadra, 8 marks; 2072 Asoj, 7 marks; 2070 Bhadra, 4 marks (asked 5 times) - 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-antenna - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3 ## Question Explain the construction, working principle and characteristics of Rhombic antenna with necessary diagram. ## Short answer Rhombic antenna is a non-resonant travelling-wave HF antenna of four long wires (2-8λ each) forming a horizontal diamond, fed at one acute corner and terminated in 600-800Ω resistor at opposite corner. Travelling waves on each leg add in phase along major diagonal giving unidirectional beam, horizontal polarization, 10-17 dBi gain, wide 2:1-4:1 bandwidth, ~50-75% efficiency due to termination loss. ## Answer 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 ```text 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: ```text H = λ / (4 sinΔ) φ = 90° − Δ l = λ / (2 sin²Δ) ``` #### Characteristics | Property | Value | |---|---| | Frequency | HF, 3–30 MHz | | Pattern | Unidirectional, along major diagonal | | Polarization | Horizontal | | Bandwidth | Very wide (2:1 to 4:1), non-resonant | | Gain | ≈ 10–17 dBi | | Input impedance | ≈ 600–800 Ω, nearly resistive and constant | | Efficiency | ≈ 50–75 % (power lost in terminating resistor) | #### 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A rhombic antenna is A. a non-resonant travelling-wave HF antenna terminated by a resistor B. a resonant standing-wave antenna with open ends C. a UHF aperture antenna D. a vertical monopole over ground Answer: A. It is fed at one acute corner and terminated at the opposite corner so the wave travels one way. (Key point: Type of antenna) 2. Why is a resistor of about 600-800 ohm connected at the far corner? A. To increase the standing wave ratio B. To absorb the wave so no reflection or standing wave occurs, giving a unidirectional pattern C. To make the antenna resonant D. To raise the polarization purity Answer: B. A matched termination removes the reflected wave, so the pattern is unidirectional, at the cost of lost power. (Key point: Purpose of termination) 3. The tilt angle is chosen so that A. all lobes point sideways B. each leg is exactly lambda/2 long C. one main lobe of each of the four legs points in the same forward direction and adds in phase D. the lobes cancel in the forward direction Answer: C. Aligned lobes add in phase along A to B, others largely cancel. (Key point: Lobe alignment) 4. For maximum field at elevation angle Delta, the tilt angle is A. phi = Delta B. phi = 2 Delta C. phi = 180° - 2 Delta D. phi = 90° - Delta Answer: D. The alignment design gives H = lambda/(4 sin Delta), phi = 90° - Delta, l = lambda/(2 sin^2 Delta). 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