--- title: "Standing vs travelling wave; Rhombic antenna" subject: "Propagation and Antenna" chapter: 3 key: "semester_6/propagation_and_antenna:standing-travelling-rhombic" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/standing-travelling-rhombic site: BCT Engineering --- # Standing vs travelling wave; Rhombic antenna Propagation and Antenna · Chapter 3: Antenna Classification - Asked in: 2071 Magh, 2+6 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/standing-travelling-rhombic - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3 ## Question What is the main difference between standing wave antenna and travelling wave antenna? Explain with a neat sketch the construction, working principle and characteristics of Rhombic antenna. ## Short answer Standing-wave antennas are open-ended and resonant, giving bidirectional narrowband patterns; travelling-wave antennas are terminated in a matched load, giving unidirectional wideband patterns. A rhombic antenna uses four long wires in a diamond, fed at one acute corner and terminated in about 600–800 Ω; travelling-wave lobes add along the major axis, giving a unidirectional beam with 10–15 dB gain, but half the power is lost in the termination. ## Answer #### Standing wave vs travelling wave antenna A **standing wave (resonant) antenna** is open at its ends, so the current reflects and forms a standing wave; a **travelling wave (non-resonant) antenna** is terminated in its characteristic impedance, so the current travels only one way with no reflection. | Point | Standing wave antenna | Travelling wave antenna | |---|---|---| | End condition | Open-ended | Terminated in matched load | | Current | Standing wave | Travelling wave | | Length | Resonant (multiple of λ/2) | Non-resonant, several λ | | Pattern | Bidirectional | Unidirectional | | Bandwidth | Narrow | Wide | | Example | Half-wave dipole | Beverage, rhombic | #### Rhombic antenna **Construction:** four straight wires (legs), each of length *L* = 2λ to 8λ, are arranged as a rhombus (diamond) and held horizontally on four poles at a height *H* above ground. One acute-angled end is fed by a balanced line; the other acute end is terminated by a non-inductive resistor of about **600–800 Ω**, equal to the characteristic impedance of the antenna. ```text B / \ feed / \ R ≈ 800 Ω --- A < > C ---/\/\--- \ / (load) \ / D Main beam: along A -> C (major axis) φ = tilt angle (half of obtuse angle at B, D) ``` **Working principle** - Because of the matched termination, each leg carries a travelling wave. A long travelling-wave wire radiates a conical main lobe tilted at a small angle to the wire, pointing toward the load. - The rhombus angles are chosen so that the main lobes of all four legs line up along the major axis (A to C). Lobes of the other directions cancel in pairs. - The ground reflection (image) adds the direct wave in phase at the wanted elevation angle Δ when *H* is chosen properly. - Result: a unidirectional, horizontally polarised beam toward the terminated end. Backward energy is absorbed in the resistor. **Characteristics** - Non-resonant, so input impedance (≈ 700–800 Ω) and pattern stay steady over a wide band (about 2:1 frequency range). - High gain (roughly 10–15 dB) and directive, used for HF (3–30 MHz) point-to-point sky-wave links. - Simple to build and adjust. - Drawbacks: needs a large land area; about half the power is lost in the terminating resistor (low efficiency); many side lobes. - Design rules (maximum-output design): H = λ/(4 sin Δ), φ = 90° − Δ, L = 0.371λ / sin² Δ. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main difference between standing wave and travelling wave antennas? A. Standing wave antennas are terminated in a matched load; travelling wave ones are open-ended B. Standing wave antennas are open-ended with reflection, travelling wave antennas are terminated in their characteristic impedance with no reflection C. Travelling wave antennas are resonant with narrow bandwidth D. Standing wave antennas are always unidirectional Answer: B. Open ends reflect current and form a standing wave; matched termination gives a one-way travelling wave and a unidirectional, wide-band pattern. (Key point: Standing vs travelling wave) 2. What is the purpose of the 600-800 Ω resistor at the far end of a rhombic antenna? A. To make the antenna resonant B. To absorb the wave so that no reflection occurs and the pattern is unidirectional C. To increase the radiation resistance to infinity D. To tilt the beam upward Answer: B. The matched load makes each leg carry a travelling wave, and backward energy is absorbed, giving a unidirectional beam. (Key point: Role of terminating resistor) 3. How is the rhombic's main beam produced along the major axis? A. The tilt angle is chosen so one lobe of each leg points along the major axis and these add in phase while other lobes cancel in pairs B. All four legs radiate in opposite directions C. The ground absorbs all lobes except one D. Standing waves in the legs produce a bidirectional beam Answer: A. Correct choice of the tilt angle aligns one lobe from each of the four legs along A to C, where they add, and the rest cancel. (Key point: Working principle of rhombic) 4. Which is a drawback of the rhombic antenna? A. Narrow bandwidth B. Needs large land area and about half the power is lost in the termination C. Very low gain D. Only vertical polarisation Answer: B. Rhombics are big, have large side lobes and waste about half the power in the resistor, though they give 10-15 dB gain and wide bandwidth. (Key point: Characteristics and drawbacks) ## Close variants set in other exams - [Standing vs travelling wave; V antenna](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Astanding-travelling-v-antenna) — Propagation and Antenna, chapter 3, asked 2× ## Related in Propagation and Antenna - [Rhombic antenna construction and working](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Arhombic-antenna) — Propagation and Antenna, chapter 3, asked 5× - [Travelling wave antenna and V antenna](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Atravelling-wave-v-antenna) — Propagation and Antenna, chapter 3, asked 2× - [Rhombic antenna working and design](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Arhombic-working-design) — Propagation and Antenna, chapter 3 - [V antenna construction and working](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Av-antenna) — Propagation and Antenna, chapter 3, asked 4× - [Rhombic antenna advantages and applications](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Arhombic-advantages-applications) — Propagation and Antenna, chapter 3 - [Single wire radiation and travelling wave long wire](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Asingle-wire-travelling-long-wire) — Propagation and Antenna, chapter 1 ## Related in other subjects - [MOSFET construction and working principle](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Amosfet-construction-working) — Basic Electronics Engineering, chapter 3, asked 2×