--- title: "Single wire antenna and field line detachment" subject: "Propagation and Antenna" chapter: 1 key: "semester_6/propagation_and_antenna:single-wire-field-line-detachment" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/single-wire-field-line-detachment site: BCT Engineering --- # Single wire antenna and field line detachment Propagation and Antenna · Chapter 1: Radiation and Antenna Fundamentals - Asked in: 2073 Bhadra, 5 marks; 2070 Magh, 2+4 marks; 2069 Bhadra, 4 marks (asked 3 times) - Video: [RADIATION MECHANISM OF AN ANTENNA](https://www.youtube.com/watch?v=PyY7yVdl0TI) — Prof. Barapate's Tutorials - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/single-wire-field-line-detachment - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-1 ## Question How does a single wire act as an antenna? During the transmission mode, explain the mechanism involved by which the electric lines of force are detached from the dipole antenna to form free space waves. ## Short answer In a centre-fed dipole, charge buildup creates field lines; when the current reverses, charges neutralize and the old lines lose their conductor ends, close on themselves, and are pushed outward by new lines. These detached closed E-field loops, with associated H field, travel at the speed of light as free-space waves. ## Answer A single wire acts as an antenna when an **alternating current** flows on it: the charges are continuously accelerated and decelerated, and accelerated charges radiate (l·dI/dt = l·q·dv/dt). A straight wire with steady DC or uniform charge motion does not radiate; a time-varying current, a bend or an open end causes radiation. #### Detachment of electric lines of force from a dipole Consider a small centre-fed dipole driven by a sinusoidal source of period T. **First quarter cycle (0 to T/4):** - Charges flow outward; positive charge builds up on one arm and negative on the other. - Electric field lines form between them and spread outward, reaching a distance of about **λ/4** by t = T/4. **Second quarter cycle (T/4 to T/2):** - The current reverses; the charges move back and **neutralize**. - The old field lines, which have already travelled out, lose their ends on the conductor. - Since there are no charges left to end on, the lines **join and close on themselves**, forming closed loops. - At the same time, new lines of **opposite direction** start forming near the dipole. **Next half cycle:** the same process repeats with reversed polarity. The new lines push the closed loops away. ```text t = T/4 t = T/2 later + ___ ___ _ _ | / \ / \ ( ) ( ) || | --> | ( ) | --> loops travel | \___/ \_____/ outward at c - (lines close) ``` The detached closed loops of E field, with an associated H field, travel outward at the speed of light as **free-space waves**. The transmission line guides energy as a TEM wave; at the open end (the dipole arms) the field lines spread out, detach and propagate. The detached fields are the **radiation field**; fields that stay near the antenna and return energy each cycle are the **reactive (induction) field**. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Under what condition does a single straight wire act as an antenna? A. It carries steady DC B. It carries alternating current so charges are accelerated and decelerated C. It is grounded at both ends D. It is very thick Answer: B. Accelerated charges radiate; DC or uniform charge motion on a straight wire does not. (Key point: Single wire as antenna) 2. In the first quarter cycle of the dipole, how far do the electric field lines reach? A. About λ/4 by t = T/4 B. About 2λ C. Only the length of the dipole arms D. About λ/100 Answer: A. The field lines spread outward to a distance of about λ/4 by t = T/4. (Key point: Field line extent at T/4) 3. Why do the old field lines form closed loops in the second quarter cycle? A. The magnetic field cancels them B. The wire absorbs them C. The charges they ended on have neutralized, so the lines join and close on themselves D. The frequency doubles Answer: C. As the current reverses and charges neutralize, the lines have no charges to end on, so they close into loops and detach. (Key point: Why the lines close) 4. What do the new lines of opposite direction do? A. Cancel the radiation B. Return energy into the wire C. Stay attached to the arms permanently D. Form near the dipole and push the closed loops outward Answer: D. New opposite lines push the detached loops away, and they travel out at the speed of light as free-space waves. (Key point: Role of new lines) 5. Which fields stay near the antenna and return energy each cycle? A. Reactive (induction) fields B. Radiation fields C. Far fields D. Detached loops Answer: A. Detached fields are the radiation field; those that stay near and return energy are the reactive (induction) field. (Key point: Radiation vs reactive field) ## Related in Propagation and Antenna - [Antenna definition and single wire radiation conditions](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Asingle-wire-radiation-conditions) — Propagation and Antenna, chapter 1, asked 4× - [Single wire radiation and omnidirectional antenna characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Asingle-wire-radiation-omnidirectional) — Propagation and Antenna, chapter 1 - [Friis equation and free space path loss](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afriis-path-loss-free-space) — Propagation and Antenna, chapter 5, asked 2× - [Free space path loss: 6 GHz, 10,500 km](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afspl-derivation-6ghz-10500km) — Propagation and Antenna, chapter 5 - [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 - [Free space propagation and Friis derivation](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afree-space-friis-complete) — Propagation and Antenna, chapter 5, asked 2× ## Related in other subjects - [Electric dipole potential and field](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Adipole-potential-and-field-derive) — Electromagnetism, chapter 2, asked 4× - [Dipole field on axial line](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Adipole-axial-field-inverse-cube) — Engineering Physics, chapter 7 - [Dipole field on equatorial line](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Adipole-equatorial-field) — Engineering Physics, chapter 7 - [Inductance of a single-phase two-wire line](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Asingle-phase-inductance-derivation) — Power System Analysis I, chapter 4