--- title: "Travelling wave antenna and V antenna" subject: "Propagation and Antenna" chapter: 3 key: "semester_6/propagation_and_antenna:travelling-wave-v-antenna" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/travelling-wave-v-antenna site: BCT Engineering --- # Travelling wave antenna and V antenna Propagation and Antenna · Chapter 3: Antenna Classification - Asked in: 2072 Magh, 2+5 marks; 2071 Magh, 1+7 marks (asked 2 times) - Video: [V Antenna (Basics, Structure, Designing, Working, Radiation, Applications, Pros & Cons) Explained](https://www.youtube.com/watch?v=FhirfLrqTGE) — 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/travelling-wave-v-antenna - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3 ## Question What is travelling wave antenna? Explain the construction, working principle, characteristics and types of V-antenna. ## Short answer A travelling wave antenna has current flowing one way because of a matched termination, making it unidirectional and wideband. A V-antenna is two long wires fed in antiphase at the apex; choosing apex angle 2θ_m makes lobes add along the bisector. Types are resonant (standing wave, bidirectional) and non-resonant (travelling wave, unidirectional). ## Answer #### Travelling wave antenna A **travelling wave (non-resonant) antenna** is one in which the current travels along the conductor in one direction only, because the far end is terminated in a matched load (equal to the characteristic impedance). There is no reflected wave, so no standing wave; the current amplitude is nearly uniform and the phase changes progressively along the wire. Such antennas are **unidirectional**, **wideband** and have nearly constant resistive input impedance. Examples: terminated long wire (Beverage), terminated V, rhombic. ```text feed travelling wave → R = Z₀ ~ o==========================================/\/\/--⏚ max radiation tilted forward, towards the load end ``` #### V-antenna: construction ```text leg 1 (l = 2λ to 8λ) /‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ (R/2) 600 Ω < apex angle = 2θₘ ===> beam feeder \___________________________ (R/2) leg 2 ``` - Two long horizontal wires forming a "V", fed at the apex in antiphase by a balanced open-wire line. - Apex angle chosen as 2θₘ, where θₘ is the main-lobe angle of a single long wire: cosθₘ ≈ 1 − 0.371λ/l (e.g. l = 4λ → θₘ ≈ 25°, apex ≈ 50°). - Far ends open (resonant) or terminated by resistors to ground (non-resonant). #### Working principle Each leg radiates a cone of radiation at angle θₘ to its wire. When the apex angle equals 2θₘ, one major lobe of each leg lies along the bisector of the V, and because the legs are fed in antiphase and are mirror images, these lobes add in phase there. Lobes in other directions are partly cancelled. Ground reflection sets the elevation angle of the beam. #### Characteristics - Directive, with gain about twice (3 dB more than) a single long wire of the same leg length; ≈ 10–15 dBi for long legs. - Horizontal polarization, HF band (3–30 MHz). - Input impedance high (≈ 500–800 Ω); wide bandwidth, especially when terminated. - Requires large space but only three supports. #### Types of V-antenna | Type | Ends | Wave | Pattern | |---|---|---|---| | Resonant V | Open | Standing wave | Bidirectional along bisector | | Non-resonant V | Terminated in R | Travelling wave | Unidirectional | | Inverted (sloping) V | Legs slope down from one mast, ends terminated | Travelling wave | Unidirectional, low-angle beam | ```text Resonant: <=== V ===> Terminated: V ====> ``` **Applications**: HF point-to-point communication and reception. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A travelling wave antenna has A. open end with standing waves B. no feed C. matched termination, so no reflected wave D. resonant length only Answer: C. Termination equals Z0, so current travels one way. (Key point: Travelling-wave antenna) 2. Apex angle of the V is chosen as A. theta_m B. 90° C. 4 theta_m D. 2 theta_m Answer: D. One lobe of each leg then lies along the bisector. (Key point: Apex angle) 3. The inverted V is A. terminated, travelling wave, unidirectional low-angle beam B. resonant and bidirectional C. an unfed array D. vertically polarized only Answer: A. Legs slope from one mast and are terminated. (Key point: Types) 4. Gain of V compared with a single long wire is A. same B. about twice (3 dB more) C. half D. ten times Answer: B. Two legs add in phase along the bisector. 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