--- title: "Helical antenna axial and normal modes" subject: "Propagation and Antenna" chapter: 3 key: "semester_6/propagation_and_antenna:helical-antenna-modes" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/helical-antenna-modes site: BCT Engineering --- # Helical antenna axial and normal modes Propagation and Antenna · Chapter 3: Antenna Classification - Asked in: 2077 Chaitra, 8 marks - Video: [Helical Antenna (Basics, Structure, Ground Planes, Modes & Applications) Explained](https://www.youtube.com/watch?v=TeZhy9hgBJY) — 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/helical-antenna-modes - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3 ## Question Explain the helical antenna with respect to structural design, working principle in axial mode and normal mode, their merits, demerits and applications. ## Short answer The helical antenna is a wire wound in a helix over a ground plane. Normal mode (small dimensions) radiates broadside with elliptical polarization; axial mode (circumference ≈λ, pitch angle 12°–14°, N>3) yields end-fire circular polarization, wide bandwidth, and gain D ≈15 N C² S/λ³, used in satellite and GPS. ## Answer A **helical antenna** is a conductor wound into a helix (like a spring), fed at one end by a coaxial cable whose outer conductor is connected to a ground plane. Depending on its size relative to λ, it works in the **normal mode** or the **axial mode**. #### Structural design ```text _____ / \ D = helix diameter \_____/ C = πD (circumference) / \ S = spacing (pitch) between turns \_____/ α = pitch angle = tan⁻¹(S/C) / \ N = number of turns \_____/ L = NS = axial length =============== ground plane ≥ 0.5λ | coax feed one turn unrolled: L₀ = √(C² + S²) ``` Special cases: α = 0° → loop; α = 90° → straight wire (monopole). #### Normal (broadside) mode - **Condition**: helix dimensions ≪ λ (NL₀ ≪ λ, D ≪ λ). - **Working**: each turn behaves like a small loop (gives E_φ) plus a short dipole of length S (gives E_θ). Both have the same doughnut pattern, so radiation is **maximum normal to the helix axis**, null along the axis. - **Polarization**: elliptical; axial ratio AR = 2Sλ/C². It becomes **circular** when C = √(2Sλ). - **Merits**: very compact; can replace a long whip (shortened monopole). - **Demerits**: narrow bandwidth, low radiation resistance and efficiency; circular polarization only at one frequency. - **Applications**: compact "rubber-ducky" antennas in walkie-talkies and handsets. ```text Normal mode Axial mode <== ||| ==> ||| ====> max ⊥ axis max along axis ``` #### Axial (end-fire) mode - **Condition**: 3/4 λ < C < 4/3 λ (C ≈ λ), S ≈ λ/4, pitch angle 12°–14°, N > 3. - **Working**: the wave travels along the helix as a travelling wave; with one wavelength per turn, the fields of successive turns add along the axis. The current rotates around the axis, so the radiated field is **circularly polarized** (sense same as winding), giving a single main lobe along the axis away from the ground plane. - **Formulas (Kraus)**: ```text HPBW ≈ 52 λ^(3/2) / (C √(NS)) degrees FNBW ≈ 115 λ^(3/2) / (C √(NS)) degrees D ≈ 15 N C² S / λ³ Zin ≈ 140 (C/λ) Ω (≈ 100-200 Ω) AR ≈ (2N + 1)/(2N) ``` - **Merits**: circular polarization, wide bandwidth (≈ 1.7:1), moderate to high gain (10–15 dBi), simple and robust, almost resistive input impedance. - **Demerits**: bulky at low frequencies, needs a ground plane and matching from ≈ 140 Ω to 50 Ω. - **Applications**: satellite and space-probe telemetry, GPS, radio astronomy, feed for parabolic reflectors. | Feature | Normal mode | Axial mode | |---|---|---| | Size | ≪ λ | C ≈ λ | | Beam | Broadside | End fire | | Polarization | Elliptical | Circular | | Bandwidth | Narrow | Wide | | Gain | Low | High | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the radiation pattern of a helix in normal mode? A. Maximum along the axis B. Maximum normal to the axis, null along the axis C. Unidirectional end fire D. Omni overhead only Answer: B. Small turns act as loop plus short dipole, giving a doughnut pattern broadside to the axis. (Key point: Normal mode pattern) 2. Which conditions give axial mode? A. Pitch angle 0 degrees B. C much less than lambda C. C about lambda (3/4 to 4/3), S about lambda/4, pitch 12-14 degrees D. D much less than lambda Answer: C. Axial mode needs circumference near one wavelength. (Key point: Axial mode condition) 3. What polarization and beam does axial mode give? A. Vertical, omnidirectional B. Linear, broadside C. Elliptical, broadside D. Circular polarization, single lobe along the axis Answer: D. A travelling wave around turns gives end-fire circular polarization. (Key point: Axial mode polarization) 4. Which is a demerit of normal mode? A. Narrow bandwidth and low efficiency B. Very large size C. High gain D. Wide bandwidth Answer: A. It is compact but narrowband with low radiation resistance and efficiency. 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