---
title: "Helical antenna"
subject: "Propagation and Antenna"
chapter: 3
key: "semester_6/propagation_and_antenna:helical-antenna"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/helical-antenna
site: BCT Engineering
---

# Helical antenna

Propagation and Antenna · Chapter 3: Antenna Classification

- Asked in: 2080 Asoj, 3 marks; 2074 Bhadra, 4 marks; 2073 Bhadra, 3 marks (asked 3 times)
- 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
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3

## Question

Write a short note on helical antenna.

## Short answer

A helical antenna is a wire helix above a ground plane, fed by coax. In axial mode, with circumference near λ and pitch angle about 13°, it radiates a circularly polarized end-fire beam. Gain is about 15NC²S/λ³ and bandwidth about 1.7:1.

## Answer

A **helical antenna** is a conducting wire wound in the form of a helix (screw), usually fed by a coaxial line against a ground plane. It is the standard antenna for **circular polarization** at VHF/UHF.

```text
        ____
       /    \   <- turn, circumference C = πD
       \____/
       /    \   spacing S between turns
       \____/   pitch angle α = tan⁻¹(S/C)
       /    \   N turns, axial length L = NS
  =====\____/=====  ground plane (≥ λ/2)
          |
        coax feed
```

**Parameters**: diameter D, circumference C = πD, spacing S, pitch angle α, number of turns N.

**Modes of operation**
- **Normal (broadside) mode**: helix dimensions ≪ λ (total wire length NL₀ ≪ λ, where L₀ = √(C² + S²) is the length of one turn). Maximum radiation is normal to the axis, like a short dipole plus a small loop; polarization is elliptical, circular when C = √(2Sλ). Low efficiency, narrow band. Used for small whips in handsets.
- **Axial (end-fire) mode**: C ≈ λ (0.75λ < C < 1.33λ), S ≈ λ/4, α ≈ 12°–14°. A single main beam along the axis, **circularly polarized**, wide band.

**Axial-mode formulas (Kraus)**

```text
HPBW ≈ 52 λ^(3/2) / (C √(NS))  degrees
D    ≈ 15 N C² S / λ³
Zin  ≈ 140 (C/λ)  Ω
```

**Advantages**: circular polarization, wide bandwidth (about 1.7:1), moderate gain (10–15 dBi), simple. **Applications**: satellite and space telemetry, GPS, radio astronomy, feeds for reflectors.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Axial (end-fire) mode of a helix needs
   A. circumference C about lambda and S about lambda/4
   B. C much smaller than lambda
   C. S much larger than lambda
   D. C = 0.1 lambda
   Answer: A. This gives a single axial beam with wide bandwidth. (Key point: Axial mode conditions)

2. Axial mode radiation is
   A. vertically linear
   B. circularly polarized
   C. horizontally linear only
   D. unpolarized
   Answer: B. The helix is the standard circular-polarization antenna. (Key point: Polarization)

3. In normal mode, maximum radiation is
   A. along the axis
   B. backward only
   C. normal to the helix axis, with small size compared with lambda
   D. absent
   Answer: C. It behaves like a short dipole plus a small loop. (Key point: Normal mode)

4. Pitch angle alpha is given by
   A. tan^-1(C/S)
   B. sin^-1(N S)
   C. cos^-1(D/L)
   D. tan^-1(S/C)
   Answer: D. Pitch angle relates turn spacing S to circumference C; axial mode uses about 12°-14°. (Key point: Pitch angle)

## Related in Propagation and Antenna

- [Helical antenna axial and normal modes](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Ahelical-antenna-modes) — Propagation and Antenna, chapter 3