---
title: "Antenna bandwidth"
subject: "Propagation and Antenna"
chapter: 2
key: "semester_6/propagation_and_antenna:antenna-bandwidth"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/antenna-bandwidth
site: BCT Engineering
---

# Antenna bandwidth

Propagation and Antenna · Chapter 2: Antenna Parameters and Arrays

- Asked in: 2075 Bhadra, 3 marks
- Video: [Antenna Theory Bandwidth](https://www.youtube.com/watch?v=eJUs1vmbGbY) — Doug LeBlanc
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/antenna-bandwidth
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-2

## Question

Explain the bandwidth of an antenna.

## Short answer

Bandwidth is the frequency range where antenna performance (VSWR, gain, pattern) stays within specified limits, often VSWR ≤2. Narrowband BW% = (f_H - f_L)/f_c ×100; broadband ratio f_H:f_L. Example: 900 MHz centre, VSWR ≤2 from 860–940 MHz gives 8.89% BW.

## Answer

**Bandwidth** of an antenna is the range of frequencies over which its performance (input impedance/VSWR, gain, pattern, beamwidth, polarization) stays within an acceptable limit of its value at the centre frequency.

```text
Narrowband:  BW% = (fH − fL)/fc × 100
Broadband:   ratio = fH : fL  (e.g. 10:1)
Resonant:    BW ≈ fc / Q
```

Common criterion: VSWR ≤ 2 (return loss ≥ 10 dB).

**Example:** an antenna centred at 900 MHz with VSWR ≤ 2 from 860 to 940 MHz has BW = 80 MHz = 8.89 %.

Types: **impedance bandwidth** and **pattern bandwidth**. Thick conductors, travelling-wave and frequency-independent designs (log-periodic, spiral, biconical) give wide bandwidth; high-Q resonant antennas (thin dipole, small loop) are narrowband.

## Check yourself

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

1. What is the bandwidth of an antenna?
   A. The width of its main lobe
   B. The range of frequencies over which its performance stays within acceptable limits
   C. The highest frequency it can radiate
   D. The length of its feed line
   Answer: B. Impedance/VSWR, gain, pattern, beamwidth and polarization must stay acceptable. (Key point: Bandwidth definition)

2. An antenna centred at 900 MHz meets VSWR <= 2 from 860 to 940 MHz. What is its percentage bandwidth?
   A. 80 %
   B. 4.44 %
   C. 8.89 %
   D. 11.1 %
   Answer: C. BW% = (940 - 860)/900 × 100 = 8.89 %. (Key point: Percentage bandwidth)

3. Which antennas give wide bandwidth?
   A. Thick conductors and frequency-independent designs such as log-periodic and spiral
   B. Thin dipoles and small loops
   C. Any high-Q resonant antenna
   D. Only electrically short antennas
   Answer: A. High-Q resonant antennas are narrowband since BW is about fc/Q. (Key point: Which antennas are wideband)