Undergraduate Antenna Engineering

Virtual Laboratory:
Dipole Antennas

Explore the fundamental principles of linear wire antennas through interactive simulations, radiation pattern analysis, and impedance calculations.

Laboratory Objectives

01

Understand Current Distribution

Analyze sinusoidal current distribution along dipole arms and its relationship to antenna length.

02

Radiation Pattern Analysis

Visualize and interpret E-plane and H-plane radiation patterns for various dipole lengths.

03

Input Impedance

Calculate and plot input impedance (resistance and reactance) as a function of dipole length.

04

Directivity & Gain

Determine directivity, radiation resistance, and antenna gain for different configurations.

05

Half-Wavelength Dipole

Investigate the characteristics of the resonant half-wave dipole antenna.

06

Bandwidth Analysis

Examine how antenna parameters vary with frequency and define operational bandwidth.

Theoretical Background

1 The Dipole Antenna Structure

A dipole antenna consists of two identical conductive elements such as metal wires or rods, oriented symmetrically and driven by a current source at the center. The total length is typically denoted as L, with each arm having length L/2.

Key Parameters:

  • • Length: L = 2 × h (where h is arm length)
  • • Radius: a (wire radius)
  • • Wavelength: λ = c/f

2 Current Distribution

For a thin dipole, the current distribution is approximately sinusoidal with zero current at the ends and maximum at the center (for odd multiples of λ/2).

I(z) = I₀ sin[k(h - |z|)] / sin(kh)

where k = 2π/λ is the wavenumber and h = L/2.

3 Radiation Pattern

The normalized radiation pattern for a dipole of length L is given by:

E(θ) = [cos(kL/2 cos θ) - cos(kL/2)] / sin θ

For a half-wave dipole (L = λ/2), this simplifies to:

E(θ) = cos(π/2 cos θ) / sin θ

4 Input Impedance

The input impedance Zin = Rin + jXin varies with dipole length:

  • Short Dipole (L << λ): Rr ≈ 80π²(L/λ)², high capacitive reactance
  • Half-Wave (L = λ/2): Zin ≈ 73 + j42.5 Ω (theoretical)
  • Full-Wave (L = λ): High impedance, anti-resonant

5 Directivity

Directivity measures how concentrated the radiation is in a particular direction:

Hertzian Dipole

1.5 (1.76 dBi)

Half-Wave Dipole

1.64 (2.15 dBi)

Experimental Procedure

1

Setup the Simulation

Launch the dipole antenna simulator. Set the operating frequency (default: 300 MHz) and observe the calculated wavelength.

λ = c/f = (3×10⁸ m/s) / (300×10⁶ Hz) = 1 meter
2

Vary Dipole Length

Adjust the dipole length from 0.1λ to 2.0λ using the slider. Observe how the current distribution changes along the antenna arms. Note the standing wave pattern.

3

Analyze Radiation Pattern

For each length setting:

  • Observe the E-plane (elevation) pattern
  • Note the number of lobes and null directions
  • Identify the direction of maximum radiation
  • Record the half-power beamwidth (HPBW)
4

Measure Input Impedance

Record the input resistance (Rin) and reactance (Xin) for each configuration. Identify the resonant length where Xin ≈ 0 (typically slightly less than λ/2 due to end effects).

5

Calculate Directivity

Use the simulation to determine the maximum directivity and compare with theoretical values. Calculate the radiation resistance using the relationship between total radiated power and input current.

6

Bandwidth Investigation

Fix the physical length and sweep the frequency to observe how impedance and radiation pattern vary with frequency. Determine the -10 dB return loss bandwidth.

Interactive Simulation

Real-time dipole antenna analysis engine

100 300 MHz 1000

λ = 1.00 m

0.1λ 0.50λ 2.0λ

Physical length: 0.50 m

0.1 1.0 mm 10

L/2a ratio: 250

Antenna Geometry & Current Distribution

Current Max
Current Min
I₀ = 1.0 A

Radiation Pattern (E-Plane)

Input Resistance

73.1 Ω

Input Reactance

+42.5 Ω

Directivity

2.15 dBi

Max: 1.64

HPBW

78°

Half-Power Beamwidth

Input Impedance vs. Normalized Length (L/λ)

Resistance (R)
Reactance (X)
Current Position

Laboratory Report Guidelines

Required Sections

  • 1

    Title and Objectives

    Clear statement of experiment goals

  • 2

    Theoretical Background

    Relevant equations and principles

  • 3

    Simulation Setup

    Parameters used for each test case

  • 4

    Results and Analysis

    Plots, tables, and observations

  • 5

    Conclusions

    Key findings and comparisons with theory

Data Collection Requirements

Minimum configurations tested 5
Include L/λ = 0.5 (resonant) Required
Pattern plots required E-plane
Impedance sweep data Table format

Grading Criteria

Technical Accuracy 40%
Analysis & Interpretation 30%
Presentation & Format 20%
Conclusions 10%

Sample Discussion Questions

1. Why does the input impedance of a dipole antenna vary periodically with length?

2. Explain why a half-wave dipole is considered resonant despite having a reactive component.

3. How does the radiation pattern change as the dipole length increases beyond one wavelength?

4. Compare the directivity of a Hertzian dipole vs. a half-wave dipole. Why the difference?