Half-Wave Dipole and Standing Wave Antennas

The half-wave dipole is one of the most common and efficient antenna designs used in communication systems. It is a special case of a dipole antenna where the length of the elements is approximately λ/2. This article explores the characteristics of the half-wave dipole, its radiation resistance, directivity, and the standing wave current distributions that define its behavior.

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Characteristics of the λ/2 Dipole

A half-wave dipole antenna consists of two conductive elements, each approximately λ/4 in length, forming a total length of λ/2. Key properties include:

• Resonant Operation: The antenna operates efficiently when its length is close to λ/2, ensuring maximum radiation.
• Sinusoidal Current Distribution: The current is highest at the center and approaches zero at the ends.
• Balanced Impedance: The input impedance of a half-wave dipole in free space is approximately 73 Ω, making it compatible with many transmission lines.
• Omnidirectional in the H-plane: The radiation pattern in the horizontal plane is nearly circular.

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Radiation Resistance and Directivity

The radiation resistance $(R_{rad})$ of a half-wave dipole represents the equivalent resistance accounting for radiated power. For a λ/2 dipole, it is given by:

$$ R_{rad} \approx 73 \Omega $$

This value makes the half-wave dipole relatively efficient, minimizing resistive losses. The directivity D of the dipole is approximately:

$$ D \approx 1.64 \ (2.15 dB) $$

This means that compared to an isotropic radiator, the half-wave dipole concentrates energy more effectively in certain directions.

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Standing Wave Current Distributions

A defining characteristic of the half-wave dipole is its standing wave current distribution. Unlike a simple wire carrying a uniform current, the half-wave dipole exhibits a sinusoidal variation of current along its length:

• Maximum current at the center: This corresponds to the point of lowest impedance.
• Zero current at the ends: Due to the open-circuit nature of the dipole tips.
• Voltage Distribution: Inversely related to the current, with voltage maxima occurring at the ends.

This behavior results in an efficient radiation pattern that is strongest broadside to the dipole and weakest along its axis.

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Conclusion

The half-wave dipole is a fundamental antenna design widely used due to its efficiency, simple construction, and predictable performance. Understanding its resonance, radiation resistance, and standing wave behavior is essential for designing effective communication systems.

In the next article, we will explore Yagi-Uda Antennas, an extension of the dipole concept used for high-gain directional applications.

Stay tuned for the next article: Yagi-Uda Antennas and Their Applications.