Advanced Amateur Radio Exam

Antenna Elevation and Performance

The performance of an antenna, particularly in terms of its radiation pattern, directivity, and impedance, is significantly influenced by its height above the ground. This article explores how the elevation of different types of antennas affects their operation and efficiency, particularly for Yagi antennas, dipoles, and vertical antennas.

Yagi Antenna and Main Lobe Takeoff Angle

For a 3-element Yagi antenna with horizontally mounted elements, the main lobe takeoff angle decreases as the antenna's height above flat ground increases. This characteristic is crucial for optimizing the antenna's directivity and range, especially for long-distance communication.

Directivity of Horizontally Polarized Antennas

Most simple horizontally polarized antennas, such as dipoles, do not exhibit significant directivity unless they are positioned at least a half-wavelength or more above the ground. This elevation ensures that the antenna's radiation pattern is less affected by ground reflections.

Effective Ground Plane

The effective ground plane for an antenna, the plane from which ground reflections are considered to take place, can range from several centimeters to up to 2 meters below the ground, varying with soil conditions. This factor influences the antenna's radiation efficiency and pattern.

Vertical vs. Horizontal Antennas for Long-Distance Contacts

A ground-mounted vertical quarter-wave antenna, in open surroundings, is often better for long-distance contacts compared to a half-wave dipole at a quarter wavelength above ground. This is because the vertical antenna typically has a lower radiation angle, making it more suited for distant communications.

Dipole Antenna Above Ground

When a half-wave dipole antenna is installed at a height of one-half wavelength above the ground, the vertical or upward radiation is effectively canceled. This attribute affects the antenna's overall radiation pattern and its suitability for different communication needs.

Antenna Height and Horizontal Radiation Pattern

The height of a horizontal dipole HF antenna significantly impacts its horizontal (azimuthal) radiation pattern. If the antenna is less than one-half wavelength high, the ground reflections can notably distort the pattern, affecting the antenna's performance.

Ideal Vertical Radiation Angle for Long Distance

For long-distance propagation, the vertical radiation angle of the antenna's energy should ideally be less than 30 degrees. This angle helps in achieving efficient long-range communication through ionospheric propagation.

Multiple-Hop Transmissions and Radiation Angle

Decreasing the vertical radiation angle of an antenna can facilitate greater distances being covered with multiple-hop transmissions. This strategy is particularly effective in HF communications, where ionospheric reflection plays a significant role.

Impedance of Dipole Antennas at High Elevation

The impedance at the center of a dipole antenna situated more than 3 wavelengths above the ground is closest to 75 ohms. This impedance level is important for matching the antenna to the transmission line for optimal energy transfer.

Horizontal Antennas for Close Range Communication

A horizontal antenna placed closer to the ground can be advantageous for close-range communications on lower HF bands. The proximity to the ground enables the ground to act as a reflector, enhancing the antenna's near-field performance.

Suitable Antenna System for NVIS

Near Vertical Incidence Skywave (NVIS) communications, which are effective for regional distances, are best achieved using a horizontal antenna less than 1/4 wavelength above the ground and operating at a frequency below the current critical frequency. This setup ensures that the radio waves are directed almost vertically upwards, facilitating skywave propagation over shorter distances.

Conclusion

The height of an antenna above the ground plays a pivotal role in defining its radiation characteristics and overall performance. Whether it's for long-distance communication, regional NVIS operations, or optimizing directivity, understanding the impact of antenna elevation is crucial for amateur and professional radio operators alike. By tailoring the antenna's height to specific communication needs and frequency bands, one can significantly enhance the efficiency and effectiveness of radio communication systems.