HF Antenna Application Note

This application note provides general engineering guidance on the selection, installation, and optimization of HF antennas. It outlines fundamental considerations such as site preparation, antenna sizing and efficiency, polarization effects, and propagation modes. The intent is to support system designers, field engineers, and operators in achieving reliable HF performance while balancing land-use, efficiency, and operational constraints.

Land Area and Site Preparation

The physical space required for an HF antenna depends on its configuration, desired frequency range, and operating mode (groundwave, or Near Vertical Incidence Skywave - NVIS).

General guidelines:

• Full-size half-wave dipoles typically span 40 - 80m at lower HF frequencies (3 – 7 MHz). Allow for a clear, linear area at least equal to the antenna length plus a few meters at each end for anchors and maintenance. A width of ~10 m is usually sufficient for safe installation and access.

• Inverted-V configurations reduce lateral space requirements; a single centre mast can support the antenna with end points about half the horizontal distance apart. A circular or diamond-shaped clearing with a diameter of 60 - 80 m is often adequate.

• Maintain clearance from tall structures, trees, and metallic objects within at least one antenna height to minimize detuning and ground coupling effects.

• Vegetation and terrain affect near-field conditions and ground conductivity. Groundwave and NVIS performance can degrade in dense vegetation or over poor soil conductivity. Where feasible, clear dense foliage within the immediate antenna footprint and along the desired propagation path.

Antenna Size, Efficiency, and Loading

Reducing antenna size below a significant fraction of a wavelength introduces efficiency trade-offs.

Key points:

• A shortened antenna (less than ~0.25 λ) exhibits reduced radiation resistance, and antenna efficiency decreases roughly with the square of the length ratio (∝ (L/λ)²).

• Using loading coils or passive matching networks with shortened antennas can restore impedance match and hence antenna efficiency but typically only over a narrow bandwidth. This is the principle used in an “Antenna Tuning Unit” (ATU).

• Lossy matching networks can restore impedance match over a wider bandwidth but with a commensurate reduction of antenna efficiency and hence gain.

• Transmit and receive performance from very short antennas can drop by several decibels (3 - 10 dB or more), depending on design and ground losses.

• Active or amplified antennas are suitable for receive-only applications and only useful where the external environmental noise is less than the downstream receiver noise.

• For transmitting, use a physically large antenna or compensate for reduced efficiency with higher transmitter power and good ground systems.

Polarization and Propagation Considerations

Combined Transmit and Receive Use

Most HF systems use the same antenna for transmit and receive, as polarization scrambling in the ionosphere minimizes the benefit of separate polarization paths.

Separate TX/RX antennas are used only when simultaneous operation requires isolation, physical constraints dictate different antenna sizes, or specialized diversity reception is desired.
Polarization diversity reception can improve fading resistance but adds complexity and is uncommon in typical HF communication systems.

Practical Installation Tips

• Maintain antenna height at approximately 0.1 - 0.25 λ above ground for NVIS operation.

• Ensure good electrical grounding or ground radial networks for vertical antennas to reduce ground loss.

• When feeding a balanced antenna (like a horizontal dipole) ensure that a balun is used to maintain impedance stability and minimize feedline radiation.

• Keep feedlines as short as possible, properly choked or shielded to avoid unwanted coupling.

• Consider seasonal and environmental effects - soil moisture, vegetation growth, and weather can influence impedance and radiation characteristics.

Summary of Trade-Offs

Examples of high frequency products from Alaris Antennas, relating to this application note are as follows:

Ø  OMNI-A0354 (Wideband HF Antenna)

Ø  RA52-0100-01 (HF Wideband Dipole Antenna)

Ø  RA01-1022-01 (HF Dipole Antenna)

Ø  RA01-0130-01 (High Frequency Wideband Dipole Antenna)

_References

_{ITU-R P.833  -  Attenuation in vegetation.}

_{A recommendation from the International Telecommunication Union (ITU) that provides models for predicting the radio frequency attenuation caused by vegetation. It details how signal strength is lost as it passes through foliage due to scattering and absorption, characterizing this loss with two main parameters: specific attenuation (dB/m) and maximum total attenuation (dB). The recommendation provides different models for different frequency ranges and path geometries.} 

_{ITU-R P.368  -  Groundwave propagation curves.}

_{Ground-wave propagation prediction method for frequencies between 10 kHz and 30 MHz}

_{ITU-R P.372  -  Radio noise characteristics.}

_{Provides information on the background levels of radio-frequency noise in the frequency range from 0.1 Hz to 100 GHz.}

_{ITU-R P.525  -  Free-space path loss.}

_{Provides methods to calculate the attenuation in free space.}

_{ITU-R F.1245  -  Antenna patterns and gains for HF.}

_{Gives the reference radiation patterns of point-to-point fixed service antennas, based on the peak envelope of side-lobe levels.}