Antenna Theory, Propagation & Siting Notes

Defence School of Transport Communications - Antenna Theory, Propagation, and Siting

Introduction

  • Purpose of Document: This material serves as a training aid and is not an authority for current requirements or policies. Official publications must be consulted for confirmation.
    • © Crown Copyright 2015
    • © Crown Copyright 2011

Importance of Antennas

  • Nursery Rhyme Analogy:
    • "For the want of a nail the shoe was lost. For the want of a shoe the horse was lost. For the want of a horse, the rider was lost…"
    • The "nail" represents antennas in radio systems, implying that without proper understanding and selection of antennas, the entire communication system fails.
  • Command and Control: Effective communications are crucial to ensure success in military operations. Without effective antennas and propagation knowledge, communication lines could fail, leading to strategic disadvantages.
  • Reference Resource: Land Components Handbook (LCH) 3:13:1 Antenna and Propagation theory provides further details on antenna fundamentals.

Components of a Radio System

  1. Power Supply

    • Primarily a 24-Volt Direct Current (DC) supply, using batteries and charging systems in the FFR (Fitted For Radio).

  2. Transceiver

    • Combination of a Radio Transmitter (TX) and Radio Receiver (RX) integrated into one unit.

  3. Transmission Line

    • Typically a coaxial cable for conveying Radio Frequency (RF) current from the radio to the antenna. Must be robust and flexible to match both antenna and radio requirements.

  4. Antenna

    • Functions as both a transmitter and receiver of RF energy.

Transmission Lines

  • Common Military Use of Coaxial Cable
    • Advantages
    • Robust
    • Flexible
    • Easily matched with dipoles and end-fed antennas.
    • Construction: Includes inner conductor (signal carrier), outer braid sheath (traps radiation), and dielectric separator (insulates inner conductor).
  • Installation Warning: Always unroll coaxial cable fully before connection to avoid resistance and performance degradation.

Purpose of an Antenna

  1. Transmit

    • Converts RF energy from the radio into electromagnetic energy for radiation.

  2. Receive

    • Captures radio waves to induce voltage, which is sent to the receiver for amplification and conversion.

Characteristics of Radio Waves

  • Three main characteristics are crucial for antenna length determination:
    1. Velocity:
    • Speed of light: $300,000,000$ meters/second in free space; slightly slower in air and other materials.
    1. Frequency:
    • Measured in cycles per second (Hz).
    1. Wavelength:
    • Distance measured from one point on a wave to the same point on the next wave.

Electromagnetic Spectrum

  • Specific Bands Used in Military Communications:
    • High Frequency (HF): $3 - 30$ MHz. Commonly known as "Short Wave" communications.
    • Very High Frequency (VHF): $30 - 300$ MHz, dominant at the lower and upper ends (30 - 88 MHz for military).
    • Ultra High Frequency (UHF): $300$ MHz to $3$ GHz, for ground-to-air communications and high-capacity data links.

Bandwidth and Modulation

  • Bandwidth: Total frequencies within a particular band. Example:
    • HF Bandwidth: $27$ MHz. VHF Bandwidth: $270$ MHz.
  • Modulation: Process of modulating data onto a carrier wave:
    • Continuous wave radiates immediately when transmitting.

Electromagnetic Wave Radiation

  • Requirements for Propagation:
    • Voltage: Ranges between $0$ - $30,000$ Volts varying by frequency.
    • Current: Alternating current (AC) flows through the transmission line.
    • Velocity, Frequency, Wavelength: Previously discussed.

Types of Antennas

  1. Resonant Antennas:

    • Typically have a matched wavelength and require an antenna tuning unit (ATU).

  2. Wideband Antennas:

    • Operates across a range of frequencies, constructed with larger cross-sections than resonant antennas, yet less efficient.

  3. Dipole and Unipole Antennas:

    • Dipole Antenna: Two equal lengths; optimal for radiation. Often constructed at $L_{ideal} = 0.5 imes rac{ ext{wavelength}}{2}$.
    • Unipole Antenna: Similar to dipole but uses a ground image; relies on ground quality for efficiency.
  • Counterpoise: Enhances unipole performance by connecting radial wires to the antenna's earth side.

Properties of an Antenna

  1. Polarisation: Orientation of the antenna plane (vertical or horizontal).
  2. Polar Diagram: Illustrates if an antenna is directional or omni-directional.
  3. Antenna Gain: Measured in decibels (dBs). Power radiated by the antenna can be boosted by:
    • Correct length, height, directionality, and tuning.
  4. Characteristic Impedance & Loss Resistance: Influenced by feedpoint resistance and coaxial cable use.
  5. Efficiency: Ensures maximum power radiation to intended areas.
  6. Physical Dimensions: Wideband antennas are preferred for mobile use; exact dimensions are critical for fixed-length dipoles.

Radio Wave Propagation

  • Types of Propagation: Governed by antenna set-up, leading to:
    1. Sky Wave (Horizontal Dipoles): Long-range communication; refracts back to earth from the ionosphere.
    2. Ground Wave (Vertical Unipoles): Short-range communication along the ground. Maximum range affected by terrain and weather conditions.
Ground Wave Propagation
  • Surface Wave: Adheres to ground contours; used predominantly in HF communications close to the ground.
Space Wave Propagation
  • Directed at VHF systems: Need to exceed the Minimum Effective Height (MEH).
  • Types of space waves: Direct waves and ground reflected waves.
Sky Wave Propagation
  • Uses the ionosphere; consists of four layers impacting communication:
    1. D Layer: At 50 km above earth.
    2. E Layer: Starts at 100 km.
    3. F1 & F2 Layers: Begin at 200 km to 350 km.
  • Night-time considerations: D and E layers thin out, enhancing noisy HF communication.
  • Seasonal Variations: Affects communication reliability based on seasonal pressure changes.

Conclusion: Communication Using Sky Waves

  • Category Ranges for Sky Wave:
    • Short (0-300 km): Near Vertical Incidence Skywave (NVIS).
    • Medium (300-1500 km): Elevated at about 45°.
    • Long (1500-3000 km): Near parallel transmission.

Antenna Selection Criteria

  1. Frequency Use: High or Very High Frequency determined by radio equipment.
  2. Range Considerations: Influenced by power, gain, and frequency type (surface or sky wave).
  3. Propagation Path: The type of wave needed for effective communication.

Siting a Radio Detachment

  • Tactical Considerations:
    • Accessibility, concealment, and tactical security are vital for operational readiness.
  • Technical Considerations:
    • Ensure effective communications without having an obvious setup; avoid large metallic objects and electrical interference.
  • Electronic Warfare Threat: Minimize direct radiation towards enemy lines; use directional antennas as needed.

Environmental Factors Affecting Communications

  1. Electromagnetic Compatibility: Ensuring minimal noise interference from external sources.
  2. Sources of Noise:
    • Man-made (e.g., generators, transmission lines).
    • Atmospheric noise increases near equatorial regions and during low frequencies.
  3. Interference Management: Good frequency management can reduce interference issues.

Mobile Electromagnetic Incompatibility (MEMIC)

  • Issues arise in moving vehicles: due to vibrations affecting radio signal integrity. Mitigation includes:
    • Correct fitting of doors and mirrors and ensuring all equipment is stowed securely.
    • Ground plane considerations may necessitate retuning antennas.