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Chapter 1: Introduction to High Frequency Propagation

Course Prerequisite: Have a basic understanding of radio waves, radio frequency and antennas.

Overview of HF Radio Waves

High Frequency (HF) radio waves refer to the range of radio frequency electromagnetic waves between 3 and 30 megahertz (MHz). They are commonly used for long-distance communication due to their ability to travel long distances by bouncing off the Earth’s ionosphere. Here is an overview of HF radio waves:

  1. Frequency Range: HF radio waves operate within the frequency range of 3 to 30 MHz.
  2. Wavelength: The wavelength of HF radio waves ranges from approximately 10 to 100 meters.
  3. Communication Range: HF radio waves can travel long distances, especially during certain atmospheric conditions. They can be used for both short-range and long-range communication.
  4. Ionospheric PropagationHF radio waves interact with the Earth’s ionosphere, a layer of charged particles in the upper atmosphere. This interaction allows the waves to be refracted and reflected, enabling long-distance communication.
  5. Advantages: HF radio waves have several advantages over lower frequency radio waves. They can penetrate obstacles such as buildings and rough terrain, making them suitable for communication in remote areas. They are also less affected by weather conditions compared to higher frequency waves.
  6. Applications: HF radio waves are used in various applications, including amateur radioaviation communicationsmaritime communicationsmilitary communications, and emergency communications.

It’s important to note that HF radio waves are just one part of the electromagnetic spectrum, which also includes other types of waves such as AM/FM radioTV signalsmicrowaves, and more.

Radio Spectrum

In radio communications, the fundamental component is a radio wave, which possesses the longest wavelengths within the electromagnetic spectrum. Similar to ripples on a pond, a radio wave comprises a recurring series of peaks and valleys. The complete pattern of a wave, encompassing a full cycle before repetition, is termed a cycle. Wavelength denotes the distance a wave covers to finish one cycle. The frequency of a wave refers to the number of cycles it completes in one second and is quantified in hertz (Hz), representing cycles per second. A kilohertz (kHz) denotes one thousand hertz, a megahertz (MHz) signifies one million hertz, and a gigahertz (GHz) indicates one billion hertz. The radio spectrum spans from 3 kilohertz to 3,000 gigahertz.

A radio wave is produced by a transmitter and received by a receiver. An antenna facilitates a radio transmitter in transmitting energy into space and enables a receiver to capture energy from space. Typically, transmitters and receivers are engineered to function within a specific range of frequencies.

Radio Frequency Spectrum

Amateur Radio makes use of various frequency bands, each possessing unique propagation characteristics.

Skywave Propagation

Skywave propagation, also known as skip propagation, refers to the phenomenon of radio waves being reflected or refracted back towards the Earth‘s surface from the ionosphere, which is an electrically charged layer of the upper atmosphere. This type of propagation allows radio signals to travel over long distances, beyond the line-of-sight range.

Here are some key points about skywave propagation:

Ionosphere: The ionosphere is a region of the Earth‘s atmosphere that contains a high concentration of ions and free electrons. It extends from about 60 kilometers (37 miles) to 1,000 kilometers (620 miles) above the Earth‘s surface.

Reflection and Refraction: When radio waves encounter the ionosphere, they can be reflected or refracted back towards the Earth. The behavior of the waves depends on factors such as the frequency of the signal and the density of the ionosphere at different altitudes.

Coverage: Skywave propagation allows radio signals to cover long distances, making it useful for long-range communication. It is commonly used in applications such as international broadcasting, long-distance communication for ships and aircraft, and amateur radio operations.

Frequency Range: Skywave propagation is most effective in the high-frequency (HF) range, typically between 3 MHz and 30 MHz. Within this range, different layers of the ionosphere can reflect or refract the waves, allowing for communication over varying distances.

Skip Distance: The skip distance is the shortest distance between the transmitter and receiver where skywave propagation occurs. Beyond this distance, the radio waves will pass through the ionosphere and continue into space. The skip distance depends on factors such as the frequency of the signal and the conditions of the ionosphere.

NVIS (Near Vertical Incidence Skywave): NVIS is a propagation technique that involves directing HF radio waves nearly vertically towards the ionosphere. This method allows for reliable communication over shorter distances, typically up to a few hundred kilometers.

It’s important to note that skywave propagation is influenced by various factors, including the time of day, solar activity, and the state of the ionosphere. These factors can affect the quality and reliability of the communication.

Skywave

Skywave propagation happens when there is a significant distance between the transmitting and receiving stations. Radio waves follow varying paths based on factors like frequency, solar activity and time of day.

Groundwave Propagation

Groundwave propagation refers to the transmission of radio waves that travel parallel to and along the surface of the Earth. These waves follow the curvature of the Earth and extend beyond the visible horizon. Here are some key points to understand about groundwave propagation:

Signal Coverage: Groundwave propagation is commonly used for regional communication, particularly in broadcasting, at lower frequencies. It allows for reliable communication over relatively short distances, typically up to a few hundred kilometers.

Frequency Range: Groundwave propagation is most effective at lower frequencies, typically below 2 MHz. As the frequency increases, the groundwave signal weakens, and other propagation methods like skywave propagation become more dominant.

Advantages: Groundwave propagation offers several advantages, including:
Reliable communication over short to medium distances.
Less susceptibility to atmospheric conditions and interference compared to other propagation methods.
Suitable for broadcasting and communication in urban areas.

Disadvantages: Groundwave propagation also has some limitations, such as:
Limited range compared to other propagation methods like skywave or satellite communication.
Susceptibility to terrain obstacles, which can cause signal attenuation or blockage.
Higher frequencies experience more signal loss due to ground absorption.

In summary, groundwave propagation is a method of radio wave transmission that allows signals to travel along the Earth’s surface. It is commonly used for regional communication, especially at lower frequencies, and offers reliable coverage over shorter distances. However, it has limitations in terms of range and susceptibility to terrain obstacles.

Groundwave Propagation

Refers to the transmission of radio waves that travel parallel to and along the surface of the Earth.

Determining the Optimal High Frequency (HF) Frequency Selection

To determine the optimal High Frequency (HF) frequency selection for propagation, you can follow these steps:

Understand the Basics: HF radio waves are affected by the ionosphere, which causes them to bounce back to Earth. Different frequencies can be better suited for different distances and times of day.

Consider the Distance: Shortwave frequencies (3-30 MHz) are suitable for long-distance communication, while high frequencies (like 30 MHz and above) are better for shorter distances.

Time of Day: The ionosphere changes throughout the day, affecting which frequencies are best for communication. Generally, higher frequencies work better during the day, while lower frequencies work better at night.

Space Weather: Solar activity can impact HF propagation. During times of high solar activity, higher frequencies may perform better due to increased ionization in the ionosphere.

Use Propagation Prediction Tools: Tools like VOACAP (Voice of America Coverage Analysis Program) can help predict the best frequencies for communication based on your location and time of day. VOACAP will be explained in detail in another chapter.

Experiment and Monitor: It’s essential to monitor the quality of communication on different frequencies and make adjustments accordingly. Try different frequencies and observe which ones provide the best results for your specific needs.

By following these steps and considering factors like distance, time of day, space weather, and using prediction tools, you can determine the optimal HF frequency selection for propagation in your specific communication scenario. This topic will be explained in other chapters

Importance of Understanding HF Propagation

Understanding HF propagation is crucial for various reasons. Here are some key points highlighting the importance of understanding HF propagation:

Optimizing Communication: HF (High Frequency) propagation plays a vital role in long-distance radio communication. By understanding HF propagation, you can optimize your communication strategies and improve the reliability and effectiveness of your transmissions.

Signal Path Prediction: HF propagation is influenced by various factors such as time of day, solar activity, ionospheric conditions, and geographical location. By understanding these factors, you can predict the signal path and determine the best frequencies, times, and locations for communication.

Avoiding Signal Loss: HF signals can experience various propagation phenomena like fading, absorption, and interference. Understanding HF propagation helps you identify potential signal loss issues and take appropriate measures to mitigate them, ensuring better signal quality and minimizing communication disruptions.

Emergency Communication: During emergencies or natural disasters, HF communication often becomes crucial when other communication methods are unavailable. Understanding HF propagation allows emergency responders and amateur radio operators to establish reliable communication links over long distances, providing vital information and assistance.

Optimal Frequency Selection: Different HF frequencies have different propagation characteristics. By understanding HF propagation, you can select the optimal frequency for your specific communication needs, maximizing the chances of successful transmission and reception.

DXing and Contesting: DXing (contacting distant stations) and contesting (competitive radio communication events) heavily rely on HF propagation. Understanding HF propagation helps DXers and contesters anticipate favorable propagation conditions, enabling them to make successful contacts and achieve their goals.
In summary, understanding HF propagation is essential for optimizing communication, predicting signal paths, avoiding signal loss, facilitating emergency communication, selecting optimal frequencies, and enhancing DXing and contesting experiences. It empowers radio operators to make informed decisions and improve the overall effectiveness of their HF communication.

Key Terminology in HF Propagation

HF Propagation refers to the way radio waves travel through the High Frequency (HF) portion of the electromagnetic spectrum. Understanding the key terminology associated with HF Propagation is essential for comprehending long-distance communication via skywaves. Here are some important terms related to HF Propagation:

Ionosphere: The ionosphere is a region of the Earth‘s atmosphere that contains a high concentration of ions and free electrons. It plays a crucial role in HF Propagation as it reflects and refracts radio waves, allowing them to travel long distances.

Skywave: Skywave propagation occurs when radio waves are reflected by the ionosphere back to the Earth‘s surface. This phenomenon enables long-distance communication over HF frequencies.

NVIS (Near Vertical Incidence Skywave): NVIS is a propagation technique that involves directing HF radio waves nearly vertically towards the ionosphere. This method allows for reliable communication over shorter distances, typically up to a few hundred kilometers.

Radio Spectrum: The Radio Spectrum refers to the range of electromagnetic frequencies used for various types of radio communication. It encompasses a broad range of frequencies that are allocated for different purposes, including broadcasting, telecommunications, radar, satellite communication, and more.

Signal Path: A Signal Path refers to the route or journey that a signal takes from its source to its destination in a communication system. In the context of telecommunications and electronics, a signal path typically involves the transmission of information, data, or a waveform from one point to another.

DX / DXing: DXing, taken from DX, the telegraphic shorthand for “distance” or “distant”, is the hobby of receiving and identifying distant radio or television signals, or making two-way radio contact with distant stations in amateur radio, citizens band radio, or other two-way radio communications. It involves the pursuit of distant stations with the goal of earning various DXCC Awards. DXing is a popular activity among amateur radio operators who challenge themselves to establish communication with stations located far away from their own location.

Radio waves: Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. They have frequencies from 300 gigahertz (GHz) to as low as 30 hertz (Hz). Radio waves are used for various communication purposes, including broadcasting, two-way radio communication, radar, satellite communication, and wireless networking. Here are some key points about radio waves:

Propagation: Radio waves can propagate through air, space, and most other materials. They travel in straight lines unless they are reflected, refracted, or diffracted by obstacles or the atmosphere.

Groundwave: The groundwave refers to radio waves that propagate parallel to and adjacent to the surface of the Earth, following the curvature of the Earth beyond the visible horizon.

Skip Zone: A radio wave skip zone, also known as a silent zone or zone of silence, is an area where a radio transmission cannot be received. It is a region located between two points: the point where the groundwave signals can no longer be heard and the point where the skywave first returns to Earth. In this skip zone, the radio waves “skip” over the intended station and do not reach it. The skip zone occurs due to the interaction of groundwaves and skywaves in radio propagation.

Radio waves play a crucial role in modern communication systems, enabling long-distance communication, wireless connectivity, and the transmission of data across various devices and networks.