High Frequency (HF)

The Shortwave Band

High Frequency (HF) radio occupies the 3-30 MHz frequency range, corresponding to wavelengths from 100 meters to 10 meters. This band is perhaps the most historically significant portion of the radio spectrum, enabling the first truly global wireless communication networks. HF signals have the unique ability to propagate around the curved surface of Earth by reflecting off the ionosphere, a phenomenon known as skywave propagation, allowing transmissions to travel thousands of kilometers beyond the horizon.

The term "shortwave" refers to the relatively short wavelengths compared to the long waves used in early radio communications. During the 1920s through the 1950s, HF shortwave radio was the primary means of international broadcasting, diplomatic communication, and long-distance naval communication. The BBC, Voice of America, Radio Moscow, and many other international broadcasters maintained extensive HF networks that could reach audiences across continents and oceans.

Ionospheric Propagation

The ionosphere is a region of charged particles (ions and free electrons) extending from approximately 60 km to 600 km above Earth's surface. Solar radiation, particularly ultraviolet and X-ray emissions, ionizes neutral atoms in the upper atmosphere, creating several distinct layers with different electron densities:

• D Layer (60-90 km) - Exists only during daylight, absorbs MF and HF signals, prevents skywave propagation at lower HF frequencies during day
• E Layer (90-150 km) - Forms during daylight, allows some HF propagation, dissipates at night
• F1 Layer (150-200 km) - Present during daylight, often merges with F2 at night
• F2 Layer (200-600 km) - Primary layer for long-distance HF propagation, exists day and night but varies with solar activity

The critical frequency, below which signals penetrate rather than reflect, typically ranges from 3-15 MHz depending on atmospheric conditions. Frequencies above the critical frequency pass through the ionosphere into space, while those below are reflected back to Earth. The Maximum Usable Frequency (MUF) for a given path can reach 30 MHz or higher during periods of high solar activity, declining to around 10-15 MHz during solar minimum.

Frequency Selection and Skip Zones

HF propagation creates distinctive phenomena that affect communication planning. The skip distance is the minimum distance from the transmitter where a skywave signal returns to Earth. Between the groundwave coverage and the first skip zone lies a "dead zone" or skip zone where no signal is receivable. This can result in situations where nearby stations are inaudible while distant ones come in clearly.

Selecting the optimal frequency requires balancing several factors. Lower frequencies (3-10 MHz) tend to work better at night and during periods of low solar activity but may be absorbed by the D layer during daylight. Higher frequencies (15-30 MHz) perform better during daylight and solar maximum but may pass through the ionosphere during nighttime or solar minimum. Experienced HF operators monitor conditions and adjust frequencies throughout the day, often using bands like 80 meters (3.5-4 MHz) and 40 meters (7-7.3 MHz) for nightime regional communication, 20 meters (14-14.35 MHz) for daytime long-distance work, and 15 meters (21-21.45 MHz) and 10 meters (28-29.7 MHz) for Sporadic E and enhanced propagation during solar maximum.

Aviation and Maritime Applications

HF radio remained essential for aviation and maritime communications well into the satellite era. Aircraft crossing oceans relied on HF single-sideband (SSB) radios to maintain contact with ground stations and receive weather information. The requirement for long-range aircraft to carry HF radios persisted until satellite communications became universally available. Maritime HF provides crucial safety communications through the Global Maritime Distress and Safety System (GMDSS), which includes HF SITOR (Simplex Teletype Over Radio) for automated message handling and distress alerting.

Amateur Radio and HF Bands

Amateur radio operators (hams) have exclusive access to several HF bands that serve as training grounds for communications professionals and enablers of emergency communication. The traditional HF ham bands include:

• 160 meters (1.8-2.0 MHz) - Challenging band due to noise, primarily nighttime
• 80/75 meters (3.5-4.0 MHz) - Regional daytime, international at night
• 40 meters (7.0-7.3 MHz) - Excellent for both regional and long-distance
• 30 meters (10.1-10.15 MHz) - WARC band, shared with other services
• 20 meters (14.0-14.35 MHz) - The most popular DX band
• 17 meters (18.068-18.168 MHz) - WARC band
• 15 meters (21-21.45 MHz) - Good for long-distance during solar maximum
• 12 meters (24.89-24.99 MHz) - WARC band, daytime long-distance
• 10 meters (28-29.7 MHz) - Excellent during solar maximum, supports FM and digital modes

Modern HF Communication

While satellite communications have largely supplanted HF for commercial long-distance traffic, the band remains vital for several reasons. Military organizations maintain HF capabilities as a resilient backup when satellites might be vulnerable. Disaster relief agencies use HF when infrastructure is destroyed. News organizations maintain HF bureaus for file transmission when internet connectivity is unavailable. Additionally, HF skywave propagation is immune to ground-based disruptions that can affect cables and satellites.

Modern HF transceivers incorporate digital signal processing for interference rejection, automatic link establishment (ALE) for frequency selection, and connectivity to computers for digital modes. HF modems can achieve data rates of thousands of bits per second using technologies like PACTOR, WINMOR, and FreeBSD's airmail. These systems enable email and file transfer over HF even when internet is unavailable.

Listen Live: WebSDR

You can tune into live shortwave and HF signals right from your browser using WebSDR — networked software-defined radios that stream audio over the internet.

• WebSDR.org — Global directory of WebSDR receivers. Tune into any frequency, hear real shortwave signals as they arrive at remote receivers worldwide.
• KiwiSDR — Browser-based SDR with waterfall display. Many public receivers worldwide offer free access to the full HF spectrum (0-30 MHz).
• SDR.hu — Another directory of public KiwiSDR receivers with real-time spectrum displays and audio streaming.

These receivers use SDR hardware (like the KiwiSDR board or RTL-SDR) connected to antennas at quiet rural locations, providing crystal-clear reception free from local interference. Select a receiver, point it at a frequency, and listen to shortwave broadcasts, amateur radio, numbers stations, and more — all from your web browser.