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 via electron-neutral collisions (absorption ∝ 1/f²). The D layer is the reason AM radio has poor reception during the day but improves at night when it dissipates. Critical for determining the Lowest Usable Frequency (LUF)
• E Layer (90–150 km): Forms during daylight. Allows sporadic-E propagation at VHF (50–150 MHz) when metallic ion patches form. Normal E supports HF up to ~10 MHz during daytime. Sporadic E can support 6m and 2m propagation over 1000+ km paths
• F1 Layer (150–200 km): Present during daylight only, often merges with F2 at night. Important for multi-hop HF paths. Electron density: 10⁵–10⁶ electrons/cm³
• F2 Layer (200–600 km): Primary layer for long-distance HF propagation. Exists day and night but varies with solar activity. Peak electron density: 10⁵–10⁶ electrons/cm³ (varies with solar cycle). The F2 layer altitude and density determine the MUF for any given path

The critical frequency (foF2) is the highest frequency that will be reflected back to Earth when transmitted vertically. It typically ranges from 3–15 MHz depending on time of day, season, and solar activity. The Maximum Usable Frequency (MUF) for oblique paths is higher than foF2 by a factor of 1/cos(θ), where θ is the angle of incidence. During solar maximum, MUF can exceed 30 MHz on paths through the sunlit ionosphere.

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.

OTH Radars of the World

Over-the-horizon radar uses HF skywave propagation to detect targets thousands of kilometers beyond the horizon. Divided by hemisphere:

• OTH Radars — Northern Hemisphere — US OTH-B, ROTHR, MADRE, UK's Cobra Mist, France's NOSTRADAMUS, Russia
• OTH Radars — Southern Hemisphere — Australia's JORN, Brazil OTH-0100, South Atlantic surveillance

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

Modulation and Digital Modes

HF communication uses several modulation techniques, each optimized for different use cases:

• SSB (Single Sideband): The dominant voice mode on HF. Eliminates carrier and one sideband, using 2.4 kHz bandwidth vs AM's 6 kHz. Upper Sideband (USB) is standard above 10 MHz; Lower Sideband (LSB) below 10 MHz. Provides 20–25 dB gain over AM
• CW (Continuous Wave/Morse Code): 50–100 Hz bandwidth. Can copy signals 20–30 dB below the noise floor using narrow filters. Standard for DX contests and emergency communication. 12–15 WPM minimum for international operation
• FT8/FT4 (developed by K1JT): Digital mode using 8-FSK modulation. 15-second decode cycles (FT8), 6-second (FT4). Near the theoretical Shannon limit for weak signals. Typically decoded at -24 dB SNR. Changed HF operation forever after 2017
• PSK31: Phase-shift keying at 31.25 baud. 31.25 Hz bandwidth — extremely narrow. Ideal for keyboard-to-keyboard chat. Can be decoded at -10 dB SNR with modern SDRs
• RTTY (Radio Teletype): FSK modulation at 45.45 or 170 Hz shift. Used for news, weather, and contest exchanges. Robust but slow compared to modern digital modes
• Olivia/Contestia: MFSK-based modes designed for ultra-weak signals. Olivia 8/500 can decode at -30 dB SNR. Used for emergency nets when conditions are severe

Modern HF Communication

While satellite communications have largely supplanted HF for commercial long-distance traffic, the band remains vital for several reasons:

• Military: HF provides resilient backup when satellites are jammed or destroyed. ALE (Automatic Link Establishment) selects the best frequency automatically. US military uses MIL-STD-188-110C for high-speed data (up to 9.6 kbps)
• Disaster relief: HF is often the only communication available after hurricanes, earthquakes, and floods. ARRL nets activate within hours of disasters
• MARSA (Maritime Satellite): HF SITOR provides the GMDSS fallback when satellite systems fail
• Aviation: Oceanic routes still use HF for ATC communication (HF SSB at 2–22 MHz)

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 JS8Call (FT8-based keyboard mode). 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.