HF RADIO: REGIONAL COMMUNICATIONS IN GUERRILLA WARFARE

The Regional Communications Problem

For a Guerrilla force operating across a broad geographical region, communications present a persistent challenge. Mountainous terrain, dense forests, damaged infrastructure, and the absence of cellular networks can isolate dispersed elements. High Frequency (HF) radio offers an alternative, allowing stations separated by considerable distances to communicate without relying on repeaters, satellites, or commercial infrastructure.

Unlike VHF and UHF systems, which are generally constrained by line-of-sight propagation, HF signals can travel beyond the horizon through interaction with the ionosphere. This makes HF particularly relevant to the historical study of irregular warfare and decentralized communications.

Understanding Skywave Propagation

Skywave propagation occurs when radio energy travels upward and is refracted by electrically charged layers of the ionosphere, returning toward Earth's surface.

The effectiveness of this process depends on operating frequency, solar activity, time of day, season, and atmospheric conditions. Long-distance HF communication can involve substantial distances between the transmitting station and the point where its signal returns to Earth. This creates a potential skip zone, where stations closer to the transmitter may receive little or no signal.

For regional communications, this limitation makes Near Vertical Incidence Skywave (NVIS) propagation particularly interesting.

NVIS: Regional HF Coverage

NVIS involves transmitting HF energy at steep angles toward the ionosphere, allowing signals to return to Earth relatively close to their point of origin. Under suitable ionospheric conditions, NVIS can support communications across distances of approximately 50–400 kilometers, although coverage varies significantly. Historically, military organizations, emergency communications networks, and remote-area operators have studied NVIS as a means of connecting geographically dispersed stations.

Common HF communication modes include:

• SSB (Single Sideband): A bandwidth-efficient voice mode widely used in HF communications.

• CW (Continuous Wave): Morse-code communication with relatively narrow bandwidth requirements.

• JS8Call: A weak-signal digital messaging system based on JS8 modulation.

• Olivia: A digital text mode designed for reliable reception under difficult propagation and interference conditions.

• Winlink: A radio-based messaging system commonly associated with amateur-radio and emergency communications applications.

Each mode involves tradeoffs involving bandwidth, equipment complexity, transmission duration, and message delivery.

One-Time Pad Encryption

One-Time Pad (OTP) encryption has a long history in military and intelligence communications.

The underlying concept is straightforward: plaintext is combined with a genuinely random key of equal length. When implemented correctly, OTP provides information-theoretic secrecy. Three conditions are essential: the key must be genuinely random, remain secret, and never be reused.

Key distribution and safeguarding are major practical challenges. A compromised pad can expose protected messages, while reusing key material can destroy the system's security guarantees. Importantly, encryption protects message content, not necessarily the existence, location, or timing of a radio transmission.

HF RADIO: SPECIAL FORCES COMMUNICATIONS IN VIETNAM

During the Vietnam War, U.S. Army Special Forces and MACV-SOG reconnaissance teams relied on High Frequency (HF) radio to maintain communications across mountainous terrain and dense jungle. Operating far beyond conventional VHF line-of-sight ranges, these teams used HF skywave propagation to communicate with distant command elements without relying on repeaters. Manpack radios, including the AN/PRC-64, provided long-range communications for clandestine reconnaissance missions. Morse code (CW) offered an efficient means of transmitting information under difficult signal conditions. HF communications enabled isolated teams to relay intelligence, report enemy movements, and maintain contact with supporting headquarters during operations deep inside contested territory.

The Radio Failure That Helped Doom the 1967 Guerrilla Campaign

During Che Guevara's 1966–1967 guerrilla campaign in Bolivia, High Frequency (HF) radio was intended to provide a critical communications link between his isolated force, Cuban intelligence, and supporting networks.

Tamara Bunke, better known as Tania, was a Cuban-trained intelligence operative who helped establish Guevara's clandestine support network in Bolivia. Trained in covert communications, she maintained contacts between the guerrillas and their external supporters.

The campaign's communications system suffered a catastrophic failure. According to a declassified CIA summary of Guevara's diary, the guerrillas' shortwave radio became damp and rusted while stored inside a cave, rendering its transmitting capability inoperative. Other accounts describe equipment damage during movement. By early 1967, Guevara's force could receive transmissions from Havana but could not reliably transmit replies. 

Cuban broadcasts continued delivering coded messages, including communications protected by one-time-pad encryption. However, the loss of a tape recorder used to capture these broadcasts further complicated reception and decoding. Tania's compromised cover and subsequent death in August 1967 compounded the isolation.

Without dependable two-way HF communications, Guevara could neither coordinate effectively with Cuba nor reliably request reinforcements and supplies. The communications breakdown became one of several critical failures contributing to the campaign's destruction in October 1967.

The Continuing Relevance of HF

HF remains valuable because it does not inherently depend on commercial telecommunications infrastructure. It also illustrates how dispersed organizations maintained regional connectivity despite difficult geography and unreliable infrastructure. Modern digital modes have expanded the capabilities of HF, but ionospheric variability, interference, equipment limitations, and radio-frequency monitoring will always remain important considerations.

The enduring lesson is that reliable regional communications depend on understanding propagation, equipment capabilities, message security, and the limitations of the electromagnetic environment.