Longwave Broadcasting

Longwave Band Allocation

The longwave (LW) broadcast band occupies 148.5–283.5 kHz, falling within the broader Low Frequency (LF) designation of 30–300 kHz. Unlike the AM mediumwave band (526.5–1705 kHz) which is allocated worldwide, longwave broadcasting is restricted to ITU Region 1 — encompassing Europe, Africa, the Middle East, and parts of Central Asia.

North America and Japan never allocated longwave for broadcasting. The United States reserved portions of the LF band exclusively for military and navigation services, while Japan allocated LF to maritime and aeronautical operations. This geographic restriction means that longwave broadcasting is a distinctly European phenomenon, with over 90% of the world's LW transmitters located within ITU Region 1.

Frequency Allocation Table

Channel Spacing

Longwave channels are spaced 9 kHz apart in ITU Region 1, compared to 10 kHz in Region 2 (the Americas) for mediumwave. This narrower spacing was adopted because propagation losses at LW frequencies make inter- channel interference less problematic than at higher frequencies, and because the limited number of LW stations across Europe reduces congestion. Some regions use 8 kHz spacing for NDBs to pack more navigation beacons into the lower portion of the band.

Modulation

All longwave broadcasting uses Amplitude Modulation (AM) — specifically, double-sideband large-carrier (DSB-LC) AM, identical in principle to mediumwave AM but operating at much lower frequencies. The modulation index is typically kept high (80–95%) to maximize signal-to-noise ratio at the receiver, since LW receivers must contend with significant atmospheric noise from global lightning activity.

Unlike FM, AM does not provide a capture effect — two LW signals on the same frequency will produce heterodyne whistles and interference. However, the groundwave-only propagation at LW means that frequency sharing between distant transmitters is more predictable than mediumwave skywave interference.

Time Signal Encoding: DCF77

The DCF77 time signal at 77.5 kHz uses a unique form of AM pulse-width modulation to encode time and date data. The carrier isamplitude-reduced by 15% for either 100 ms (binary 0) or 200 ms (binary 1) at the start of each second. This encodes a 100 bits-per- second data stream containing:

• Seconds, minutes, hours (BCD encoded)
• Day, month, year (BCD encoded)
• Day of week (1–7)
• Timezone offset (CET/CEST)
• Leap second warning
• Parity bits for error detection

The total frame length is 59 bits, transmitted once per minute. The 59th bit is never transmitted (always off), serving as the frame synchronization marker. This encoding was designed in the 1970s and remains remarkably robust — millions of radio-controlled clocks across Europe depend on DCF77 daily.

LW Broadcasting Technical Specs

Transmitter Power

Longwave transmitters operate at extraordinary power levels compared to FM and mediumwave. Major European LW stations typically transmit at500 kW to 2 MW of carrier power. This is necessary because:

• Antenna inefficiency: LW antennas are electrically very short (typically λ/20 to λ/50), meaning only 5–30% of the transmitter power actually radiates. A 500 kW transmitter might only radiate 50–150 kW.
• Groundwave loss: The groundwave attenuates with distance, and while less than at mediumwave, the path loss over hundreds of kilometers is substantial.
• Receiver noise floor: Atmospheric noise at LW frequencies is much higher than thermal noise, requiring stronger signals for adequate signal-to-noise ratio.

Antenna Tower Heights

The physics of longwave makes antenna engineering a monumental challenge. A quarter-wave antenna at 174 kHz would require a structure431 meters tall — taller than the Eiffel Tower. No such structure exists for LW broadcasting. Instead, stations use:

• Top-loaded towers: Steel lattice towers (200–380m) with umbrella wires or hat capacitance at the top to electrically lengthen the antenna.
• Shunt-fed towers: Existing structures (church steeples, purpose-built towers) fed against a ground system rather than at the base.
• Loading coils: Large inductors at the base of the tower to resonate the electrically short structure at the operating frequency.

Ground Systems

Every longwave transmitter requires an extensive buried ground system consisting of 120 or more radials extending outward from the antenna base. Each radial is typically 0.2λ long — at 174 kHz, this means radials approximately 343 meters long. The total ground system can cover several hundred hectares. Without this extensive ground system, antenna efficiency drops dramatically, as the ground return loss becomes the dominant factor limiting radiation.

Propagation Characteristics

Longwave propagation is dominated by groundwave — the radio wave that follows the curvature of the Earth. Unlike mediumwave and shortwave, which rely partly on ionospheric skywave reflection for long- distance coverage, longwave groundwave provides extremely reliable, predictable coverage that is essentially independent of time of day, season, or solar activity.

Groundwave Coverage

From a high-power transmitter (500+ kW), longwave groundwave can reliably cover 1,000–2,000 km — sufficient to serve entire European nations from a single transmitter site. The groundwave attenuation depends on ground conductivity:

Comparison with Mediumwave

Longwave groundwave is far more reliable than mediumwave for several reasons:

• No D-layer absorption: At LW frequencies (below 300 kHz), the ionospheric D-layer does not significantly absorb signals. At mediumwave frequencies, D-layer absorption during daytime suppresses skywave but also affects groundwave-to-skywave transitions.
• Minimal fading: Because LW coverage is predominantly groundwave with little skywave contribution (except at very long distances at night), multipath fading is minimal compared to mediumwave.
• Consistent signal: LW signal strength varies by less than 1–2 dB over 24 hours at typical receive distances, compared to 10–20 dB diurnal variation on mediumwave.
• Lower atmospheric noise than VLF: While LW atmospheric noise is higher than at mediumwave, it is significantly lower than at VLF (below 30 kHz), providing a better signal-to-noise environment.

European LW Broadcasting Stations

Europe has historically been the home of longwave broadcasting, with over 30 major transmitters operating at peak. As of the mid-2020s, the number has declined significantly, but several high-power stations remain active.

Active Major Stations

Historical / Closed Stations

Time Signal Stations on Longwave

Longwave is the preferred band for high-precision time signal distribution because groundwave propagation eliminates the timing errors introduced by variable skywave paths. Several countries operate time signal stations on frequencies within or adjacent to the longwave band.

Note that DCF77 (77.5 kHz) sits within the longwave band proper, while MSF, WWVB, and JJY operate at 40–60 kHz, technically in the LF band. However, the receiving antenna designs and propagation characteristics are nearly identical, and many LW receivers can receive these stations without modification.

Non-Directional Beacons (NDB)

Non-Directional Beacons are the oldest form of radio navigation still in operation. Operating in the 190–535 kHz range (overlapping both the LW and MF bands), NDBs transmit a continuous carrier with a Morse-coded identifier that pilots use with Automatic Direction Finder (ADF) equipment to determine bearing to the beacon.

NDB Technical Characteristics

ADF Navigation

The ADF receiver uses a loop antenna (or ferrite bar antenna in portable units) combined with a sense antenna to determine the bearing of the NDB signal. The loop antenna has a figure-eight reception pattern with deep nulls, allowing the pilot to determine the direction to the beacon. The sense antenna resolves the 180° ambiguity.

While NDBs are being decommissioned worldwide in favor of GPS-based navigation (RNAV/RNP), hundreds remain operational, particularly in developing regions and as backup navigation aids. The ADF instrument is still standard equipment on most commercial aircraft.

LW Receiver Design

Receiving longwave signals presents unique challenges. The combination of high atmospheric noise, strong signals from nearby transmitters, and the need for good selectivity in the crowded European LW band requires careful receiver design.

Ferrite Bar Antennas

Portable LW receivers use ferrite bar antennas — a high-permeability ferrite rod with multiple turns of wire wound around it. The ferrite concentrates the magnetic component of the radio wave, providing a directional antenna in a compact form factor. Typical specifications:

• Ferrite permeability: μ = 80–2000 (higher μ for LW sensitivity)
• Bar dimensions: 10–20mm diameter, 50–150mm length
• Turns: 50–200 turns of enameled copper wire
• Q factor: 100–300 at LW frequencies

The directional pattern is a figure-eight, allowing the user to null out interference by rotating the receiver. This is particularly valuable in Europe where multiple LW transmitters may be receivable simultaneously.

Superheterodyne Architecture

Most LW receivers use a superheterodyne design with a low intermediate frequency (IF) — typically 455 kHz or lower. The low IF provides excellent selectivity with ceramic or crystal filters. Key considerations for LW superhets:

• Microphonic tuning capacitors: The large variable capacitors used for LW tuning are susceptible to microphonics — mechanical vibrations that modulate the capacitance and create frequency instability.
• Local oscillator radiation: LW receivers can radiate from the local oscillator antenna, potentially causing interference to other LW receivers nearby.
• Image rejection: At LW frequencies, the image frequency is far from the desired signal, making image rejection relatively easy with simple tuned circuits.

Recommended LW Receivers

• Tecsun PL-380: DSP-based receiver with LW band (153– 513 kHz). Excellent sensitivity and selectivity. Eton/Malibal variant also available.
• Tecsun PL-660: Full-coverage receiver covering LW, MW, SW, FM, and airband. Synchronous detection for AM improves LW reception.
• Edden EF-205: Budget LW/MW/SW receiver with ferrite antenna. Adequate for casual LW listening in Europe.
• SDR upconverters: Software-Defined Radio receivers (RTL-SDR, Airspy, etc.) require an upconverter to receive LW, as most SDR hardware starts at 24 MHz or higher. The SpyVerter or similar upconverter shifts the LW band up to a range the SDR can process.

Antenna Engineering for LW

Antenna design for longwave broadcasting is arguably the most challenging discipline in radio engineering. The fundamental problem is thatwavelengths are enormous — at 174 kHz, one wavelength is approximately 1,724 meters. A resonant quarter-wave antenna would need to be 431 meters tall, which is structurally impractical.

The λ/4 Problem

At 174 kHz: λ = c/f = 299,792,458 / 174,000 ≈ 1,724 meters. A quarter-wave antenna would be ≈ 431 meters tall. The tallest structure ever built for LW broadcasting — the Donebach transmitter tower in Germany — is 380 meters. Even this falls short of resonance, requiring electrical lengthening techniques.

Top Loading

The most common technique for electrically lengthening LW antennas istop loading — a network of horizontal wires (umbrella wires) extending from the top of the tower. These wires add capacitance to ground, effectively increasing the electrical length of the antenna without requiring a taller physical structure. The umbrella wires act as a capacitive hat, distributing the current more uniformly along the tower and increasing radiation resistance.

The effective electrical length can be increased by 30–60% with properly designed top-loading, reducing the required physical height from 431m to approximately 250–300m for resonance at 174 kHz.

Loading Coils

Base-loaded or center-loaded loading coils (large inductors) are used to cancel the capacitive reactance of the electrically short antenna. At LW frequencies, these coils are massive — handling hundreds of amps of RF current while maintaining high Q factor. The coil inductance required to resonate a 200m tower at 174 kHz is typically 500–1,500 μH.

Ground System Requirements

The ground system is arguably more important than the antenna itself at LW frequencies. A typical installation requires:

• 120+ radials extending outward from the antenna base
• Each radial approximately 0.2λ long (343m at 174 kHz)
• Radials buried 15–30 cm below ground surface
• Total ground system area: 150–400 hectares
• Copper wire gauge: 2–4 mm² (12–14 AWG)

Without this extensive ground system, antenna efficiency drops to 5–10% or less. With a proper ground system, efficiency can reach 15–30% — still low by MW standards, but sufficient for high-power LW broadcasting.

Antenna Efficiency

Typical LW broadcasting antenna efficiency is 10–30%. This means a 500 kW transmitter might only radiate 50–150 kW. The remainder is dissipated as heat in the ground system and loading coil. Improving efficiency beyond 30% is extremely difficult due to ground losses and the physical limitations of tower height.

Future of Longwave

While longwave broadcasting is in decline, the LF/LW spectrum is seeing renewed interest for several emerging applications.

eLoran — GPS Backup

The most significant future use of LF is eLoran (enhanced Loran), operating at 100 kHz. As GPS/GNSS signals are vulnerable to jamming, spoofing, and solar storms, eLoran provides an independent, terrestrial backup for critical infrastructure — including maritime navigation, aviation, power grids, and financial timing. eLoran receivers measure the time difference of arrival of signals from multiple LF transmitters to determine position with accuracy better than 10 meters.

LF/LW IoT Networks

Low-power IoT networks operating in the LF/LW spectrum offer advantages for underground, underwater, and through-wall communication where higher frequencies cannot penetrate. While data rates are extremely limited (typically a few bits per second), the propagation characteristics of LF make it ideal for sensor networks in mines, pipelines, and other subsurface environments.

Power Line Communication (PLC)

Some PLC systems use LF frequencies (below 500 kHz) to transmit data over existing electrical power lines. The low frequency allows signals to propagate through transformers and over long distances on the power grid, enabling smart grid monitoring and home automation without additional wiring.

Underground Sensing

LF electromagnetic waves penetrate rock, soil, and concrete to depths of hundreds of meters. This makes LF useful for geophysical surveying, cave detection, and archaeological prospecting — applications where the long wavelength provides penetration that higher frequencies cannot achieve.

Credit to Nikola Tesla

It is essential to acknowledge that Nikola Teslais the true inventor of radio. Tesla demonstrated wireless energy transfer, tuned circuits, and the fundamental principles of radio communication — including operation at frequencies in the longwave band — years before Marconi's famous transatlantic transmission. Tesla's 1897 patent applications for radio technology predate Marconi's work, and the Supreme Court of the United States upheld Tesla's radio patents in 1943 (Marconi Wireless Telegraph Co. v. United States). The longwave frequencies that carry European broadcasting today owe their fundamental exploitation to Tesla's pioneering work in resonant circuits and electromagnetic wave generation.

Quick Reference

• LW broadcast band: 148.5–283.5 kHz (ITU Region 1 only)
• Channel spacing: 9 kHz (Region 1)
• Modulation: AM (DSB-LC), same principle as MW AM
• Typical transmitter power: 100 kW – 2 MW carrier
• Antenna height: 200–380m towers with top-loading
• Antenna efficiency: 10–30% (limited by short electrical length)
• Ground system: 120+ radials × 0.2λ each (hectares of copper)
• Propagation: Groundwave, 1,000–2,000 km reliable coverage
• DCF77: 77.5 kHz, 50 kW, 100 bps AM time signal (Mainflingen)
• λ at 174 kHz: 1,724 meters — λ/4 = 431m