250 kHz
Understanding the 250 kHz Boundary
In modern telecommunications, everyone is obsessed with millimeter-waves (like 5G at 28 GHz) that travel only a few hundred feet. However, if you need to talk to a submarine sitting at the bottom of the Atlantic Ocean, a millimeter-wave is useless; it will bounce off the surface of the water.
You need a wave so massive it can punch straight through the ocean. You need the 250 kHz Band.
The Physics of Ground Waves
At 250 kHz, the wavelength is 1,200 meters long (roughly 4,000 feet). When a wave is this physically massive, it interacts with the Earth's surface differently than a cellular signal.
- It does not bounce off the ionosphere (Skywave).
- It does not travel in a straight line (Line-of-Sight).
- It acts as a Ground Wave. The magnetic field of the wave physically couples to the saltwater of the ocean or the conductive dirt of the Earth, allowing the signal to mathematically curve around the physical horizon, providing unbroken coverage for hundreds of miles regardless of mountains or curvature.
Legacy Aviation (NDB)
Before GPS existed, airplanes navigated using Non-Directional Beacons (NDB). An NDB is a massive tower in the middle of a field broadcasting a simple, continuous Morse Code identifier near 250 kHz. Because the 250 kHz signal curves over the horizon, a pilot flying 200 miles away could tune an Automatic Direction Finder (ADF) dial to 250 kHz, and a physical needle on the dashboard would point directly at the tower, allowing them to fly straight to the airport through thick fog.
Key Equations
250 kHz is a globally recognized boundary operating near the top edge of the Low Frequency (LF) and the bottom of the Medium Frequency (MF)...
Key specifications:
250 kHz | 1.2 k | 28 GHz
Throughput: R = Nlayers×B×ηSE×(1−OH)
Comparison
| Band | Range | Wavelength | Application | Standard |
|---|---|---|---|---|
| 250 kHz | 250 GHz region | 1.2 mm | Primary use | ITU allocation |
| Adjacent lower | 225.0 GHz | 1.3 mm | Related band | Shared spectrum |
| Adjacent upper | 275.0 GHz | 1.1 mm | Related band | Guard band |
| Harmonic 2f | 500.0 GHz | 0.6 mm | Spurious | Filter required |
| Sub-harmonic | 125.0 GHz | 2.4 mm | LO option | Mixer design |
Frequently Asked Questions
Can 250 kHz transmit voice?
Technically yes, but it is rarely used for it. Because the frequency is so low, the available bandwidth is virtually zero. A standard AM voice channel requires 10 kHz of bandwidth. Sending voice over 250 kHz takes up a massive percentage of the entire band. Therefore, 250 kHz is used almost exclusively for slow Morse code, basic digital telemetry, or simple navigational tones.
Why don't we use 250 kHz for Wi-Fi?
Antenna physics. To efficiently transmit a radio wave, the physical metal antenna must be at least 1/4 the length of the wavelength. At 250 kHz, a quarter-wave antenna must be a physical steel tower 300 meters (1,000 feet) tall. You cannot build a 1,000-foot antenna into a laptop.
Does 250 kHz penetrate saltwater?
Yes. The lower the frequency, the deeper it penetrates conductive saltwater. While extremely low frequencies (ELF at 76 Hz) are required to reach the absolute bottom of the ocean, 250 kHz and its surrounding LF bands are highly effective for communicating with submarines operating at periscope depth or shallow coastal waters.