MF

Medium frequency signals occupy the band from 300 kilohertz to 3 megahertz, nestled between low frequency (LF) below and high frequency (HF) above. The MF band carries AM broadcast radio (535 to 1705 kHz in most regions), maritime distress signals, and aeronautical navigation beacons. At these frequencies, ground wave propagation is reliable out to roughly 200 kilometers during daylight, making MF practical for regional coverage without relying on the ionosphere.

The wavelengths in the MF range span from about 100 meters down to 100 meters, which shapes antenna design directly. A full-wavelength dipole antenna at 1 MHz would be impractically long, so MF transmitters typically use shorter radiating elements with electrical tuning networks to achieve resonance. Receiving antennas in the MF band are often ferrite-loaded loops or whips just a few meters tall; the trade-off between compactness and sensitivity is always present.

Propagation and industrial use

MF signals propagate efficiently along seawater and salt marshes because seawater is a good conductor. This is why maritime distress frequencies center on MF; a ship transmitting at 500 kHz will reach shore stations and other vessels reliably even in poor visibility or rough conditions. In contrast, industrial RF heating and induction systems rarely use MF because the frequencies are too low for efficient coupling into most work material geometries.

Interference is a chronic problem in the MF band because broadcast stations run kilowatt-level power. Receivers in industrial or laboratory settings must filter aggressively to isolate weak signals from distant transmitters. Selective filters with high Q factors and narrow bandwidth are standard in any MF receiver design. Adjacent-channel rejection is critical; a 10 kHz bandwidth filter might reject a strong station just 20 kHz away by 20 decibels or more.

The term medium frequency dates to early radio nomenclature when the spectrum was divided into broad categories. Although the boundaries between LF, MF, HF, and VHF are now formalized in international regulation, the names persist because they remain intuitive: MF signals are the middle ground between very long waves and short waves in terms of both frequency and practical propagation behavior.