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LF and MF experimenting
2200 m and 630 m.
2200 m and 630 m.
Below the 160 m band lie two small, experimental amateur bands: 2200 m (about 135.7 to 137.8 kHz, "LF") and 630 m (about 472 to 479 kHz, "MF"). With wavelengths of about 2,200 m and 630 m, they are the longest waves an American ham may transmit. The activity is experimental: building huge antennas, learning ground-wave and night-time propagation, and decoding signals buried in atmospheric noise.
The limits are tiny and measured as EIRP: the power as radiated by the antenna, gain included. They apply in the US, with extra rules (for example, notifying power-utility systems that share these frequencies, and limits in parts of Alaska). Read the current Part 97 text before transmitting, and note that other countries differ.
A wavelength is so long that any practical antenna is a tiny fraction of one. A 30 m mast at 137 kHz is about 1.4% of a wavelength tall, and its radiation resistance is under a tenth of an ohm. Losses in the ground and coil are measured in ohms, so almost all the transmitter power heats the ground. A transmitter for a legal 1 W EIRP can therefore need tens to hundreds of watts, and the antenna carries amps through a very high reactance, producing kilovolts of RF at the top.
Builders respond with a top hat (a wire umbrella that adds capacitance and current), a large loading coil, and a huge buried or radial ground system. Receiving is far easier, since at these frequencies atmospheric noise is stronger than receiver noise, so a small loop or whip works fine.
By day, ground wave covers a few hundred kilometres. At night, when the D layer fades, sky wave carries 630 m and 2200 m signals much further, sometimes across oceans, mostly in winter. Lightning static is a constant background, so narrow modes help most: QRSS (CW so slow that a dit lasts seconds and a computer spectrogram reads it) and weak-signal digital modes such as WSPR and ones designed for these bands.