Medium frequency

Ground waves are progressively absorbed by the Earth, so the signal strength decreases exponentially with distance from the transmitting antenna.

Typical MF radio stations can cover a radius of several hundred kilometres/miles from the transmitter, with longer distances over water and damp earth.

At night, especially in winter months and at times of low solar activity, the ionospheric D layer can virtually disappear.

When this happens, MF radio waves can easily be received hundreds or even thousands of miles away as the signal will be refracted by the remaining F layer.

In recent years, some limited amateur radio operation has also been allowed in the region of 500 kHz in the US, UK, Germany and Sweden.

[10] Many home-portable or cordless telephones, especially those that were designed in the 1980s, transmit low power FM audio signals between the table-top base unit and the handset on frequencies in the range 1600 to 1800 kHz.

Ground wave propagation, the most widely used type at these frequencies, requires vertically polarized antennas like monopoles.

The most common transmitting antennas, monopoles of one-quarter to five-eighths wavelength, are physically large at these frequencies, 25 to 250 metres (82 to 820 ft) requiring a tall radio mast.

Commercial radio stations use a ground system consisting of many copper cables, buried shallowly in the earth, radiating from the base of the antenna to a distance of about a quarter wavelength.

Lower power transmitters often use electrically short quarter wave monopoles such as inverted-L or T antennas, which are brought into resonance with a loading coil at their base.