Antenna Height Considerations - Longwave/Low Frequency Antennas

Longwave/Low Frequency Antennas

At VLF, LF and MF the radio mast or tower is often used directly as an antenna. Its height determines the vertical radiation pattern. Masts and towers with heights around a quarter wave or shorter, radiate considerable power towards the sky. This allows only a small area of fade-free reception at night, because the distance at which groundwave and skywave are of comparable strength and can interfere with each other is severely restricted (approximately 40 kilometres to 200 kilometres from the transmission site, depending on frequency and ground conductivity).

For high power transmitters, masts with heights of about half the radiated wavelength are preferred because they concentrate the radiated power toward the horizon. This enlarges the distance at which selective fading occurs. However, masts with heights of around half a wavelength are much more expensive than shorter ones and often too expensive for lower power mediumwave stations.

For longwave transmitters, however, the construction of halfwave masts is infeasible in most cases, either for economic reasons or because of problems with flight safety. The only radio mast for longwave with a height of half a wavelength built to date was the Warsaw Radio Mast (which did not survive). For frequencies lower than the longwave range, masts have to be electrically enlarged by base loading coils or structures on the top, because the heights required for masts of even a quarter wavelength are too large to realize physically.

Use is not normally made of masts higher than five-eighths (5/8) of a wavelength, because such masts (except for some special constructions for high power mediumwave broadcasting) exhibit poor vertical radiation patterns. The heights of masts for mediumwave transmitters normally do not exceed the 300 metre (1000 foot) level.

Sometimes cage aerials or longwire aerials are used for LF and MF transmission. In this case the height of the tower may be greater than is usually the case. Because towers or masts used for cage or long wire aerials are grounded at the base, they are especially suitable for supporting antennas for UHF or VHF broadcasting.