12.6 HF SURFACE WAVE ANTENNAS

Since surface waves must be vertically polarized and use a low frequency. A vertical dipole at surface-wave frequencies is very large, however—a half-wave dipole at 1 MHz would have to be 150 m high and poses significant problems in construction, particularly for mobile applications. The half-wave dipole, depending on its height above ground, can be at least twice as efficient as the quarter-wave monopole, although the latter is much more suited to mobile use because it is half the height and can be deployed on either a self-supporting mast or the vertical wire supported by trees or masts

For mobile applications, large monopoles are too unwieldy and antennas must be limited to approximately 3 m. For the highest surface-wave frequencies of around 3 MHz, a quarter-wave monopole should be 25-m long. A 3-m antenna therefore radiates much less power than an antenna of the correct length, and more transmitter power is required to achieve the same radiated power. Much of this additional power is wasted because excess power must be absorbed in the ATU.