In order to resonate at a particular frequency, an antenna needs to be at least one half wave length long. At 30 MHz the length required is 5 metres or 16.4 feet, but at 2 MHz it is as much as 75 metres or 246 feet, and at LF/MF frequencies, the required lengths are very long indeed.
Although the 1/2 wave antenna does not need an earth system to be able to resonate per se, it is very necessary for radiation efficiency. Antennas below a 1/2 wave are dependent on a ground system for resonance as well. Therefore, to keep the resistance of the earth or earth losses to a minimum it is essential to provide a good ground system for all vertical antennas less than a wavelength in length.
Where antennas are physically short (especially at MF/LF frequencies) and the percentage of power radiated is therefore low, the quality of the ground system will be a major factor in determining quality and range of communications.
When an antenna radiates, a circulating current is created with current flowing between the antenna and the earth. Losses occur when the radiation is absorbed by the earth rather than returned.
As not all types of terrain are able to conduct electricity equally, how much current is returned to the radiator depends on the actual degree of conductivity of the ground. The use of a good ground system ensures that these losses are kept to a minimum so that the maximum possible power is available for radiation.
At sea, the required antenna lengths for HF communciations are frequently impractical. If a ground plane can be used, however, the required length can be shorted to about half. This is known as the 1/4 wave grounded mode. The antenna is made 1/4 wavelength long and the ground plane acts like a mirror reflecting the quarter wave above to make a total of 1/2 wavelength.
In the case of a ship at sea, the sea provides an ideal ground plane. In fact, grounded vertical antennas at sea have the best possible earth system. With metal vessels the ship itself provides the connection with the sea, whereas with wooden and fibreglass vessels, a separate earthing plate needs to be provided - the better the connection with the sea, the better the performance.
In making this connection, it is not the actual size of the plate that counts, but rather the total surface area of the plate. The plate must be installed in a location where it is below the water line at all times for the connection to be made. This is especially relevant where speed boats are concerned. And naturally it needs to be able to withstand being immersed in salt water year after year.
Earth plates are normally constructed from copper alloy. The Moonraker E Plate uses 94% low corrosion copper alloy, suitable for all waters including tropical. However, where vessels have aluminium outboard legs, and where bronze fittings may not be used, copper is not suitable, as electrolysis will occur over time. The Moonraker aluminium E Plate overcomes this problem. For similar reasons it is always a good idea to keep the earth leads separate from other electrical wiring.
For land based communications, on vehicles it is the metal chassis with capacity coupling to ground, and in the open or on buildings, in the absence of a metal roof of sufficient size, wire ground planes are used. Siting a marine base station close to the sea will greatly enhance performance, as will connection with the water table where it is very high.
While it is a good idea to have as many radials as possible in a ground system, each installation will have an optimum number directly related to the antenna length, working frequency and type of ground. Above this number, the increase in performance is minimal in relation to the cost of the additional radials.
At Moonraker we are always happy to design individual ground systems, especially in the case of MF/LF communications where it is most important.