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A rhombic antenna is a type of antenna that covers a wider range of frequencies than most antennae. It is usually used for high frequencies, which are also known as shortwave. The most common use of a rhombic antenna is in amateur radio relay and broadcasting.
The rhombic antenna is a broadband directional antenna. The broadband has nothing to do with internet access, but rather refers to the fact that it can work with an unusually wide range of frequencies. Directional means that the antenna receives signals best from a specific direction, meaning it should be pointed toward the source with which it is communicating.
The name of the rhombic antenna comes from the fact that it consists of four conductors, in the form of wires, arranged in a diamond shape. To work to full effect, these wires have to be symmetrical. This means all four should be the same length, and the angles at which the pair of wires on either side of the diamond must be the same. The reason this shape works so well is that the two wires at the side that connects to the rest of the equipment acts in a similar fashion to a funnel.
Although the rhombic antenna must be pointed in a specific direction, its wide range means that it is easy to combine multiple antennae effectively. One example of this is to combine six antennae so they face in different directions across all three dimensions. In effect, the antennae are pointed in the same way as the six faces of a die, although it's not possible to literally set them up to form a cube. That's partly because of the potential for interference, and partly for the simple fact that rhombus shapes can't form a cube. With this set up of six antenna, depending on the user's location, it may be possible to receive signals from the majority of populated areas on the planet.
The main advantages of the rhombic antenna include its cheapness. This is because only four poles are needed, one to support the wires at the points where they meet. Another benefit is that the broad range of frequencies means there is more room for error in setting up and adjusting the antenna.
The main disadvantage of the rhombic antenna is that it requires a comparatively large area on which to be erected. It is also subject to earth losses below the antenna, which can significantly reduce efficiency.