Rhombic antenna

A rhombic antenna is a broadband directional wire antenna co-invented by Edmond Bruce and Harald Friis, in 1931, mostly commonly used in the high frequency (HF) or shortwave band.

It consists of one to three parallel wires suspended above the ground in a "rhombic" (diamond) shape, supported by poles or towers at each vertex to which the wires are attached by insulators. Each of the four sides are the same length, typically at least one wavelength (λ) or longer. A horizontal rhombic antenna (picture, top right) radiates horizontally polarised waves. Its principal advantages over other types of antenna are its simplicity, high forward gain and wide bandwidth, the ability to operate over a wide range of frequencies.

It is typically fed at one of the two acute (sharper angle) vertices through a balanced transmission line. Less commonly, it can be fed with coaxial cable through a balun transformer. The end of the wires meeting at the opposite vertex is either left open (unconnected), or terminated with a non-inductive resistor. When resistor-terminated, the radiation pattern is unidirectional, with the main lobe off the terminated end, so this end of the antenna is oriented toward the target country or geographical region. When unterminated the rhombic is bidirectional, with two opposite lobes off the two acute ends, but is not perfectly bi-directional. This is because of energy losses caused by radiation, conductor resistance, and coupling to the lossy soil below the antenna.

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