Rhombic Antenna

a type of traveling-wave antenna. The conductors forming the antenna are arranged along the sides of a rhombus. This design was suggested by the American engineer Bruce in 1931. Rhombic antennas are used as transmitting or receiving antennas, predominantly for decameter-wave (shortwave) radio communications and broadcasting.

The antenna is usually suspended in a horizontal position at a height of 15-40 m (Figure 1,a; see bottom of page 154). The length l of each side of the rhombus is 50-160 m. A radio transmitter or receiver is connected to one of the acute angles of the rhombus. A resistor is connected to the other acute angle, which is directed toward the receiver or transmitter with which communication is carried on. The impedance of the resistor is close to the characteristic impedance of the antenna (600-700 ohms). For this reason, the antenna operates as a traveling-wave antenna. Maximum intensity of radiation or reception is observed at some angle to the horizon (angle of elevation). In rhombic antennas designed for very long communication links (2,500-3,000 km or more), the acute angle of the rhombus is between 30° and 50°, and the angle of elevation of the direction of maximum radiation or reception may vary within the operating wavelength range from 5° to 20°. The minimum wavelength λ_min of the operating range is usually chosen to be ~(0.1–0.18)l; the maximum wavelength λ_max ≈ (2-4)λ_min. The front-to-rear factor of a rhombic antenna can reach 160-200. The gain (relative to a half-wave dipole in free space) is 25-80.

The important disadvantages of a rhombic antenna are that the resistor absorbs a substantial part (20-40 percent) of the power that the side lobes of the radiation pattern enters the antenna and that represent a large part (up to 50 percent) of the radiated power. These power losses can be reduced if a double rhombic antenna is used, as proposed by the Soviet scientist G. Z. Aisenberg (Figure 1,b). Such a double antenna consists of two rhombuses that are displaced with respect to each other by (0.17–0.25)l in the horizontal plane. The sides of the rhombus each consist of two or more parallel conductors. The gain of a double rhombic antenna is 1.5-2 times greater than the gain of a single rhombic antenna.