The Rhombic Antenna

Figure 31. The free-space radiation pattern for a lossless wire several wavelengths long, and terminated so that there are no standing waves, is as shown. The angle θ at which radiation is maximum is as given by Fig. 32.

The radiation pattern from a long wire in free space is⁶

where E is the field strength in volts per meter, I₀ is current input to the wire in amperes, d is the distance in meters, θ is the angle with respect to the axis of the wire, L is the wire length in wavelengths, and p = (2π/λ)(l - cos θ). Neglecting the minor lobes, Fig. 31 gives the free-space radiation pattern. The angle θ can be found

Figure 32. For a lossless wire several wavelengths long, and in free space, maximum radiation occurs at the angle θ = (90° - φ).

from Fig. 32, where θ = (90 - φ). This curve shows that the angle of maximum radiation is almost independent of the length of the wire for wire lengths of over several wavelengths, a very important point as will be explained later. Before continuing the discussion of the rhombic antenna, two other antennas should be considered.

Figure 33. The two wires of the tilted-wire antenna are held above the earth by a vertical pole at the center. For the proper height, the two lobes add to produce a strong signal in the direction of transmission. The actual radiation pattern would be affected considerably by reflection from the earth. The terminating resistor prevents wave reflection, giving a non-resonant antenna.

A tilted-wire antenna can be arranged as in Fig. 33. This simple structure is useful at the upper end of the high-frequency band. By the proper selection of wire length and angle φ, the antenna can be designed³⁷ so that two of the lobes combine for directional transmission and so that the other two largely cancel.

A non-resonant Vee antenna is arranged as shown in Fig. 34. The two wires are horizontal above the earth. For the proper wire length and angles, a highly directional antenna is obtained. The wires are several wavelengths long and terminated so that no reflection occurs.

Figure 34. The non-resonant Vee antenna is composed of two wires several wavelengths long, and connected to ground through terminating resistors. If the wires are arranged at the proper angles, the signals radiated, as indicated by the lobes, add, giving a strong signal in the desired direction. The plane of the two wires is parallel to the surface of the earth. The actual radiation pattern is affected by the earth.

Figure 35. The non-resonant rhombic antenna is composed of four wires each several wavelengths long and arranged as indicated. A terminating resistor R is used to prevent reflection. The radiation from each wire is as shown by the lobes and the individual radiations combine to give the directional patterns of Fig. 36. This is the top view of the structure. It is supported in a plane parallel to the surface of the earth.

Figure 36. As shown in the upper illustration, the horizontal rhombic antenna having the specifications noted is most sensitive in reception (and also in transmission) to a wave or ray coming at an angle of about 10° with the horizontal. As the lower illustration indicates, the antenna is highly directional in the horizontal plane. (Reference 54.)

Horizontal rhombic antennas are shown in Fig. 35, and also in Fig. 20 of Chapter 13. This is one of the most satisfactory of- all types of directional antennas. The relative radiations in vertical and horizontal planes are shown in Fig. 36 for a typical rhombic antenna composed of wires each 4λ in length, held horizontally one wavelength above the earth, and with φ = 65° (see Fig. 32). The radiation of each wire, as indicated by the lobes in Fig. 35, combine to give the patterns of Fig. 36.

The horizontal rhombic antenna requires no connection to ground, is non-resonant, and will operate well over a considerable band of frequencies because the angle φ of Fig. 32 varies but little with wavelength. This antenna is low in first cost, easy to maintain, and can be designed to meet a variety of operating conditions. It is important to note that it sends and receives best at some vertical angle with the surface of the earth. This angle can be varied by proper design. This adapts the horizontal rhombic antenna to sky-wave transmission and reception at high frequencies.