The commonest practical form of high-frequency antenna using the traveling-wave principle is the horizontal rhombic antenna, constructed as shown in the figures below. It is widely used in high-frequency applications for both transmitting and receiving. It has some marked advantages and disadvantages.
The advantages include simple construction, low-cost supporting structures, low cost of material, relatively high gain for the cost, broad frequency response from the impedance standpoint, minimum antenna potentials and currents for the power transmitted, inconspicuousness, easy maintenance and repair, almost no field adjusting required after installation, and the ease with which the height can be changed to obtain the optimum vertical angle as layer heights change through the sunspot cycle.
The disadvantages include large amount of land required; loss of power in the terminating load; a multiplicity of lobes of radiation in almost all directions, in addition to some rather large secondary lobes under the best conditions of design; compromises necessary from a propagation standpoint as the radiation pattern changes with frequency; limitation of gain and signal-to-noise ratio due to the multiplicity of radiation lobes; difficulty of predetermining its complete performance due to the complications of computation and to the effects of attenuation and partial standing waves always present in practice; and inability to control the horizontal and vertical patterns separately.
For optimum performance, a rhombic antenna should be designed for use at one frequency or a very small band of frequencies, the pattern for which is best suited to the propagation conditions of the space circuit.
Horizontal rhombic antenna (common three-wire form)
Horizontal rhombic antenna with feeders arranged for reversing pattern