Traveling-wave Antenna for Vertically Polarized Transmission

Author: Edmund A. Laport

Figure 3.96 shows the construction of an inverted-V antenna. This is essentially one-half of a rhombic antenna split along its major axis and then turned so that its plane is vertical. In this form, the main lobe of radiation is vertically polarized. The system is structurally simple and uses only one supporting pole.

The angle of the wires with respect to ground is a function of the length of the legs. The optimum slope angle is tabulated in Table 3.5. These angles are not the same as one-half of the acute angle of the equivalent horizontal rhombic antenna because, in the latter, the acute angle is adjusted to bring the intersection of the two cones of first maximums a few degrees above the plane of the rhombus, while in this case the angle is that which will maximize the pattern in the plane of the antenna. With this exception, we can say that the vertical pattern for the inverted-V antenna is the same as that of a free-space rhombus in the plane of the rhombus. In the same sense the horizontal-plane pattern for the inverted-V antenna is the same as that in the major axial plane normal to a free-space rhombus.

 FIG. .3.96. Inverted-V traveling-wave antenna.

Circuitally, the inverted-V antenna is unbalanced, the ground forming one side of its input and output circuits. It is desirable to use a system of ground wires at the input end and also at the far end where the antenna is connected to a dissipation line for transmitting or to a resistor for receiving. Its characteristic impedance is one-half that of the equivalent balanced rhombic antenna.

 TABLE 3.5. OPTIMUM SLOPE ANGLES FOR INVERTED-V ANTENNAS Leg Length Slope Angle Wavelengths Degrees 2 36 3 29 4 24.5 5 22 6 20.5 7 19.5

To feed the inverted V, it is usually preferred to use a balanced feeder, similar to those which would be employed for other balanced antennas, and to make a balanced to unbalanced transformation with the proper impedance ratio to excite the antenna.

At the terminal end, the dissipation line can be of the unbalanced type, using the ground itself as the dissipator. If the attenuation per unit length is small, owing to high ground conductivity or low operating frequency, the required length of the line may be inconveniently large. In such a case, dissipative conductors will increase the attenuation rate by adding conductor loss to the ground loss.

Last Update: 2011-03-19