Crossed Dipole Array in Front of Reflector

In mobile communication systems the position and orientation of the receiving and transmitting antennas change continually. This can effect the signal strength at the receiver, even when the antennas are pointing at each other, as the orientation of the antennas may result in a polarization mismatch. A simple way to ensure that the polarization mismatch is not more than 3dB is to use a circularly polarized antenna at one end (e.g. transmitter) and a linearly polarized antenna at the other (e.g. receiver). However, achieving circular polarization is difficult and practically an elliptical polarization with an axial ratio close to unity is used. Here a four element crossed dipole array with reflecting metallic plate is modeled in FEKO to determine gain patterns and axial ratios.

Figure 1 shows the FEKO model of a crossed dipole antenna designed for 300MHz. The dipoles are normally orientated with respect to each other. Circular polarization is achieved by driving the two dipoles with a phase difference of 90°. In Figure 1 the top antenna is shown to lead by 90°, which means a right-hand-circular polarization is expected (along the positive z-axis). The spacing between the dipoles is in the order of λ/50.

Figure 1: FEKO model of a crossed dipole antenna

To increase the gain along the z-axis a linear array consisting of four crossed-dipoles with a spacing of λ/2 is constructed. The array is shown in Figure 2. Note the numbering of the elements. Odd numbered elements are driven with equal phase (0°) even numbered elements are driven with equal phase (90°). Again right-hand-circular polarization is expected.

Figure 2: A four element crossed dipole array

Finally the gain is increased further by placing a metal plate λ/4 behind the crossed dipoles. Figure 3 shows the model of the array with a reflector. The 3D gain pattern of the antenna is also shown.

Figure 3: Crossed dipole array with metallic reflector

The gain patterns of the crossed dipole, four element array and array with reflector are compared in Figure 4. Figure 4a shows the total gain in the xz-plane (phi = 0) and Figure 4b shows the total gain in the yz-plane (phi = 90). A forward gain of about 2dB is achieved with the crossed dipole, 8dB with the four element array and about 12.5dB with the metallic plate placed behind the array. Side-lobes in the xz-plane (for the array and the array with reflector) are more than 10dB below the main lobe; the back lobe (for the array with reflector) is almost 20dB below the main lobe. Half-power beamwidth (for the array with reflector) in the xz-plane is around 25° in the xz-plane and 70° in the yz-lane.

Figure 4: Comparison of gain patterns
(a) xz-plane (phi = 0) (b) yz-plane (phi = 90)

Axial ratios for the three antennas are compared in Figure 5. An axial ratio of nearly 0.9 along the positive z-axis is obtained with the single crossed dipole. Using the four element array degrades this axial ratio to 0.5 and with a reflector increases it to nearly 0.6. A sudden change in the axial ratio occurs at theta = 30 (phi = 0) which corresponds to the first null in the patterns for the arrays.

Figure 5: Comparison of axial ratios

Finally, Figure 6 shows the input impedance for element #1 (numbering as shown in Figure 2) in the array (and the bottom element in the single crossed dipole). Although coupling between elements affect the input impedance the element is nearly matched to a 75Ω system.

Figure 6: Input impedance for element #1 in array