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 affect 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 polarisation mismatch. A simple way to ensure that the polarisation mismatch is not more than 3dB is to use a circularly polarised antenna at one end (e.g. transmitter) and a linearly polarised antenna at the other (e.g. receiver). Since it is difficult to achieve circular polarisation, elliptical polarisation with axial ratio close to unity is used. A four-element crossed dipole array with a reflecting metallic plate is modelled 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 polarisation is achieved by driving the two dipoles with a phase difference of 90°. The top antenna in Figure 1 leads by 90°, which means a right-hand-circular polarisation 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 is constructed, consisting of four crossed-dipoles with a spacing of λ/2. The array is shown in Figure 2. Similarly aligned elements are driven with equal phase (the bottom antennas with a phase of 0° and the top antennas with 90°). Again right-hand-circular polarisation is expected.

Figure 2: A four element crossed dipole array

The gain is increased further by placing a metal plate at a distance of λ/4 behind the crossed dipoles. Figure 3 shows the model of the array with the 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, about 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-plane.

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 it increases to 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 reflection coefficient for element #1 in the array (and the bottom element in the single crossed dipole). Although coupling between elements affect the input impedance, the element is well matched to a 75Ω system.

Figure 6: Input impedance for element #1 in array