2.1.2 Slot antennas for circularly polarized

Single-feed MPAs for CP are usually achieved by using traditional simple changes in the shape of the patch such as truncating corners, using nearly square or nearly circular patches, cutting a diagonal slot in the square or circular patches, protruding or inserting a pair of symmetric perturbation elements at the boundary of a circular patch [33–35]. However, this type produced narrow axial ratio (AR) bandwidth. The dual-fed and sequential array structure produced wider AR bandwidth, but this requires more complicated design and may occupy larger space. Slot antennas are expected to overcome the limited bandwidth problem as well as similar changes in the slot shape could be used to achieve CP.

Rectangular notches etched in the rectangular slot are used as a way to improve the 3 dB AR bandwidth as shown in Figure 5(a). The notch width C1 = C2 = 0.7 mm gives the best wideband AR. The comparison of AR between measured and simulated is also shown in Figure 5(a).

In the proposed design, a square stub with dimension side K was added to the circular shaped slot to achieve the CP performance. The stub is added at a radial distance t from square stub. The locations of this stub set the AR bandwidth without degrading the antenna performance as shown in Figure 5(b). This figure shows that the stub with notch at a distance t = 3 mm gives the best performance AR

Figure 4. Antenna |S11| response with different slot shapes (a) rectangular, (b) circular, and (c) triangular [27].

Figure 5. Axial ratio of the antennas with different slot shapes (a) rectangular, (b) circular, and (c) triangular [27].

bandwidth. However, AR bandwidth discontinuities appear from 40 to 55 GHz. The comparison results of AR values simulated and measured at t = 3 mm is shown in Figure 5(b). Finally, for the triangular slot shape, a truncated corner was used. To improve the AR bandwidth, an L-shaped strip was added at the other triangular corner with width 0.1 mm as shown in Figure 3. The bandwidth for simulated and measured AR values for the triangular CP antenna is shown in Figure 5(c). From previous shapes, it appears that rectangular shaped slot with notches gives wide axial ratio bandwidth without degrading the antenna bandwidth.
