Interference rejection capabilities of different types of array antennas in cellular systems - Electronics Letters

Experimental results and conclusions: Several designs with the feed position at point A for left-hand CP operation were constructed and experimentally studied. Fig. 2 shows the measured return loss against frequency for the cases with e = 0, 8, 10.5 and 1 1.5 mm; the case with e = 0 represents a corresponding regular circularly polarised microstrip antenna with an unslotted ground plane. The corresponding measured data are listed in Table 1 for comparison. The measured results of the axial ratio in broadside direction (0 = Oo) are shown in Fig. 3. From the results obtained, it is clearly seen that the resonant frequency is decreased with increasing slot length. For the case of e = 1 1.5 mm, the centre frequency (fc), defined as the frequency with minimum axial ratio, is only about 74% of that for a corresponding regular CP microstrip antenna (2190 against 2970 MHz). This corresponds to an antenna size reduction of about 45% for the proposed antenna at a fixed operating frequency. It is also noted that the impedance bandwidth and the CP bandwidth, determined from 3 dB axial ratio, of the proposed antenna are increased with the lowering of the centre frequency, which is different from that observed for the compact designs with a slotted radiating patch [l-41. This characteristic suggests that the embedded slots in the ground plane are more effective than the slots in the radiating patch in lowering the quality factor of the microstrip antenna, and widened CP bandwidth is thus obtained for the proposed design.