The annular-ring patch antennas with a PBG ground plane and a slotted ground plane were constructed and studied. A reference antenna with a regular ground plane (no embedded slots) was also constructed. Measured return loss of the constructed antennas is shown in Fig. 2. The corresponding measured data for the antennas with a regular ground plane, a slotted ground plane, and a PBG ground plane are also listed in Table I for comparison. The annular-ring patch selected is with R1 = 55 mm and R2
= 23.2 mm, and the ground-plane size is 150 × 150 mm2 (G ×G). The thickness of the air substrate is 3.2 mm. For all the antennas studied, the TM21 mode is excited to obtain broadside-null or conical-pattern radiation. The PBG period is determined by Eq. (1) to be 62.5 mm to generate a stopband centered at about 2.4 GHz. To obtain the value of r/S to be 0.25, the 3 × 3 circular slots with a radius of 15.6 mm are embedded in the antenna’s ground plane to form a PBG structure. On the other hand, for the slotted ground plane, four circular slots with the same slot radius of 15.6 mm and the adjacent spacing of 62.5 mm as the PBG structure are embedded. From the obtained results, it is clearly seen that the resonant frequency of the antenna with a slotted
ground plane was greatly lowered, similar as that observed in [1], by about 20%, from 2380 to 1920 MHz. This resonant frequency lowering suggests that a 36% antenna size reduction can be achieved for the antenna with a slotted ground plane, compared to the antenna with a regular ground plane.
Conversely, the resonant frequency of the antenna with a PBG ground plane was slightly increased, from 2380 to 2460 MHz. This characteristic is probably due to the decreased effective pemittivity of the substrate caused by the embedded periodic circular slots. It is also observed that the impedance bandwidths, determined from 10-dB return loss, for both antennas with the slotted and PBG ground planes are larger than the antenna with a regular ground plane. This increased impedance bandwidth is largely due to the embedded circular slots in the ground plane, which causes a lowered quality factor for the antenna.
Figure 3 presents the measured radiation patterns in two principal planes. For the three antennas studied, good conical patterns are observed. The measured antenna gain is presented in Fig. 4. Among the three antennas studied, the antenna with a slotted ground plane has the largest peak antenna gain. However, the gain variations within the impedance bandwidth are also larger. As for the antenna with a PBG ground plane, the peak antenna gain is also slightly increased, compared to the reference antenna (the case with a regular ground plane), and the gain variations (less than 0.5 dBi) are smaller than those of the antenna with a slotted ground plane.
It is also noted that the backward radiation of the antenna with a slotted ground plane is greatly increased. From the experiments, the backward radiation is increased by about 7 dB, compared to the reference antenna. This behavior is partly owing to the embedded slots in the ground plane and partly because the electrical size of the ground plane is decreased with the decreasing of the antenna’s resonnat frequency. On the other hand, it seems that the PBG ground plane can provide as a metallized surface, and a F/B ratio of about 9 dB larger than that of the antenna with a slotted ground plane (12.1 vs. 3.2 dB) is obtained. The obtained F/B ratio for the antenna with a PBG ground plane is even larger than that of the antenna with a regular ground
plane.
Table I: Performances of the antennas with regular, slotted, and PBG ground planes;
antenna parameters are given in Fig. 2. fr is the resonant frequency of the TM21 mode.
F/B ratio denotes the front-to-back maximum radiation intensity ratio.
dp
Regular ground 28.5 2380 76, 3.2 7.9 10.2
Slotted ground 36 1920 100, 5.2 8.5 3.2
PBG ground 30 2460 95, 3.9 8.1 12.1
4. CONCLUSIONS
We have presented an experimental study of the characteristics of an air-substrate annular-ring patch antenna with a slotted and a PBG ground plane. From the experimental results, it is observed that the antenna with a slotted ground plane can have the advantages of size reduction and bandwidth enhancement. However, decreased F/B ratio is also obtained. Conversely, although no size reduction is obtained for the antenna with a PBG ground plane, bandwidth enhancement is also observed. In addition, small gain variations within the obtained impedance bandwidth and improved F/B ratio are seen for the antenna with a PBG ground plane.
REFERENCES
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Figure Captions Figure
1. (a) Geometry of an air-substrate annular-ring patch antenna. (b) The PBG ground plane. (c) The slotted ground plane.
2. Measured return loss against frequency for the antennas with a regular ground plane, a slotted ground plane, and a PBG ground plane; R1 =55 mm, R2 = 23.2 mm, G = 150 mm, S = 62.5 mm, r = 15.6 mm, h = 3.2 mm.
3. Measured radiation patterns in two principal planes. (a) Antenna with a regular ground plane at 2380 MHz. (b) Antenna with a slotted ground plane at 1920 MHz.
(c) Antenna with a PBG ground plane at 2460 MHz.
4. Measured antenna gain against frequency. (a) Antenna with a regular ground plane. (b) Antenna with a slotted ground plane. (c) Antenna with a PBG ground plane.