The configuration of the initial planar printed dipole investigated in this work is shown in Figure 1. It is composed of a rectangular printed dipole, a coaxial feeder connected to a SMA connector and a PEC ground. The planar rectangular dipole, with a dimension of d × t, is printed on the top surface of a FR4/Epoxy substrate, which is suspended on a PEC ground plane and supported by Nylon poles. The substrate has a height of h1, a dielectric constant of 4.4 and a thickness of 0.8 mm , as shown in Figure 1(a). The substrate and the ground have the dimensions of L1 × W1 and L × W respectively. The dipole is fed by a coaxial balun. Figure 1(b) shows the feeding mechanism. Two coaxial lines are placed between the substrate and the ground. The outer copper shields of both coaxial lines are connected to the PEC ground, as well as the dipole through two metal poles. One of inner copper cores is connected to a 50 W SMA connector underneath the ground and a small metallic strip, printed on the top surface of the substrate. Another copper core is an open stub and connected to the other end of the strip and has a length of h2 + ht. Compared to the conventional printed dipole antenna, the two copper cores and the small strip on the top surface of the substrate are served as the balun and the copper shields are the corresponding ground connected to the printed dipole. The antenna has a planar structure and an additional ground to increase the directivity above the ground.

The design of this planar printed dipole antenna with a coaxial feeding is similar to that of a conventional dipole antenna. A l/4 short-circuited conductor is needed to connect one of the l/4 printed strips. The outer conductors or the copper shields of the coaxial line are employed as the l/4 short-circuited conductor. Therefore, the length of the coaxial line, h1, should be about l/4 of the operating frequency. The length of the open copper core, h2, should be about l_g/4 (l_g is the guided-wavelength in coaxial line) in order for the balun to feed the planar dipole antenna. The dipole length, d, should be about l₁/4 (l₁ is the wavelength on the FR4 substrate). The antenna is designed for the wireless base station antennas operating at about 2 GHz. Based on the design strategy above, the dimensions of the planar dipole antenna are optimized and listed in Table 1. In the design, the parameter h1 and h2 are two important parameters to tune the impedance matching of the antenna. The parameter h1 is normally chosen to be as small as possible, to keep a low profile. The length of the open copper core, h2, can be adjusted for good impedance matching. The simulated VSWRs for h1 = 50 mm and different h2 values, obtained from commercial software Anoft HFSS, are plotted in Figure 2.

As can be seen, adjusting the length of the open copper core can minor tune the operating band and the impedance matching. The simulated realized gain for h2 = 16 mm , also plotted in Figure 2, is maintained about 6.7 dBi over the whole operating band, which is determined by VSWR < 2, ranging from 1.75 GHz to 2.32 GHz, about 28%.

Further study on the planar printed dipole antenna with a coaxial feed was carried out and it is found that the bandwidth can be improved by changing the shape of the printed dipole, without compromising the radiation performance. One of the examples is the planar printed dipole with a petal shape. Figure 3 shows the geometry of the petal dipole. It can be seen that the antenna geometry is similar to that in Figure 1, except for the shape of the dipole. The parameters of the petal dipole are optimized and the final dimensions are listed in Table 2. The simulated VSWR and realized gain are plotted in Figure 4. The bandwidth for VSWR < 2 is 35%, from 1.74 GHz to 2.48 GHz. Compared to that of the dipole in Figure 1, which is 28%, the bandwidth is improved by 7%. The realized gain is also improved a little, though it slightly decreases with frequency.

Dual-polarized dipoles are normally used to develop base station antennas, to reduce the effect of multi-path fading and increase channel capacity in mobile communication systems. Two planar printed petal dipoles are employed in this section to develop a dual-polarized antenna. Figures 5(a)-(c) show the top view, the 3D view and the side view of the configuration of the concerned planar printed antenna, fed by coaxial lines, respectively. Two planar petal dipoles are cross-printed on a substrate and two pairs of coaxial lines are used to feed them. The two petal dipoles have the same dimension but different feeding structures. The configuration of one of the two petal dipole antennas and its feeding mechanism is the same as that shown in Figure 3. Another crossed petal dipole is on the same surface but the coaxial feeding is a little different. As shown in Figures 5(a)-(b), two small metallic strips are cross-printed on the top and the bottom surfaces of the substrate respectively. The copper cores of the two coaxial lines, used to feed the second dipole, are connected to the two ends of the strip on the bottom surface of the substrate and the outer copper shields are connected to the petal dipole through two

metallic poles, as shown in Figure 5(c). Thus, the two dipoles can be cross-arranged and fed by two pairs of coaxial lines, exhibiting dual-polarization performance.

In the design, FR4/Epoxy substrate with the permittivity of 4.4 and a thickness of 1.5 mm is employed. The height (h1) of the substrate, the lengths of the two open copper cores and the dimensions of the dipoles determine the impedance matching and its operating bandwidth. The target of the design is the wide bandwidth and the low profile of the antenna. Based on the simulation and optimization from HFSS, it is found that the antenna exhibits a better bandwidth and a lower profile, compared to the two single-polarized cases shown in Figure 1 and Figure 3 respectively. The reason might be the strong mutual coupling between the two dipoles. The final optimized dimensions of the dual-polarized design are listed in Table 3. It can be seen that the height (h1) of the substrate is only 32 mm , lower than those in the designs shown in Figure 1 and Figure 3. The VSWRs of the two ports of the dual-polarized planar printed dipole antenna, calculated using Ansoft HFSS, are plotted in Figure 6. One can see that wide operating bands at the two ports, determined by VSWR < 2 and ranging from 1.54 GHz to 2.78 GHz and 1.6 GHz to 2.8 GHz respectively, are obtained. The bandwidths at the two ports are 57.4% and 54.5% respectively and both cover the Chinese TD-SCDMA band (1880 - 2400 MHz). Compared to those of the single-polarized designs above, which are 28% and 35% respectively, the common bandwidth, which is about 54%, is significantly improved, due to the mutual coupling between the two dipoles.

The fabricated prototype for the designed dual-polarized printed dipole was measured using Vector Network Analyzer Agilent E 8363C and the measured results are also plotted in Figure 6, for comparison. The measured common bandwidth for the two ports of the antenna are 42%, ranging from 1.76 GHz to 2.68 GHz. Figure 7 shows the simulated and measured isolations between the two ports, which are 30 dB and 25 dB over the whole operating bandwidth, respectively. The difference between simulations and measurements might come from the difficulty when tuning open copper cores and other unpredicted factors during the fabrication. The realized gain, calculated using Ansoft HFSS and shown in Figure 8, is maintained about 7 dBi over the bandwidth, for both ports. The antenna prototype is also measured and the measured gains for both ports are almost same. Therefore, only one measured gain curve is given in Figure 8. Figure 9 shows the prototype pictures of the proposed dual-polarization planar antenna. The radiation patterns are also measured and plotted in Figure 10. It can be seen that stable radiation patterns at 1.8 GHz, 2.2 GHz and 2.4 GHz are obtained.