Broadband low cross-polarization patch antenna

Authors


Abstract

[1] A broadband 180° microstrip balun is employed as a feed network for the L-probe square patch antenna. The broadband balun delivers impedance matching, equal amplitude power division, and consistent 180° (±10°) phase shifting over a wide band (∼45%). We demonstrate that the use of the 180° broadband balun affords significant H-plane cross-polarization suppression across the wide impedance bandwidth designed for L-probe patch antennas. The antenna using the 180° broadband balun exhibits a measured impedance bandwidth (SWR < 2) of 37.15%, from 1.6 to 2.33 GHz. Across our bandwidth of interest (1.7 to 2.3 GHz), the measured boresight gain ranges from 6.16 to 8.5 dBi, and the measured H-plane cross-polarization levels are consistently well suppressed (<−20 dB).

1. Introduction

[2] Patch antennas are very popular because they possess many advantageous features such as being low profile and light weight, easy to fabricate, conformable to mounting structures, and compatible with integrated circuit technology. However, patch antennas are inherently constrained by their narrow impedance bandwidth, especially when the radiating elements are printed on thin dielectric substrates. An established method for overcoming this limitation is to use thick low permittivity dielectric substrates that allow for loosely bound electromagnetic fields. A probe feed, which can couple with a radiating patch positioned above the antenna substrate, is commonly used in this bandwidth-widening approach [Chang et al., 1986]. However, the probe inductance limits the impedance bandwidth to less than 10% [Chang et al., 1986]. This probe inductance can be compensated in several ways [Hall, 1987; Huynh and Lee, 1995; Luk et al., 1998]. The L-probe proximity-feed technique [Luk et al., 1998] compensates for probe inductance and extends the achievable impedance bandwidth for probe-fed patch antennas on thick (∼0.1 λo) low-permittivity dielectric substrates. Typically, this design yields 30% impedance bandwidth (SWR ≤ 2) and an average gain of 7.0 dBi. Extensive investigations have been dedicated to this category of wideband patch antennas [Mak et al., 2000; Lo et al., 2000; Guo et al., 2001, 2002; Wong et al., 2004]. Wideband dual or circular polarization patch antennas can be easily achieved using this L-probe feed approach [Guo et al., 2002; Wong et al., 2004]. The L-probe fed patch antenna is simple in structure and has been used in base stations for various mobile phone systems and other wireless communication systems.

[3] On the other hand, the probe leakage radiation, which has serious implications on the radiating performances, remains an important issue for L-probe proximity-fed patch antennas. The L-probe feed, though effective in widening the impedance bandwidth, emits probe leakage radiation that causes high cross-polarization levels, especially in the H-plane. A second L-probe feed, provided with an equal amplitude and 180° out-of-phase excitation, can be added to reduce any unwanted probe radiation [Mak et al., 2000; Wong et al., 2004]. Cross-polarization is suppressed when the probe leakage radiation from the added L-probe feed cancels out the probe leakage radiation from the original L-probe feed. Much work has been done to address similar problems [Petosa et al., 1999; Levis et al., 2000; Chen and Chia, 2003]. In prior arts [Petosa et al., 1999; Levis et al., 2000; Mak et al., 2000; Wong et al., 2004], balanced feed networks were used to reduce the cross-polarization. However, the conventional balanced feed networks used in the above cases only provide a consistent 180° (±10°) phase shift over a very narrow band (∼10%), thus limiting the frequency range across which proper cancellation of probe leakage radiation can take place. For probe-fed patch antennas, the wideband suppression of cross-polarization has been a very challenging problem critical in base station applications.

[4] In this paper, we propose the use of a novel 180° broadband microstrip balun [Zhang et al., 2005] as a feed network for the dual L-probe square patch antenna. The broadband balun delivers impedance matching, equal amplitude power division, and consistent 180° (±10°) phase shifting over a wide band (∼45%). We demonstrate that the use of the proposed 180° broadband balun in place of the conventional 180° narrowband balun affords significantly improved H-plane cross-polarization suppression across a wide impedance passband (∼30%) [Khoo et al., 2005]. As an important application example, we present a wideband patch antenna targeted for emerging broadband mobile base station applications covering three bands, i.e. DCS1800 (1710–1880 MHz), PCS1900 (1850–1990 MHz) and UMTS2000 (1920–2170 MHz).

2. Feed Network Configuration

2.1. Conventional 180° Narrowband Balun

[5] The conventional 180° narrowband microstrip balun, as shown in Figure 1, is used commonly in antenna designs as a phase shifting feed network. To provide a 180° phase shift, the lengths of the microstrip branches, d1 and d2, must be such that d1 − d2 = λg/2, where λg refers to the guided wavelength at a center operation frequency. Here, the characteristic impedances of the two microstrip branches d1 and d2 are given by Z2 = Z0 = 50 Ω.

Figure 1.

Schematics of the conventional 180° narrowband microstrip balun.

2.2. Proposed 180° Broadband Balun

[6] First introduced in [Zhang et al., 2005], the 180° broadband microstrip balun shown in Figure 2 delivers balanced power splitting and consistent 180° (±10°) phase shifting across a wide band. This specially designed feed network comprises of a 3-dB Wilkinson power divider for wideband balanced power splitting, cascaded with a broadband 180° phase shifter for wideband 180° phase shifting.

Figure 2.

Schematics of the proposed 180° broadband microstrip balun.

2.3. Simulated Results and Verification

[7] All simulations presented in this paper were performed using IE3D 12.0, a commercially available electromagnetic field solver based on the Method of Moment (MoM). The feed networks were modeled on a substrate of thickness 0.8 mm and dielectric constant 3.38. For convenient analysis, the input and output ports of the feed networks were all set to 50 Ω.

[8] Figure 3 shows the simulated and measured return loss comparison between the two baluns. The 180° broadband balun exhibits a wide measured impedance bandwidth (S11 < −10 dB) of 67.3%, from 1.39 to 2.8 GHz, while the 180° narrowband balun exhibits a relatively wider measured impedance bandwidth (S11 < −10 dB) of 150.15%, from 0.41 to 2.88 GHz. Figure 4 shows the simulated and measured output port amplitude response comparison between the two baluns. The 180° broadband balun exhibits balanced measured output ports power distribution (S21 = S31 = −3 dB (±1.0 dB)) over a wide band of 44.73%, from 1.51 to 2.38 GHz, while the 180° narrowband balun exhibits balanced measured output ports power distribution (S21 = S31 = −3 dB (±1.0 dB)) over a relatively wider band of 55.29%, from 1.23 to 2.17 GHz. Figure 5 shows the simulated and measured output ports phase difference comparison between the two baluns. The 180° broadband balun exhibits consistent measured 180° (±10°) output ports phase difference over a considerably wide band of 48.84%, from 1.47 to 2.42 GHz, while the 180° narrowband balun exhibits consistent measured 180° (±10°) output ports phase difference over a much narrower band of 11.43%, from 1.65 to 1.85 GHz. The simulated and measured results are in generally in good agreement. However, there exist a small frequency shift between simulation and measurement for the narrow band balun, as shown in Figure 5. This may be due to the slightly different reference plane adopted in the actual measurement.

Figure 3.

Simulated and measured return loss comparison between the 180° narrowband and broadband baluns.

Figure 4.

Output ports amplitude response comparison between the 180° narrowband and broadband baluns.

Figure 5.

Simulated and measured output port phase difference comparison between the 180° narrowband and broadband baluns.

[9] Combining the measured results in Figures 35, it is observed that the proposed 180° broadband balun delivered low input port return loss (S11 < −10 dB), balanced output ports power distribution (S21 = S31 = −3 dB (±1.0 dB)), and consistent 180° (±10°) output ports phase difference over a wide band of 44.73%, from 1.51 to 2.38 GHz; hence we term it a “broadband” balun. The conventional 180° narrowband balun delivered both low input port return loss and balanced output ports power distribution over a relatively wider band. However, its overall performance was inherently limited by its narrowband 180° phase shifting capability; hence we term it a “narrowband” balun.

3. Patch Antenna With a Broadband Balun

3.1. Antenna Structure

[10] The single L-probe rectangular patch antenna has been found to deliver a wide impedance bandwidth (SWR < 2) of over 30% [Luk et al., 1998]. However, the L-probe feed emits probe leakage radiation which results in high cross-polarization and pattern distortion, especially in the H-plane.

[11] The dual L-probe square patch antenna, as shown in Figure 6, is designed to suppress the H-plane cross-polarization. A second L-probe feed is symmetrically positioned at the opposite radiating edge (Wx) of the patch element. Probe leakage radiation can be cancelled out by providing the two L-probe feeds with equal amplitude and 180° out-of-phase excitations. Prior to this work, a dual L-probe patch antenna using a conventional 180° narrowband balun has been reported in [Wong et al., 2004]. The use of a feed network with wideband 180° phase shifting capabilities is necessary in order for the probe leakage radiation to cancel out across the wide impedance passband (∼30%) afforded by the L-probe fed patch antenna.

Figure 6.

Dual L-probe square patch antenna.

[12] We fabricated the dual L-probe single-element square patch antenna with the proposed 180° broadband balun. The antenna and feed network parameters were optimized for a wide impedance bandwidth centering 2.0 GHz. The square copper patch, of dimensions Wx = Wy = 53.5 mm (0.357 λ0), was positioned at a height above the dielectric substrate to create an air substrate of thickness H = 23.5 mm (0.157 λ0). The feed network and square copper ground plane of length G = 250 mm (1.5 λ0), were respectively printed on the top and bottom of the dielectric substrate. The L-probe feeds, of diameter 2R = 1 mm, with vertical length Lh = 12 mm and horizontal length Lv = 26.5 mm, were positioned S = 3 mm away from the edge of the patch, and respectively soldered at the output ports of the feed network. A 50 Ω SMA connector was soldered at the input port of the feed network. The feed substrate used was a Rogers RO4003 laminate of dielectric constant ɛr = 3.38 and thickness t = 0.8 mm.

3.2. Simulated and Measured Results

[13] The Agilent E8364B vector network analyzer and MiDAS far-field measurement software package were used in the impedance and radiation measurements. Figure 7 shows the simulated and measured SWR comparison for the dual L-probe square patch antenna using either baluns. The antenna with the broadband balun exhibits a wide measured impedance bandwidth (SWR < 2) of 37.15%, from 1.6 to 2.33 GHz, while the same antenna with the narrowband balun exhibits a slightly wider measured impedance bandwidth (SWR < 2) of 39.22%, from 1.64 to 2.44 GHz. The simulated and measured SWR results are in good agreement. From 1.7 to 2.3 GHz (30%), it is observed that the impedance matching is good (SWR < 2) for the antenna using either the narrowband or broadband balun. This common impedance passband, sufficient to cover the DCS 1800, PCS 1900 and UMTS 2000 bands, will be the designated bandwidth of interest when we compare the radiation performance of the antenna utilizing either baluns.

Figure 7.

Simulated and measured SWR comparison for the dual L-probe square patch antenna using either the 180° narrowband or broadband balun.

[14] Figure 8 shows the simulated and measured boresight gain for the dual L-probe square patch antenna using either baluns. Within the bandwidth of interest (1.7 to 2.3 GHz), the measured boresight gain of the antenna utilizing the broadband balun ranges from 6.16 to 8.5 dBi, while that of the same antenna utilizing the narrowband balun ranges from 6.49 to 8.46 dBi.

Figure 8.

Simulated and measured boresight gain comparison for the dual L-probe square patch antenna using either the 180° narrowband or broadband balun.

[15] Figure 9 shows the simulated radiation patterns for the dual L-probe square patch antenna using the 180° narrowband balun at the start frequency (1.7 GHz), center frequency (2.0 GHz), and end frequency (2.3 GHz) of our bandwidth of interest. Across this passband, the antenna using the 180° narrowband balun exhibits symmetrical E- and H-plane co-polarization patterns, and consistently low E-plane cross-polarization levels (<−32 dB). The H-plane cross-polarization levels are generally higher than that of the E-plane and can be seen to appreciate considerably at the end frequency point (up to −11 dB).

Figure 9.

Simulated radiation pattern for the dual L-probe square patch antenna utilizing the 180° narrowband balun.

[16] Figure 10 shows the simulated radiation patterns for the dual L-probe square patch antenna using the 180° broadband balun at the start frequency (1.7 GHz), center frequency (2.0 GHz), and end frequency (2.3 GHz) of our bandwidth of interest. Across this passband, the antenna using the 180° broadband balun exhibits symmetrical E- and H-plane co-polarization patterns and consistently low E- and H-plane cross-polarization levels (<−28 dB). From Figure 5, it is observed that at 1.7, 2.0, and 2.3 GHz, the simulated phase shift for the narrowband balun, are respectively 210°, 177°, and 215°, while the simulated phase shift for the broadband balun are respectively 178°, 178°, 180°. These simulated results suggest that H-Plane cross-polarization levels can be kept sufficiently low when the phase difference between the two L-probe feeds are kept within 180° (±10°).

Figure 10.

Simulated radiation pattern for the dual L-probe square patch antenna utilizing the 180° broadband balun.

[17] Figure 11 shows the measured radiation patterns for the dual L-probe square patch antenna using the 180° narrowband balun at the start frequency (1.7 GHz), center frequency (2.0 GHz), and end frequency (2.3 GHz) of our bandwidth of interest. Figure 12 shows the measured radiation patterns for the dual L-probe square patch antenna using the 180° broadband balun at the start frequency (1.7 GHz), center frequency (2.0 GHz), and end frequency (2.3 GHz) of our bandwidth of interest. While still maintaining symmetrical E- and H-plane co-polarization patterns and considerably low E-plane cross-polarization, it is evident that the use of the broadband balun provides the added advantage of H-plane cross-polarization suppression, across the bandwidth of interest.

Figure 11.

Measured radiation pattern for the dual L-probe square patch antenna utilizing the 180° narrowband balun.

Figure 12.

Measured radiation pattern for the dual L-probe square patch antenna utilizing the 180° broadband balun.

[18] Across this passband, the antenna using the 180° broadband balun exhibits symmetrical E- and H-plane co-polarization patterns and consistently low E- and H-plane cross-polarization levels (<−21 dB). Consistent with our analysis from the simulated results, it is observed that as long as the phase difference afforded by the balun is kept within 180° (±10°), the measured H-plane cross-polarization levels will not exceed −20 dB. Table 1 provides a summary of the simulated and measured H-plane cross-polarization levels for the dual L-probe square patch antenna utilizing either baluns, across our bandwidth of interest. Both simulated and measured results indicate that, unlike the narrowband balun, the broadband balun consistently provides good H-plane cross-polarization suppression throughout the 30% passband.

Table 1. Simulated and Measured H-Plane Cross-Polarization Comparison for the Dual L-Probe Square Patch Antenna Using Either the 180° Narrowband or Broadband Balun
Frequency, GHzWith 180° Narrowband BalunWith 180° Broadband Balun
Simulated X-Pol, dBMeasured X-Pol, dBSimulated X-Pol, dBMeasured X-Pol, dB
1.7−27.2−26.7−28.6−24.1
1.8−27.0−24.9−28.3−27.2
1.9−27.9−22.9−28.6−22.8
2.0−27.2−15.6−31.9−26.3
2.1−20.6−12.9−31.0−24.4
2.2−6.4−8.3−27.5−22.1
2.3−10.1−12.1−32.1−21.1

4. Conclusion

[19] A broadband 180° microstrip balun has been employed as a feed network for the L-probe square patch antenna. The broadband balun provides impedance matching, equal amplitude power division and consistent out-of-phase excitations over a wide band (∼45%). We have shown that for the dual L-probe square patch antenna, the use of the proposed 180° broadband balun as a feed network affords good H-plane cross-polarization suppression across a wide passband. In this way, we can now achieve high gain, stable and symmetrical E- and H-plane co-polarization patterns, and consistently low E- and H-plane cross-polarization levels, throughout the wide impedance bandwidth (∼30%) designed for L-probe patch antennas.

Ancillary

Advertisement