L1/E1/B1 and L5/E5a/B2a Band Dual-Polarized Microstrip Antenna for GNSS-R

Abstract

This paper presents the design of a dual-band and dual-polarized microstrip antenna for global navigation satellite systems reflectometry (GNSS-R) for the L1/E1/B1 and L5/E5a/B2a bands. In order to allow advanced GNSS-R signal processing and sensing techniques, the design has been carried out for dual-band and dual-polarization operation with isolated ports to receive both left and right-hand circular polarizations. The design procedure to allow receiving both bands independently and with high isolation is described in detail. Numerical and experimental results show that the antenna presents good performance in terms of impedance matching, circular polarization purity and isolation between the ports. The measured levels of axial ratio are 0.86 dB and 1.89 dB for the L1/E1/B1 and L5/E5a/B2a bands, respectively. The measured isolation levels are larger than 30 dB and 40 dB in the L1/E1/B1 and L5/E5a/B2a bands, hence proving that the proposed antenna concept can be properly used in dual-band dual-polarized GNSS-R applications.

Index Terms
GNSS-R; dual-polarization antennas; dual-band antennas; microstrip antennas.

I. INTRODUCTION

The analysis of global navigation satellite systems (GNSS) signals by GNSS reflectometry (GNSSR) has been widely used to observe climatic variables, ocean levels, polar ice caps, soil moisture, sea streams or even in the detection of leaks from oil platforms. This technique consists of receiving GNSS signals, both in line-of-sight (LOS) and after reflection by a surface of interest [¹] -[⁵]. A typical GNSS-R scenario is shown in Fig. 1. The signal radiated by the satellite impinges upon the Earth at the position indicated by LOS and is reflected at point P by the reflecting surface G. An antenna located at A is pointed to P, in order to receive the reflected signal. Depending on the material that composes the reflecting surface G, the reflected signal may exhibit right-handed circular polarization (RHCP), as the direct signal received in the LOS, or left-handed circular polarization (LHCP). The angle θ stands for the elevation of the satellite, ϑ is the angle of reflection related to the normal direction N, and ι_a is the antenna installation angle [⁶].

Fig. 1
GNSS-R scenario.

In order to achieve GNSS-R with enhanced sensitivity, multi-band and dual-polarized antennas can be employed [⁷], [⁸]. In addition to the dual-polarization reception, these radiators must operate with the possibility to receive several frequency bands independently with high isolation between the ports, good impedance matching, and broad radiation pattern [⁹]. While most of the papers report on the use of GPS L1 (centered at 1575.42 MHz) and L2 (centered at 1227.60 MHz) bands for GNSS-R, the use of the GPS L5 signal (centered at 1176.45 MHz) for GNSS-R has not been reported yet in the literature [¹⁰]. Its use brought greater accuracy, reliability, and better penetration through vegetative and around obstructions, as well as allowing operation at higher power levels than the L1 and L2 bands [¹¹], [¹²]. As most GNSS are interoperable, also the signals in the E1 and E5a bands of Galileo, as well as in the B1 and B2a bands of Beidou can be received as they have the same center frequencies as L1 and L5, respectively. Thus, an antenna design for the L1 and L5 bands has the advantage that up to three GNSS are transmitting signals in both bands, while for the L1 and L2 bands only GPS signals can be received in both bands. Hence, multi-constellation multi-frequency GNSS-R signal processing can be achieved.

In the open literature, there are several papers reporting on applications of GNSS-R signals using different receiver and antenna topologies. Most of the GNSS-R setups operate with two antennas: a top antenna, which is used to receive the direct GNSS signal with RHCP, and the bottom antenna, which is pointed to the reflecting surface (normally the ground) with LHCP [¹³]-[¹⁹]. All these antennas are designed to operate only in L1 or in L2 band with single polarization [²⁰]. Other authors report on GNSS-R applications employing antennas with dual-circular polarization, but still operating in the L1 band only.

If dual-band radiators for general applications are explored, antennas with two linear polarizations [²¹], [²²] and with circular polarization in one band (L1) and linear polarization in the other (Wi-Fi) can be found [²³] -[²⁶] in the open literature. Antennas operating in the L1/L2 bands for satellite navigation systems are reported in [²⁷], [²⁸]. Additionally, antennas with RHCP for satellite digital multimedia broadcasting (SDMB) and L-band services were reported on [²⁹]. Likewise, an antenna with LHCP operation in the L1 band and RHCP in the S band is reported in [³⁰]. Other contributions present radiators with circular polarization (CP) operation in GPS bands and dual-linear polarization for wireless local-area network (WLAN) [³¹], [³²]. Applications in radio base stations of the mobile phone systems require antennas with dual-band and dual-linear polarization. The most used bands are for global system for mobile communication (GSM), digital communication system (DCS) and universal mobile telecommunications system (UMTS) [³³], [³⁴]. Structures with dual-band and dual-linear polarization are designed to operate in the WLAN and worldwide interoperability for microwave access (WiMAX) bands [³⁵], [³⁶].

In the papers cited above, both operation bands are available in one output port and the separation between the two operation bands (generally L1 and L2) must be done using a diplexer. However, for the sake of reducing costs and volume, an optimum solution would be to use a dual-band dual-polarized GNSS-R receiving system with an antenna with diplexing and polarization selectivity, which is shown schematically in Fig. 2. In this setup, the L1/E1/B1 and L5/E5a/B2a signals with RHCP and LHCP are received independently and simultaneously. Each receiving channel is composed of a low-noise amplifier (LNA), a demodulation and down-conversion stage and a digital demodulator, which allows separating the in-phase and the quadrature signals (I/Q). Finally, the necessary correlation with a replica signal can be performed in the digital domain. In [³⁷], [³⁸], the use of a diplexer could be avoided, but the antenna was not developed for dual-polarized operation and, therefore, the receiver was composed of only two channels. While most of the papers report on GPS L1 and L2 bands, the use of GPS L5 signal (centered at 1176.45 MHz) suits better for safety-of-life applications, since it has been developed for aviation safety.

Fig. 2
Simplified block diagram of an dual-band GNSS-R receiver.

This work proposes a new structure that operates in two bands for GNSS open service signal transmission: in the L1/E1/B1 (1563.42-1587.42 MHz) and in the L5/E5a/B2a (1164.45-1188.45 MHz) bands. The proposed antenna allows receiving these two bands independently, with dual circular polarization (RHCP and LHCP) and with high isolation. This feature could be achieved by designing a special feeding system, which was optimized to provide good in-band impedance matching and to prevent the coupling between the operation bands.

In the next section, the geometry of the proposed radiator is depicted. In section III, the design procedure is described. In contrast to previous designs [³⁹], [⁴⁰], the novelty of the proposed antenna is that the L1/E1/B1 and L5/E5a/B2a bandwidths can be improved by detuning slightly the decoupling stubs. Section IV presents the experimental validation of the built prototype, whereby very good performance in terms of impedance matching, polarization purity and isolation between the bands can be verified.

II. ANTENNA STRUCTURE

The stack-up considered for the construction of the designed antenna is shown in Fig. 3. The multilayer structure has three low-loss microwave laminates of the type RO3006 with dielectric constant of 6.15 and loss tangent of 0.0025. The top laminate supports the patch that is tuned to operate in the L1 band centered at 1575.42 MHz. The middle laminate contains the patch that resonates at 1176.42 MHz (L5 band). The ground plane (GND) and the feeding lines are printed in the bottom laminate. Air layers have been added between the laminates, in order to increase bandwidth. Both patches have square shape and are fed by two independent pairs of feeding lines. The connection between the patches and the lines is done by means of metallic vias.

Fig. 3
Cross-sectional view of the dual-band antenna with highly isolated ports.

The schematic top view of the antenna can be seen in Fig. 4. Operation in the L1 band is achieved with the top square patch with edge sizes L_L1 = 70 mm and in the L5 band with the bottom square patch with edge size L_L5 = 100 mm. The outer edges of the antenna are 15 x 15 cm long.

Fig. 4
Schematic top view of the antenna.

The schematic view of the antenna feeding system is shown in Fig. 5. There are four ports, whereby the bottom patch is fed by ports 1 and 2, whilst ports 3 and 4 feed the top radiator. This feeding technique was employed in order to obtain circular polarization in each operating band with good axial ratio and the possibility to receive the L1 and L5 bands independently.

Fig. 5
Schematic view of the antenna feeding system to yield highly isolated ports.

The impedance matching for the proposed antenna cannot be realized only by adjusting the position of the vias due to the intrinsic series inductance introduced by the vias. Among other possibilities, the single-stub impedance matching can be applied by adjusting the length and the distance of the red stubs from the load to be matched. The length l_mL5 and distance d_mL5 are used in the red stubs of ports 1 and 2 for impedance matching in the L5-band. Likewise, the length l_mL1 and distance d_mL1 can be used at ports 3 and 4 for impedance matching in the L1-band.

Considering a transmission line section with characteristic impedance Z₀ terminated with a load impedance Z_L, the input impedance Z_in is given by

(1) $$Z_{in}=Z_0\frac{Z_L+j{Z}_0\tan \beta l}{Z_0+j{Z}_L\tan \beta l}.$$

Once the stub is open-ended, the load impedance Z_L approaches infinity (Z_L → ∞). In this case, the input impedance will be given by

(2) $$Z_{in}=-j{Z}_0\cot \beta l.$$

Since we want the input impedance to be zero, the cotangent term in (2) must vanish, which occurs if l=λ_g/4, where λ_g is the guided wavelength. Therefore, when the open-ended stub has a physical length of λ_g/4, it effectively creates a short-circuit. This technique is commonly used to provide isolation between two frequency bands.

Isolation between all the ports can be improved by inserting decoupling stubs, as described by the transmission line theory above. These stubs are depicted in blue in Fig. 5. According to [³⁹] and [⁴⁰], high reflection at the L5-band results, if l_dL1 ≅ λ_gL1/4 at ports 1 and 2. Similar approach can be used at ports 3 and 4 by choosing l_dL5 ≅ λ_gL5/4 to cause high reflection at the L1-band. In the above statements, λ_gL5 and λ_gL1 are the guided wavelengths at the center frequencies of the L5 and L1 bands, respectively.

III. ANTENNA DESIGN

The first step was to build the electromagnetic model in Ansys HFSS [⁴¹]. The HFSS Estimate tool was then used to determine each patch initial dimensions and the via location. With the clculated edge lengths for the square patches to ensure proper operation in both L5 and L1 bands, the optimization process has been carried out. The next step involved selecting the positions of the vias before incorporating the matching and decoupling stubs. This was achieved through an initial impedance matching optimization, with the results shown in Fig. 6, obtained using the electromagnetic simulator. The curves indicate that direct impedance matching in the L1 band is not feasible.

Fig. 6
Simulated input impedance prior to the inclusion of the matching and decoupling stubs. Red curve stands for impedance at port 1 or 2 (L5 band) and blue curve for port 3 or 4 (L1 band). The dots indicate the center frequencies of each band.

The coupling between ports with the same polarization (ports 1 and 4, and 2 and 3) is plotted in Fig. 7 only for the L5 band, because good isolation has been achieved in the L1 band. The curves show that S₁₄ is unacceptably large inside the L5 band. Following the procedure highlighted in the previous section, the isolation between the ports can be improved by adding the decoupling stubs. After this step, the S-parameters are shown in Figs. 8 and 9, whereby excellent isolation between all the ports can be verified in both bands of interest. However, the drawback is that both input impedance curves have been moved to the border of the Smith chart, as shown in Fig. 10. This occurs because the decoupling stubs include large reactances also in the pass-band. This problem can be overcome by slightly detuning the decoupling stubs, so as to yield acceptable values for the input impedance in each band to be matched.

Fig. 7
Simulated S-parameters showing isolation between ports in the L5 band. Bandwidth is hatched.

Fig. 8
Simulated S-parameters showing isolation between ports in the L5 band after the inclusion of tuned decoupling stubs.

Fig. 9
Simulated S-parameters showing isolation between ports in the L1 band after the inclusion of tuned decoupling stubs.

Fig. 10
Simulated input impedance after the inclusion of the decoupling stubs. Red curve stands for impedance at port 1 (L5 band) and blue curve for port 4 (L1 band). The dots indicate the center frequencies of each band.

Small variations in the length of each decoupling stub were simulated in order to demonstrate their effect on impedance matching. Fig. 11 shows the input impedance in each band for different values in the length of the decoupling stubs. It is possible to notice that as the stubs are detuned, the antenna input impedance moves away from the edge of the Smith chart, as expected. Fig. 12 and 13 show that a slight detuning of the stubs do not affect significantly the good isolation between all the ports.

Fig. 11
Simulated input impedance varying the length of the decoupling stubs. Red curve stands for impedance at port 1 (L5 band) and blue curve for port 4 (L1 band). Solid lines are the built dimensions.

Fig. 12
Simulated S-parameters showing the effect by the variation of the length of the decoupling stubs in the L5 band. The blue curves represent the S₁₂ parameter, red curves are S₁₃ and green are S₁₄.

Fig. 13
Simulated S-parameters showing the effect by the variation of the length of the decoupling stubs in the L1 band. The blue curves represent the S₄₁ parameter, red curves are S₄₂ and green are S₄₃.

Fig. 14 shows the input impedance in the Smith chart considering the insertion of the matching stubs (dashed lines). The final step is done by using the the single-stub impedance matching technique and the final results are shown in solid lines in Fig. 14. Table I shows all the dimensions of the designed feeding system. Figs. 15 and 16 present the axial ratio of the antenna in the L5 and L1 bands, respectively. It can be noted that good purity of polarization along the bands of interest which is especially important for GNSS-R applications.

TABLE I
DIMENSIONS FOR THE ANTENNA AND FEED SYSTEM

Parameter	Value (mm)		Parameter	Value (mm)
ldL5	20.00		ddL5	104.00
lmL5	19.00		dmL5	120.00
ldL1	34.00		ddL1	85.00
lmL1	10.00		dmL1	125.00
dvL1	20.00		dvL5	25.00
LL1	70.00		LL5	100.00
L	150.00		W	150.00

Fig. 14
Simulated input impedance. Dashed curves were obtained after detuning the decoupling stubs and inclusion of the matching stubs. Solid curves are the final results. Red curves stand for the impedance at port 1 (L5 band) and blue curves for port 4 (L1 band).

Fig. 15
Simulated axial ratio for the L5 band.

Fig. 16
Simulated axial ratio for the L1 band.

IV. EXPERIMENTAL CHARACTERIZATION

A prototype of the designed antenna has been built in order to validate the proposed concept. The top view is presented in Fig. 17, where the top square patch can be seen. The feeding system, located at the bottom of the antenna, is shown in Fig. 18. The comparison between simulated and measured Sparameters are shown in Figs. 19 and 20 for the L5 and L1 bands, respectively. The isolation between the bands can be verified through the measured S-parameters. For the L5 band (at 1176.42 MHz) S₁₂ = -29.82 dB, S₁₃ = -40.32 dB and S₁₄ = -22.54 dB. For the L1 band (at 1575.42 MHz) S₄₁ = -56.22 dB, S₄₂ = -51.84 dB and S₄₃ = -40.68 dB. The impedance matching was achieved by measuring the reflection coefficients at each port on both bands. For the L5 band the reflections at ports 1 and 2 are S₁₁ = -21.35 dB and S₂₂ = -24.28 dB respectively. For the L1 band the reflections at ports 3 and 4 are S₃₃ = -21.81 dB and S₄₄ = -21.93 dB, respectively.

Fig. 17
Top view of the built prototype detailing the top square-shaped patch.

Fig. 18
Bottom view of the built prototype detailing the feeding system.

Fig. 19
Simulated and measured S-parameters in the L5 band.

Fig. 20
Simulated and measured S-parameters in the L1 band.

Radiation pattern measurements were conducted in an anechoic chamber to test the performance of the designed antenna to generate circular polarization. At 1.176 GHz, the antenna operated in receiving mode, with ports 1 and 2 connected to a 90^◦ hybrid coupler, while ports 3 and 4 were terminated with matched loads. At 1.575 GHz, the pattern measurements were taken with ports 3 and 4 connected to the 90^◦ hybrid coupler, and ports 1 and 2 terminated with matched loads. The dual-band (L1/L5) 90^◦ hybrid coupler used in the measurements was developed by [⁴²].

To investigate the polarization purity of the antenna, the axial ratio (AR) was calculated by [⁴³]. The terms |E_co| and |E_xp| are the linear amplitudes (not in dB) of the measured fields for the main and cross-polarization, respectively.

(3) $$(AR{)}_{dB}=10\log {\left[\frac{\left|E_{co}\right|+\left|E_{xp}\right|}{\left|E_{co}\right|-\left|E_{xp}\right|}\right]}^2$$

Fig. 21 presents the comparison between the simulated and the measured patterns, whereby one can see very good agreement between the curves. Since the antenna geometry is symmetrical, only the measurements considering RHCP as the main polarization have been carried out. From the measured fields, the calculated axial ratio for L5 band is 0.86 dB. For the L1-band the comparison between simulated and measured results are shown in Fig. 22, where an overall good agreement between the results can be verified. The calculated axial ratio for L1 band is 1.89 dB.

Fig. 21
Gain pattern at 1.176 GHz. Solid lines are simulated results and markers are measured.

Fig. 22
Gain pattern at 1.575 GHz. Solid lines are simulated results and markers are measured.

Tables II and III presents experimental values for the isolation between main and cross polarization for the L5 and L1 bands, respectively. It can be seen that for a large range of angles the antenna operates with good isolation between the main and cross polarization, that is desirable in GNSS-R applications.

TABLE II
ISOLATION BETWEEN MAIN AND CROSS POLARIZATION AT L5 BAND

θ (◦)	Isol. ϕ =0◦ (dB)	Isol. ϕ =90◦ (dB)
0	26.06	27.2
30	24.76	19.99
45	19.35	15.94
60	13.36	12.23
75	9.77	10.67
90	8.85	12.59

TABLE III
ISOLATION BETWEEN MAIN AND CROSS POLARIZATION AT L1 BAND

θ (◦)	Isol. ϕ =0◦ (dB)	Isol. ϕ =90◦ (dB)
0	16.95	19.29
30	13.77	21.64
45	6.82	18.91
60	2.56	16.90
75	1.00	6.86
90	5.89	5.79

Generally, the antennas employed in the GNSS-R setups exhibit axial ratio values below 3 dB and isolation between ports of around 20 dB [⁴⁴]-[⁴⁹]. Therefore, the proposed design fulfills these performance constrains.

Table IV compares the results of this work and those reported in previous bypublished paper available in the open literature. It highlights key parameters such as operating bands, main polarization, gain, physical dimensions, topology, and cross-polarization decoupling (XPD). The term “two antennas” refers to scenarios where two-antennas are employed: one to receive the line-of-sight (LOS) satellite signal (up-looking antenna) and another for the signal reflected from a given surface (down-looking antenna).

TABLE IV
COMPARISON BETWEEN PROPOSED ANTENNA AND PREVIOUS WORK

Ref	Bands	Polarization	Gain (dBic)	Dim. (mm x mm)	XPD (dB)	Topology
[3],[17]	L1	RHCP	-	-	-	single element
[45 ] [16],[18]	L1/L2	RHCP up-looking LHCP down-looking	-	-	-	two antennas
[3]	L1	RHCP up-looking LHCP down-looking	-	80x80	-	two antennas
[9]	L1	dual-LP	8	74.5x74.5	19	single element
[14[]	L1	RHCP up-looking LHCP down-looking	9	-	-	two antennas
[22]	L1	RHCP/LHCP	-	132x132	-	single element
[27]	L1	RHCP	-	135x135	20	single element
	L2	RHCP	-		20
[28]	L1	RHCP	4.5	100x100	10	single element
	L2	RHCP	2.5		10
[29]	L1	RHCP	2.3	60x60	-	single element
	SDMB	RHCP	5		-
[33]	L1/E1	RHCP	9.5	200x200	19.3	2x2 array
	L5/E5	RHCP	8.2		15
[35]	L1/E1	RHCP	5.0	70x70	20	single element
	L5/E5	RHCP	5.0		20
[45]	L1	RHCP	-	80x80	15	single element
this work	L1/E1	RHCP/LHCP	7.01	150x150	19.29	single element
	L5/E5	RHCP/LHCP	7.32		27.2

V. CONCLUSION

This paper presented the design procedure and experimental validation of a microstrip antenna that can be used to receive the L1/E1/B1 and the L5/E5a/B2a bands of GNSS independently. This feature has been achieved by means of a feeding technique employing a set of matching and decoupling stubs. Good agreement between measured and simulated S-parameters has been verified.

Large isolation between the L1 and L5 bands has been obtained experimentally. The measured Sparameters show that the isolation in the L5 band is about 40 dB (between ports 1 and 3) and 22 dB (between ports 1 and 4). In the L1 band the isolation is about 51 dB (between ports 4 and 1) and 50 dB (between ports 4 and 2). The measured gains at the boresight are 7.01 dBic and 7.32 dBic for the L1 and L5 bands, respectively. The experimental axial ratio is 0.86 dB, for the L5-band, and 1.89 dB for the L1-band. This proves that the proposed antenna concept can be properly used in dual-band dual-polarized GNSS-R applications also considering up to three GNSS constellations. The proposed antenna is also suitable to compose an antenna array, since good radiation characteristics along with good isolation between the two bands of operation have been obtained.

ACKNOWLEDGMENTS

Authors would like to thank and acknowledge funding received from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the development of this research, through the following projects: 312394/2021-7 PQ-2, 406517/2022-3, 407245/2022-7 and 405889/2021-6. Additionally, the authors thank Prof. Daniel B. Ferreira from Instituto Tecnológico de Aeronáutica (ITA) for the gain measurements.

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Publication Dates

• Publication in this collection
  04 Nov 2024
• Date of issue
  2024

History

• Received
  17 May 2024
• Reviewed
  01 July 2024
• Accepted
  06 Sept 2024