GPS/BDS High-Gain Dual-System Antenna: LX-GB-38 Engineering Guide

In high-gain GNSS antenna design, gain, bandwidth, interference rejection, and physical size form a set of mutually constraining parameters. This article takes the Luxun Antenna (Lxelec) LX-GB-38 GPS/BDS dual-system antenna as an example. It analyzes the engineering trade-offs behind achieving a 38±2 dBi system gain, ≤2.5 dB noise figure, and third-order filter out-of-band rejection within the 1550–1610 MHz band, starting from the noise chain of active arrays and the mechanisms of out-of-band suppression. The antenna covers GPS L1 (1575.42 MHz), BDS B1 (1561.098 MHz), and 1602 MHz in a compact Φ95×112 mm form factor. It offers a highly reliable receive-front-end solution for rail transit, precision agriculture, and power grid synchronization.

1. Introduction: The SNR Dilemma in GNSS Receive Chains

The power density of GNSS signals reaching the ground is extremely low. For GPS L1 C/A code, the minimum received power is approximately -160 dBm, and BDS B1I signal power is comparable. This level is far below the thermal noise floor of a typical receiver—at 1 MHz bandwidth and 290 K, the thermal noise power is about -114 dBm. In other words, the GNSS signal is already buried in noise by about 46 dB when it enters the receiver.

This physical fact determines the core contradiction in GNSS antenna design: the antenna must maintain sufficient signal-to-noise ratio under extremely weak signal conditions while suppressing interference from the ground and adjacent bands. The former requires low-noise amplification; the latter requires high out-of-band rejection. Both impose constraints on the antenna’s physical size and structure.

From the perspective of array antenna theory, a GNSS antenna is essentially a “receive-end SNR optimizer.” Its gain is not about “amplifying the signal” but about concentrating received energy in the upper hemisphere where satellites reside, while suppressing ground thermal noise and multipath components reflected from the ground. This design logic is fundamentally different from that of a transmit antenna—the latter focuses on power efficiency and coverage, while the former focuses on SNR and phase center stability.

2. Active Antenna Architecture: Noise-Figure-First Design Principle

2.1 System Gain Distribution Logic

The LX-GB-38 has a nominal gain of 38±2 dBi. This figure is not the gain of a single radiating element but the cascaded system gain of the antenna’s radiation gain and the built-in LNA gain. According to product data, the passive antenna portion has a gain of ≤5 dBi, meaning the LNA contributes approximately 33 dB of amplification.

This gain distribution is common in GNSS active antennas, but the key lies in how the noise figure is allocated along the chain. According to the Friis cascaded noise formula:

F_total = F1 + (F2-1)/G1 + (F3-1)/(G1·G2) + …

where F1 and G1 are the noise figure and gain of the first stage (typically the connection between the antenna radiating element and the first LNA). In GNSS applications, the antenna radiating element itself is a passive device, and its “noise figure” equals its insertion loss. Therefore, the noise figure of the first-stage LNA and the transmission loss between the antenna element and the LNA jointly determine the system noise figure.

The LX-GB-38 specifies a noise figure of ≤2.5 dB. This means the total noise figure from the antenna port to the LNA output is kept within 2.5 dB. From an engineering standpoint, this requires extremely low loss (typically below 0.2 dB) in the microstrip or coaxial connection between the radiating element and the LNA, along with low-noise devices and optimized bias design in the LNA.

2.2 Engineering Significance of 38 dBi System Gain

From a link budget perspective, a 38 dBi system gain means the signal level at the antenna output is about 38 dB higher than the raw signal received by the antenna. For a -160 dBm GPS L1 signal, after the antenna system, the output to the receiver is approximately -122 dBm. This level is still below the receiver’s thermal noise floor (-114 dBm/1 MHz), but it has entered the range that the receiver front end can handle—the receiver baseband processing extracts the signal from noise through spreading gain (43 dB for C/A code).

Therefore, the engineering significance of 38 dBi is not “making the signal stronger than noise,” but providing sufficient signal level to the receiver so that its front-end LNA and mixer can operate with a lower noise figure. If the antenna system gain is insufficient, the receiver front-end noise will dominate the total link noise figure, and the spreading gain cannot compensate.

From a deployment perspective, 38 dBi system gain is adequate for RF cable runs up to 50 meters. Using RG-58 cable with about 0.5 dB/m loss at 1.5 GHz, a 50-meter cable introduces about 25 dB loss, leaving 13 dB of gain to cover the receiver front-end requirements. This is also why such antennas are typically installed near the receiver or connected via medium-length cables.

2.3 The Mutual Coupling Cost of Dual-System Compatibility

The frequency spacing between GPS L1 (1575.42 MHz) and BDS B1 (1561.098 MHz) is only 14.3 MHz, a fractional bandwidth of about 0.9%. In traditional narrowband antenna design, two such closely spaced frequencies are usually covered simultaneously by a dual-resonant structure or a broadband matching network. However, this approach has a widely overlooked issue: the current distribution at the two frequencies is not exactly the same, leading to slight differences in phase center and radiation pattern.

For centimeter-level applications such as RTK (Real-Time Kinematic), this difference directly translates into positioning error. The LX-GB-38 solution uses a wide-beam circularly polarized radiating structure in the radiating element design, maintaining stable right-hand circular polarization (RHCP) across the 60 MHz bandwidth from 1550 to 1610 MHz. The passive antenna portion has a VSWR of <2.0, indicating acceptable impedance matching across the band.

However, it should be noted that joint processing of GPS and BDS signals imposes higher requirements on group delay consistency. If the group delay difference between the two frequency points is too large, the receiver will introduce additional ranging bias when performing dual-system joint solutions. The product data does not provide group delay specifications—this is a parameter worth attention in practical high-precision applications.

3. Out-of-Band Rejection: Engineering Logic of Third-Order Filtering

3.1 Interference Environment Analysis

The main sources of out-of-band interference for GNSS receivers include:

• Cellular bands: Transmit signals from LTE Band 3 (1805–1880 MHz) and Band 1 (2110–2170 MHz) can cause intermodulation interference through antenna nonlinearity or receiver front-end saturation.
• Ground-reflected multipath: Reflected signals from buildings, water surfaces, or the ground superimpose on the direct signal, causing phase errors.
• Strong signals in adjacent bands: Such as satellite communication uplink bands and microwave relay links near 1.5 GHz.

The LX-GB-38 out-of-band rejection specifications are:

These specifications correspond to a third-order filtering architecture: the first stage provides coarse broadband filtering before the LNA (about ±100 MHz), the second stage provides medium-bandwidth filtering after the LNA (about ±50 MHz), and the third stage provides narrowband filtering at the output (about ±30 MHz). The engineering logic of this staged filtering strategy is: perform necessary pre-filtering before the LNA to prevent strong interference from saturating the LNA, and perform fine filtering after the LNA to suppress intermodulation products generated by LNA nonlinearity.

3.2 Comparison with Lens Antenna Solutions

From an array antenna perspective, out-of-band rejection can also be achieved through directional filtering—using the antenna’s spatial filtering characteristics (i.e., nulls in the radiation pattern in undesired directions) to suppress interference. Research by Professor Shiwen Yang’s group on lens antennas shows that high-gain lens antennas based on stepped dielectrics can achieve high gain and low sidelobes while maintaining wide-angle scanning capability. If this approach were applied to GNSS reception, it could theoretically improve out-of-band rejection without increasing filter order.

However, the engineering challenge of lens antenna solutions in GNSS applications is that the lens physical size is proportional to gain, while GNSS antennas need to achieve as much gain as possible within a limited volume. For the LX-GB-38 with Φ95×112 mm, a lens solution would significantly increase space occupancy, conflicting with compact installation requirements. Therefore, the product chooses an active filtering solution rather than a spatial filtering solution—a pragmatic choice under size constraints.

4. Environmental Adaptability and Mechanical Design

4.1 IP67 and 48-Hour Salt Spray

GNSS antennas often face harsher deployment environments than communication antennas. Rail transit car roofs, precision agriculture machinery tops, and power substations—these scenarios involve continuous vibration, temperature cycling, and corrosive media.

The LX-GB-38 has an IP67 rating, a 48-hour salt spray test duration, and an ASA (acrylonitrile styrene acrylate) radome. Compared with ABS, ASA offers significantly improved UV resistance and weather resistance, with an outdoor service life typically exceeding 10 years. The 48-hour salt spray test corresponds to the severe level in IEC 60068-2-52, suitable for coastal and industrial pollution environments.

From a mechanical design perspective, the 493 g weight (including bracket) and 140 km/h wind resistance indicate structural validation against vibration and wind loads. For applications mounted on railway locomotive roofs, this wind resistance covers the aerodynamic loads during train operation.

4.2 Lightning and ESD Protection

The built-in lightning protection complies with IEC 61000-4-5, tested with an 8/20 μs waveform, 5 kA peak current, and 5 positive and negative polarity events each. ESD protection complies with IEC 61000-4-2 Class 3. For GNSS antennas installed on high towers or building rooftops, these protective measures are necessary—lightning surges can enter the receiver through the antenna cable and cause permanent damage.

5. Engineering Adaptability Analysis for Application Scenarios

5.1 Rail Transit

In train control systems, GNSS signals are used for train positioning and speed calibration. Tunnels, elevated bridges, and dense urban buildings cause satellite signal blockage and multipath. The LX-GB-38’s 38 dBi system gain and out-of-band rejection enable it to maintain usable SNR in these obstructed environments. In addition, the antenna’s wide-beam circular polarization ensures stable satellite tracking even when the train tilts or turns.

5.2 Precision Agriculture

RTK systems require centimeter-level positioning accuracy. The stability of the antenna phase center directly affects RTK solution accuracy. The LX-GB-38’s wide-beam design helps maintain phase center stability in multipath environments, but its group delay characteristics (not provided in product data) are key parameters for evaluating its suitability in RTK applications.

5.3 Power Grid Synchronization

Power grid synchronization requires microsecond-level timing accuracy. The performance of GNSS timing receivers depends on the signal quality and interference rejection provided by the antenna. The LX-GB-38’s 70 dB@±100 MHz out-of-band rejection can effectively suppress interference from nearby communication base stations, ensuring a clean timing signal.

6. Selection and Deployment Recommendations

Confirm frequency coverage. The LX-GB-38 covers 1550–1610 MHz, including GPS L1, BDS B1, and 1602 MHz (an extension of GLONASS L1). If the application only requires GPS L1, the antenna remains compatible, and the extra 40 MHz bandwidth does not cause performance loss.

Evaluate cable length. The 38 dBi system gain provides about 38 dB of margin for cable loss. At 1.5 GHz, RG-58 cable loss is about 0.5 dB/m, meaning the theoretical maximum cable length is about 70 meters (retaining the signal level required by the receiver front end). In practice, keeping it under 50 meters is recommended.

Pay attention to group delay specifications. For centimeter-level RTK applications, it is advisable to request group delay test data from the supplier. The group delay difference between GPS and BDS frequency points should be controlled within the nanosecond range.

Choice of installation location. The antenna’s upper-hemisphere reception pattern means it should be installed in an open location as much as possible, avoiding obstruction of the upper hemisphere by metal objects. At the same time, the antenna’s ground plane (connected to the metal structure via the mounting bracket) significantly affects reception performance—on non-metallic mounting surfaces, an artificial ground plane is recommended.

7. Conclusion

The engineering value of the LX-GB-38 lies in achieving 38 dBi system gain and 70 dB@±100 MHz out-of-band rejection within a compact physical size. This performance relies on the comprehensive optimization of low-noise LNA design, staged filtering architecture, and wide-beam circularly polarized radiating elements. For higher-precision applications (such as RTK and timing), group delay consistency and phase center stability are directions for further verification and optimization.