The signals transmitted by the wireless base station and the mobile station are both handled by the antenna feeder system. As a result, the performance and condition of this system directly impact call quality, signal coverage, and the overall operation of the transceiver. If there is a fault in the transmitting antenna feeder, it can lead to signal loss, which affects the base station's coverage area. In such cases, the base station may shut down the connected transceiver to prevent further issues.
When the receiving antenna feeder fails, the signal strength received by the mobile device decreases, leading to situations where the mobile station cannot access the base station’s channel even in areas with strong signal reception. This can result in poor call quality or dropped calls. Currently, base stations only monitor the transmitting antenna feeders, but not the receiving ones. Therefore, if a receiving antenna feeder fails and affects network service, no alarm is triggered, making it difficult for maintenance teams to identify and resolve the issue quickly, resulting in wasted time and resources.
When the isolation between antennas is insufficient, the signal from one transmitter may interfere with another, causing intermodulation distortion. This generates unwanted frequencies that mix with the desired signal, creating interference at the receiver. Regular maintenance and testing of the antenna feeder system—especially the isolation between the receiving antenna and the antenna itself—are essential to detect potential problems early and avoid service disruptions.
Common failures in the antenna feeder system occur at the antenna, cables, and connectors. Poor installation, inadequate drainage, or improper sealing can allow water to enter the antenna during rain, leading to damage over time. Similarly, faulty joints can become wet due to weather conditions, further compromising the system. In densely populated urban areas, limited space often makes it challenging to install antennas properly, which may result in insufficient side and back lobe isolation, affecting overall performance.
Testing the antenna feeder primarily involves measuring the standing wave ratio (VSWR), return loss, and isolation. These metrics help assess the quality of installation and system performance. The VSWR is calculated using the following formulas:
- Forward power (W): Pf or Pf (dBw) = 10 log Pf
- Reverse power (W): Pr or Pr (dBw) = 10 log Pr
- Return loss: ar = 10 log (Pf - Pr)
- Reflection coefficient: Ï = |(Pr/Pf)|
- Standing wave ratio: VSWR = (1 + |Ï|) / (1 - |Ï|)
- Conversion formula between return loss and VSWR: Return loss = -20 log |Ï|
The VSWR alarm threshold for a base station's transmitting antenna feeder is typically set at 1.5. Isolation requirements vary depending on the type of base station. For example, the RBS200 base station requires more than 40 dB of isolation between its transmitting antennas, while the isolation between its transmit and receive antennas should be greater than 20 dB. For the RBS2000 base station, the isolation between the transmit antenna and other antennas should be above 30 dB.
Several instruments can be used to test the antenna feeder system, including spectrum analyzers, Time Domain Reflectometers (TDR), and the Site Master. Spectrum analyzers measure forward and reverse power to determine return loss, then use a lookup table to find the corresponding VSWR. TDR provides a rough waveform to identify faults, while the Site Master offers direct measurements of VSWR, isolation, and distance to fault, allowing for quick troubleshooting.
The Site Master is a handheld cable and antenna analyzer produced by Anritsu, Japan. It is compact, user-friendly, and ideal for on-site testing by technicians. It uses frequency domain reflectometry to measure VSWR, return loss, insertion loss, and distance to fault. It can also connect to a computer for data management and analysis. Optional accessories like a power meter allow for measurement of the base station's transmit power.
Site Master models include S331A (single port), S120A, S235A, and S251A (dual port). Each model operates across different frequency bands, so the appropriate one must be selected based on the base station’s operating frequency. Key components of the Site Master include precision calibration kits, test port extension cables, adapters, and power detectors. These tools ensure accurate and reliable measurements during testing.
To test and maintain the antenna feeder using the Site Master, follow these steps:
1. Measure the standing wave ratio and return loss of the antenna feeder to locate any faults.
2. Use the Site Master to analyze the results in both the frequency and distance domains. Figures 1 and 2 show the standing wave ratio and return loss, respectively, while Figures 3 and 4 display the same data in the distance domain. These visualizations help identify where the problem lies along the feeder line.
FD-xxxE Series
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