Method for testing base station antenna feeder system

The vertical axis of Figure 1 is the value of the standing wave ratio, and the vertical axis of Figure 2 is the value of the return loss.

The standing wave ratio and return loss of the antenna and feeder can also be measured in the distance domain, as shown in Figures 3 and 4.

Before testing with Site Master for the first time, or when the measurement frequency range is changed, or when the temperature and environment have changed significantly from the last test, the test port needs to be calibrated with a calibration kit to ensure the accuracy of the test value. When testing, pay attention to the connection between the test port or the 3-meter extension cable and the base station indoor jumper connector. There are three main types of base stations currently in use in our province: analog base station RBS883, digital base station RBS200 and RBS2000. The indoor receiving jumpers of analog base station RBS883 and digital base station RBS200 can be directly connected to the Site Master test port or a 3-meter extension cable. When testing the transmitting antenna feeder of the analog base station, first turn off the transceiver connected to the transmitting antenna feeder, and then unscrew the transmitting jumper connected to the directional coupler. The transmitting jumper needs to be connected to the Site Master test port or a 3-meter extension cable with an adapter. This adapter can be found in the adapters provided with TDR or HP spectrum analyzers. The indoor transmitting jumper of the digital base station RBS200 and the indoor receiving and transmitting jumpers of RBS2000 must be connected to the Site Master test port or a 3-meter extension cable through a 510--90 N (m) --7/16 (f) adapter. When testing RBS2000, When testing the antenna feeder, you can also start the test from the cable connected to the RF output port of the CDU, so that the quality of the cables inside the rack can be measured. In daily testing, it has been found that the signal transmission and reception are affected by the poor quality of the cables inside the rack. When testing the transmitting antenna feeder, temporarily turn off the transceiver connected to the transmitting antenna feeder to avoid leakage of RF signals. During the test, you can first test the standing wave ratio or return loss of the antenna feeder in the frequency domain to observe whether the antenna feeder is normal in the frequency band in which it works. If an abnormality is found, you can enter the distance domain to locate the fault. As shown in Figure 5, the standing wave ratio of the antenna feeder at 38 meters is close to 1.6, indicating that there is a problem there.

Figure 5: Site Master distance domain graph of a faulty antenna feeder

During routine testing, it was found that common problems with antenna and feeder systems include water ingress into the antenna due to improper installation, loose connectors or water ingress, and damage to jumpers.

2. Measurement of feeder length

At present, most of the feeders and jumpers used in base stations in our province are products of RFS Company. The feeder type is LDF5-50A, and its relative propagation velocity (RelativePropagation Velocity) Vf is 0.89, and the cable loss (Cable Loss) is 0.043 dB/m; the jumper type is LDF4-50A, and its relative propagation velocity Vf is 0.88, and the cable loss is 0.077dB/m. When measuring the length of the feeder more accurately, the cable loss Loss and the relative propagation velocity PROP VEL values ​​must be correctly entered in the settings of the cable parameters of Site Master. When measuring, first preliminarily estimate the length of the feeder, enter the starting position D1 and the ending position D2 in the distance domain, and then measure. Since the standing wave ratio at the feeder and jumper joint is generally higher than that of the feeder, the length of the feeder can be roughly obtained, and then D2 can be adjusted to an appropriate position according to the actual situation to obtain the length of the feeder more accurately.

At present, most of the outdoor jumpers used in base stations are 3 meters long. The standing wave ratio at the connection point with the feeder and antenna is higher than that at other places. As shown in Figure 6, the distance between point A and point B is about 3 meters, so d is the length of the feeder. Since the cable parameters of the feeder are used at this time, there will be a certain error in the length of the jumper. For different types of feeders, the corresponding cable parameters must be correctly entered before measurement.

3. Antenna isolation test

Only the dual-port Site Master can test the antenna isolation. When testing, enter the 'MODE' menu, select 'GAIN/INSERTION LOSS', and then calibrate the REFL PORT and TRANS PORT according to the prompts on the screen. Then connect the REFL PORT and TRANS PORT ports to the transmitting antenna feeder and the receiving antenna feeder respectively to measure the antenna isolation. Figure 7 is an isolation test chart. It can be seen from the figure that the isolation is greater than 60dB in the range of 935-960MHz, which meets the isolation requirements.

Figure 7 Antenna feeder isolation diagram

4. Software Application

Site Master is connected to the computer via RS232 port, and its data can be managed and analyzed by software running in Windows environment. By transferring the test data of antenna and feeder stored in the host to the computer, a database of the operation status of the antenna and feeder of the base station can be established. By quickly comparing the new and old test data by "Drag & Drop", it can be observed whether the operation status of the antenna and feeder has a downward trend, so as to find problems and eliminate hidden dangers early. The software can also be used to convert the graph measured in the frequency domain into a Smith graph. The software also provides a measurement calculator, which can be used to easily convert the standing wave ratio, return loss and reflection coefficient. The specific usage is to enter the "Tools" option, select "Measurment Calculator", enter the value of any one of the standing wave ratio, return loss and reflection coefficient, and you can get the values ​​of the other two and the percentage of transmission power.

5. Issues that should be noted when using Site Master testing

(1) When testing the standing wave ratio, return loss and feeder length of antenna feeders, the cable parameters of the feeders must be correctly input, otherwise the measured values ​​will be inaccurate. For different types of feeders, the parameters provided by the manufacturer must be correctly input to ensure the accuracy of the test values. Even for the same type of feeders, their technical indicators will continue to improve, so after using a new feeder, the latest cable parameters must be input.

(2) When a 3-meter test port extension cable is used, if calibration is performed at the end of the cable using a calibration kit, the start position D1 confirmed by the Site Master is at the end of the 3-meter test port extension cable; otherwise, the start position D1 is at the test port of the Site Master.

(3) The calibration components used to calibrate the test port are relatively precise and cannot withstand large power, otherwise the calibration components will be damaged, affecting the test accuracy.

(4) The maximum measurement distance of Site Master is determined by the frequency band, data points and relative propagation rate. The formula is as follows:

F1: Starting frequency F2: Ending frequency Vf: Relative propagation velocity When testing, the appropriate frequency range should be selected according to the actual situation.

(5) If the screen is completely black when you turn on the computer, it may be that the screen contrast is inappropriate. In this case, you can use the ∧ key or ∨ key on the right side of the keyboard to adjust the contrast to an appropriate level.

(6) When the car's output current is unstable, you should try to avoid charging the Site Master with a car charger to avoid burning the host.

(7) Be as careful as possible during use to avoid damaging the tester.