Practical microwave electromagnetic environment test method

Electromagnetic environment testing is a basic work of electromagnetic spectrum management. It is widely used in various aspects of electromagnetic spectrum management, such as radio station site selection, frequency assignment, radio control and electromagnetic environment assessment. Mastering scientific testing methods and constantly accumulating practical experience are crucial for microwave electromagnetic environment testing. This article introduces the basic methods of microwave electromagnetic environment testing by taking the "HP8593 monitoring and direction finding system" as an example (other systems can be used as reference) in combination with specific work practices.

1 Test standards

The standards for microwave electromagnetic environment testing mainly include GB 13616-92 "Electromagnetic Environment Protection Requirements for Microwave Relay Stations" and GB 13619-92 "Interference Calculation Method for Microwave Relay Communication Systems".

2 Test system composition

Building a corresponding test system according to different test tasks is the first step to successfully complete the electromagnetic environment test task.

The HP8593 microwave electromagnetic environment test system (as shown in Figure 1) with the HP8593 spectrum analyzer as the main body is composed of a wide-band standard horn antenna, a high-frequency attenuator, a microwave low-noise amplifier, a microwave low-loss cable, an HP8593E spectrum analyzer, a portable computer and other equipment. It can complete the electromagnetic environment monitoring work in the frequency band of 1 GHz to 18 GHz.

It is worth noting that test instruments such as spectrum analyzers should comply with the provisions of GB6113 and be calibrated by the metrology department to ensure the accuracy of the test data.

 

Figure 1 HP8593 microwave test system

3. Feasibility demonstration of test system

According to the sensitivity of the microwave frequency band receiving equipment under test, the feasibility of the constructed test system is demonstrated to confirm whether the system meets the test requirement that "the sensitivity of the test system must be higher than the sensitivity of the microwave receiving equipment". The feasibility demonstration is the premise to ensure that the electromagnetic environment monitoring work is scientific, true and effective.

(1) Test system performance demonstration

The performance of the electromagnetic environment test system is mainly reflected in the receiving sensitivity, that is, the ability to receive weak signals. The receiver noise factor and sensitivity are two important parameters to measure the receiver's ability to receive weak signals, and they can be converted to each other.

Receiver sensitivity is the ability of the receiver to monitor weak signals in a specified bandwidth, with units of μV or dBμV; while the noise factor refers to the ratio of the additional noise generated inside the receiver (or spectrum analyzer) converted to the theoretical thermal noise of the input end and the input itself, which is a dimensionless parameter, generally in dB. That is:

FN=NO/GNI ①

Where: FN is the noise figure;

NI is the input theoretical thermal noise power, NI=kT0B, k is the Boltzmann constant, T0 is the absolute temperature of room temperature, and B is the effective noise bandwidth of the receiver (or the resolution bandwidth of the spectrum analyzer);

NO is the output noise power;

G is the circuit system gain.

The output noise of the circuit divided by the gain is the equivalent noise input of the circuit, so NI FN is the input noise power of the equivalent circuit. That is:

NI FN =kT0B FN

In receiver applications, kT0B FN represents the noise power at the receiver input, and the signal level must exceed this noise power. If S/N = 1, then the input signal power is equal to kT0B FN, which is expressed in logarithmic form as:

10lgNI=10lgkT0B=-174+10lgB (dBm) ②

If B = 1Hz, then: 10lgNI = -174 + 10lgB = -174 (dBm/Hz);

If B=1kHz, then: 10lgNI=-174+10lgB =-144(dBm/kHz).

If S/N=1, the receiver noise factor is NF=10lgFN, then the sensitivity of the receiver (or spectrum analyzer) is:

SN=10lgkT0BFN=-174+10lgBFN (dBm/Hz) ③

If B=1Hz, then: SN=-174+NF (dBm/Hz);

If B=1kHz, then: SN =-144+NF (dBm/kHz).

The typical value of the noise factor of the HP8593E spectrum analyzer is 32dB, that is, NF=10lgFN=32. Then, under the condition that there is no attenuation at the front end of the spectrum analyzer, its receiving sensitivity is (if the front end attenuation is set to 10dB, the sensitivity of the spectrum analyzer will decrease by 10dB):

S=10lgkT0BFN=-174+10lgBFN =-174+32+10lgB.

If B = 1Hz, then: S = -142 (dBm/Hz);

If B=1kHz, then: S =-112 (dBm/kHz).

For weaker signals, adding a low-noise amplifier to the front end of the spectrum analyzer will significantly improve the sensitivity of the receiving system.

(2) Test system sensitivity demonstration

In this test system, the spectrum analyzer receiving sensitivity is: S = -142 (dBm/Hz), which meets the requirements of GB13616-92 "Electromagnetic Environment Protection Requirements for Microwave Relay Stations". The DH microwave low-noise amplifier gain G is 30dB and the noise factor is 5. Then, the noise power (i.e., the minimum signal level power) at the output of the high-frequency low-noise amplifier is obtained by formula ③, which can be expressed logarithmically as:

SNo=10lgkT0BGFN

=-174+10lgBFN+G =-174+5+30+10lgB

If B=1Hz, then: SNo=-174+5+28=-141 (dBm/Hz);

If B=1000Hz, then: SNo=-174+5+28+30=-111 (dBm/kHz).

This value is higher than the spectrum analyzer receiving sensitivity, that is, the HP8593E spectrum analyzer can reliably receive the air signals amplified and output by the low noise amplifier. The receiving sensitivity of the low noise amplifier is 169dBm. Considering the gain of the antenna, the ability of this test system to receive small signals, that is, the minimum signal level Pr that can be received at the antenna surface, can be expressed by the formula:

Pr = low noise amplifier receiving sensitivity - antenna gain G2 + cable loss 1.5dB

Right now:

＊1300 MHz～1800 MHz frequency band:

Pr =-169-8.5+1.5=-176 (dBm/Hz);

＊3700 MHz～4200 MHz frequency band:

Pr =-169-10.8+1.5=-178.3 (dBm/Hz);

＊4400 MHz～5000 MHz frequency band:

Pr=-169-11.1+1.5 =-178.6 (dBm/Hz);

＊12.55 GHz～12.85 GHz frequency band:

Pr=-169-12.7+1.5 =-180.2 (dBm/Hz).

According to the "DH30010T2 broadband double-ridged horn antenna technical indicators" in Table 1, the Pr of other frequency bands can be calculated. Table 2 shows the average noise level of the HP8593E spectrum analyzer.

The above demonstration shows that the sensitivity of the test system is higher than that of radar and microwave receiving equipment operating in the microwave frequency band to be measured, and can meet the relevant test requirements.

DH30010T2 broadband double-ridged horn antenna technical specifications#e#Table 1 DH30010T2 broadband double-ridged horn antenna technical specifications

 

 

Table 2 Average noise level of HP8593E spectrum analyzer

4 Test methods and steps

(1) Select the test point

The test points are determined based on the organization and use of relevant microwave communication systems and the requirements of the microwave station electromagnetic environment test on the surrounding terrain and landforms, and the basic test conditions are recorded in the "Microwave Electromagnetic Environment Test Record Materials - Test Basic Conditions Record" (as shown in Table 3).

 

Table 3 "Microwave Electromagnetic Environment Test Record Data - Test Basic Situation Record"

(2) Test preparation and equipment connection

Use a multimeter to measure the power supply voltage, which is required to be AC ​​220±10V. If a diesel generator is used for power generation, a voltage stabilizer must be connected. As shown in Figure 1, connect each test device without adding a low-noise amplifier. The antenna is at least 1.5 meters above the ground at the test point, and the relevant instruments are grounded.

(3) Protective testing and system verification

Protective testing and system calibration are to ensure the safety of the test instrument. The tester is unfamiliar with the electromagnetic environment of the new test point, so a 30dB attenuator must be added to the RF input of the spectrum analyzer for conservative testing, and then the appropriate attenuator or no attenuator must be selected according to the signal size on site.

The test process is as follows: turn on the spectrum analyzer, set the test center frequency and sweep width, and test in maximum hold mode; set the horn antenna pitch angle to 0°, and perform a sweep test by slowly rotating it clockwise in horizontal and vertical polarization modes; analyze the test results, if there is a large signal, a suitable attenuator must be added (including the internal attenuation of the spectrum analyzer), otherwise the attenuator can be removed (Note: to protect the instrument, be sure to turn off the spectrum analyzer before removing the attenuator) for subsequent tests.

(4) Large signal test

To avoid nonlinear distortion, do not add a low-noise amplifier first. Run the HP8593 automatic monitoring software, set the center frequency, sweep width and resolution bandwidth of the test frequency band, set the spectrum analyzer to the maximum hold mode, set the horn antenna pitch angle to 0°, and use horizontal and vertical polarization modes to slowly rotate clockwise for scanning tests. Stop rotating when a signal is found, adjust the sweep width and resolution bandwidth to the best (until the signal can be distinguished), record the signal level, occupied bandwidth and azimuth, and signal characteristics (also record the time to save the instantaneous spectrum and the maximum hold spectrum), and refer to the "Microwave Frequency Band Electromagnetic Environment Test Record Table" (see Table 4).

 

Table 4 "Microwave frequency band electromagnetic environment test record"

(5) Weak signal environment test

For some test points with weak signal strength or in some important test frequency bands, the sensitivity of the test system can be improved by adding a low-noise amplifier. First, turn off the spectrum analyzer, connect the low-noise amplifier as shown in Figure 1, then turn on the spectrum analyzer and perform the test according to step (4). In addition, since the test system becomes more sensitive after adding the low-noise amplifier, it is necessary to determine the authenticity of the received test signal when necessary. This article provides two methods for identifying true and false signals. Method 1: When the test signal is significantly higher than the background noise (usually more than 30dB), if the low-noise amplifier is removed and the tested signal disappears, it is determined to be a false signal. Method 2: Add a 10dB attenuator to the front end of the low-noise amplifier. If the amplitude of the original tested signal drops by much more than 10dB, it is determined to be a false signal.

(6) Background noise test

The background noise test can be combined with step (4) and step (5). However, in view of the consistency and comparability of the test results, the resolution bandwidth of the background noise test should be unified (RBW is 30kHz). For the value of background noise, the instantaneous level value should be read at the position without test signal in the required frequency band, and the background noise level should be recorded in the "Microwave Band Electromagnetic Background Noise Test Record Table" (as shown in Table 5).

 

Table 5 "Microwave frequency band electromagnetic background noise test record"

Main parameter settings of spectrum analyzer (for reference only, can be adjusted appropriately according to specific conditions during testing):

Frequency band range: the frequency band to be measured.

Resolution bandwidth: 100kHz.

Sweep width: Set according to the frequency band to be tested.

Input attenuation: 10dB.

Hold time: 30 seconds.

Scan time: Automatic.

It should be noted that the above is a general method for microwave frequency band electromagnetic environment testing. One cycle can usually complete the test of one frequency band, and the test of multiple frequency bands can be repeated from steps 3 to 6. After the test of a test point is completed, the basic conclusions of the test of each frequency band at the point at different time periods can be recorded in the "Microwave Electromagnetic Environment Test Record Data - Test Basic Conclusion Table" (as shown in Table 6).

 

Table 6 "Microwave Electromagnetic Environment Test Record Data - Test Basic Conclusion Table"

5 Test Requirements

 

(1) Test period requirements

In order to reflect the fading characteristics of communication channels in each frequency band, the test time for each item refers to the national standard and relevant regulations of the International Telecommunication Union (ITU) on radio monitoring, requiring that the effective test time for each region and frequency band should include different time periods such as morning, afternoon and evening.

(2) Convert the port voltage to antenna field strength

The data collected at the test site (background noise and signal level values) are all port voltage values ​​U of the spectrum analyzer, in dBm. When writing the test report for data analysis, they need to be converted into the field strength value E at the antenna port, in dBμV/m.

The specific method is as follows: considering the antenna K factor and the low-noise amplifier gain G (if no low-noise amplifier is added, it will not be considered), the loss d of the feeder and the adapter, and the conversion factor F (when the input impedance of the spectrum analyzer is 50Ω, F=107dB) and other factors, referring to the corresponding antenna technical indicators, through the conversion formula E=U+K-G+d+F, the port voltage value U (dBm) of the spectrum analyzer can be equivalent to the field strength value E (dBμV/m) of the antenna surface, and the power flux density can be converted according to GB 13616-92 "Electromagnetic Environment Protection Requirements for Microwave Relay Stations".

(3) List of microwave electromagnetic environment test equipment (see Table 7)

 

Table 7 Microwave electromagnetic environment test equipment list

6 Conclusion

The five aspects described in this article are the main contents of microwave electromagnetic environment testing. Testers can add the task source, test plan (including test location, personnel and time), test purpose, test content and equipment list to form a "××× microwave electromagnetic environment test plan", or add the task source, test purpose, test content, basic situation, data analysis, basic conclusions and suggestions to form a "××× microwave electromagnetic environment test report". The test methods and record forms provided by the author from a practical perspective are for reference only by relevant testers. In actual electrical testing work, specific situations should be analyzed specifically, and special situations should be handled specially. Do not copy and paste.