Interference Analysis Guide
Key words
WCDMA, interference, main, diversity, RTWP, inter-modulation
Abstract The document describes how to locate and solve interference issue in WCDMA network optimization.
Acronyms and abbreviations: Acronyms and abbreviations
Full Spelling
PIM
Passive Interactive modulation
RTWP
Received Total Wideband Power
BCCH
Broadcasting Channel
FNE
Fixed Network Element
AOA
Angle of Arrival
1.1 Judging Types of Interferences 1.1.1 Interference Types The interference includes internal interference and external interference.
The interference occurring on NodeB to the antenna-feeder system is internal interference.
The external interference includes in-band signal interference and out-band strong signal interference. The typical types are Personal Handy phone System (PHS) interference, repeater interference, and interference from handset interferer.
1.1.2 Criteria The interference belongs to external interference if it meets the following judgment criteria:
The interference to main or diversity is relevant. Namely, in terms of time, the interference to main or diversity trends similarly, and the difference between them is within 5 dB.
The external interference affects multiple cells that are geographically bordering.
In terms of time feature of RTWP, the external interference is mutational, the interference occurs at a regular point and in a regular period, and lasts for a regular period (exceptions are microwave interference, improperly configured gain of repeaters, so the RTWP is not mutational) The interference which is not external interference is internal interference, so it follows the internal interference processing procedures. Locating external interference takes more effort and time than locating internal interference. Therefore, if the interference is not confirmed to be internal interference, it must be rechecked. The inter-modulation interference which takes a high ratio in internal interference features typically as below:
The RTWP of main and diversity is usually irrelevant. If the RTWP is relevant, there must be special causes, such as the main and diversity are combined at some point.
The interference is related to traffic. The interference occurs less probably when traffic is lower.
The RTWP fluctuates sharply, as great as about 10 dB, or even greater than 10 dB.
The interference will last for a period, without mutational change, which is different from that of external interference.
In terms of time feature of RTWP, the RTWP changes irregularly. The inter-modulation usually meets one or more of the previous five features. If the five features are all met, it must be inter-modulation. For better understanding of the previous judgment criteria, the following examples provide direct phenomena of various interference from actual networks. Therefore no specific locating process is provided.
1.2 Equipment and Documents Needed In Interference Test 1.2.1 Sturcture of Interference Test The interference test uses the following structure of test equipment. Figure 1.1 Structure of interference test
Directional Antenna
1.2.2 Equipment and Documents 1.2.2 lists the equipment and documents used in interference test. Table 1.1 Equipment and Documents Equipment or document
Type of connector
Directional antenna
N-type female connector, For more info, see 4.1.3
Omnidirectional small antenna
SMA-type female connector, For more info, see 4.1.3
Equipment or document
Type of connector
Bandpass filter
N-type female connector, For more info, see 4.1.1
YBT250 spectrum analyzer
N-type female connector
1/2 jumper x3
N-type male connector
1/2 jumper x2
N-type male connector/SMA male connector
50 Ohm Dummy Load x2
N-type male connector
DIN-type male connector -> N-type female connector x2 DIN-type female connector -> N-type female connector x2 N-type dual-female connector x2 Laptop (installed with NodeB LMT software) GPS Compass Test car FNE map of sites Historic RTWP map of sites Distribution map of adjacent sites Camera PHS handset (if to locate PHS interference)
Preparation for different connectors is a little complex and patient work. It’s better try each connection before go to test. Otherwise, you have to waste your time on spot.
2
Locating Interference
2.1 Locating Internal Interference Locating internal interference includes initial location and on-site location.
2.1.1 Initial Location The initial location proceeds as below: Step 2
Check the configuration of diversity reception if you fail to observe the diversity signals. 2.1.1 shows the RTWP variation when the diversity reception is not configured. Figure 1.1 RTWP variation when the diversity reception is not configured
Step 3
If the uplink RF channel has not been adjusted, check whether the configured gain (especially TMAs are used) of RF channel is correct. It is better to adjust uplink RF channel gain so that these problems will not bother locating interference. To check UL RF channel gain , the following steps has to be taken:
Install dummy load to the port to be checked;
Start NodeB LMT, Input MML command: SET TXSW: TXSW = OFF to switch off
related PA in case of unexpected damage;
The RTWP normal value range is -105～-107dBm ( Tolerance ), Check whether measured RTWP value is in this range or not; If not, set attenuation of this channel to adjust RTWP to normal value. MML command: SET RXATTEN: ATTEN=***;
Uplink RF channel gain adjustment is not routine operation, which should be performed by RAN engineer when RAN engineer is available. Step 4
If a DCS1800M network and a WCDMA network are combined, you must check the frequency configuration with operators. Meanwhile you must check whether the third order intermodulation (2f1-f2 and 2f2-f1) of the combined DCS1800M frequency is within the RX band (1920 MHz to 1980 MHz). If yes, negotiate with operators to change the improper frequency configuration.
If the interference remains after the previous operations, you must locate interference on site.
2.1.2 On-site Location The on-site location proceeds as below: Step 5
Start NodeB LMT and measure the real-time RTWP of the cell to be located. This allows you to observe real-time RTWP variation after using consequent locating methods.
Step 6
If a DCS network is combined to a WCDMA network, you must know the DCS carrier features (the carriers on a channel, the channel number, and the channel where BCCH is) and mark the BCCH channel.
Step 7
If a DCS network is combined to a WCDMA network, you need adjust BCCH to the channel where interference is located under assistance by the operator according to the result of interference. The reason is that if BCCH does not use the problematic channel (The GSM network might transmit signals in both channel, but the BCCH uses only one channel), it is very hard to locate DCS-traffic-related interference.
Step 8
Knock every RF connector gently on the channel (especially the connectors of jumper, load, and antenna) and check the RTWP variation. If RTWP changes, the connector is problematic. Tasks to improve project quality, such as fastening connectors and reconnections, must be perform under cooperation of the operators' engineers. Ensure to power off power amplifiers of corresponding cells before performing tasks to avoid radiation injury.
Step 9
When the connector are normal and interference is present, use YBT250, filter, and directional antenna to check at WCDMA antenna whether interference signals are received (for requirements on filter and directional antenna, see Appendix 4.1 In special situations, you must customize the filter according to the local WCDMA receiver band and other radio network transmission frequency band). If YBT cannot detect special interference, you need change the NodeB antenna and check whether the interference is caused inside the antenna. If the interference still exists after changing antennas, turn to judgment of interference types.
Step 10
If interference signals are receives at the WCDMA antenna by using YBT250, filter, and directional antenna, you can solve the problem by Locating External Interference.
Step 11
If the interference cannot be located after repeated checks, solve it by Judging Types of Interferences. Stop on-site location and restore the original configurations.
Step 12
Record the previous locating steps in the form of "xx Interference Location Detailed Record".
If successful in locating the interference, you can summarize the problem in the form of interference location cases based on "xx Interference Location Detailed Record". Send the cases to the Headquarter for filing.
If failing in locating the interference, you can send the "xx Interference Location Detailed Record" to technical support engineers in the Headquarter for help.
2.2 Locating External Interference 2.2.1 Preparations before On-site Location It is hard to know when the external interference appears or disappears, so detailed preparations and analysis must be performed before on-site location. Otherwise, the on-site location will be less efficient.
2. Needed Data You need the following data:
The RTWP data for 7 (days, at least 3 days) x 24 (hours) of cells to be located The data is obtainable in "Collecting Data and Confirming Interference" section.
The MapInfo map of site distribution, the relative location of sites, and the distance between sites You can use Nastar to obtain these information.
Antenna azimuth and height of cells
Photos for surveying sites
Whether the cell to be located is the host cell of a repeater
The distribution of 2G and 3G repeaters around the cell to be located
The distribution of PHS BTSs around the cell to be located
The antenna-feeder structure diagram of the cell to be located
3. Needed Analysis and Initial Conclusion Analysis: the long-time feature and short-time feature of RTWP data for the cell to be located in different periods Conclusion: the locating time (the periods when interference occurs intensively is obtainable according to RTWP time feature. Analyze the following aspects:
Analyze the long-time feature and short-time feature of RTWP data for the cell to be located in different periods
Analyze the environment of the cell to be located with cell distribution diagram and surveying photo
Analyze the relativity of main and diversity of the cell to be located according to the antenna-feeder structure diagram
Use angle of arrival (AOA) to summarize the RTWP data of the cell to be located, the RTWP data of adjacent cells, antenna azimuth, and antenna height so that the location of the interference source can be estimated. Locate the direction of the interference source by cell antennas of multiple NodeBs. Draw on a map, the crossing point of the direction of each antenna is the interference source.
4. Methods and Procedures for On-site Location On-site location proceeds as below: Step 1
Start NodeB LMT and monitor realtime RTWP of the cell to be located for the features and time when the external interference occurs.
Step 2
Check the environment of the antenna for metal blockings, antenna of other networks or systems, the antenna distribution of other operators. Check the potential adjacent blockings to signals.
Step 3
Measure the interference strength, direction, and frequency spectrum by using YBT250, filter, and antenna.
Step 4
Find the rough location of the interference source. For more interference-locating methods and experience, see chapter Method and experience in external interference location.
Step 5
Fix the potential interference source according to the previous analysis.
Step 6
Verify the relationship between the interference and the state variation of the potential interference source (such as on, off, starting, and stopping) For the equipment that is controlled by the operator, such as repeaters, you can verify the relations between the equipment and the interference by powering on or off the equipment in a proper time. For the uncontrollable equipment, you need to wait to observe the interference.
Step 7
Record the previous locating steps in the form of "xx Interference Location Detailed Record".
If successful in locating the interference, you can summarize the problem in the form of interference location cases based on "xx Interference Location Detailed Record". Send the cases to the Headquarter for filing.
If failing in locating the interference, you can send the "xx Interference Location Detailed Record" to technical support engineers in the Headquarter for help.
3
Case study
3.1 Cases of Internal Interference 3.1.1 Multi-frequency Intermodulation Due to Load In an indoor distributed system, the 3G signals, 2G signals of the operator S, and 2G signals of the operator P are combined. The operator P uses the absolute radio frequency channel number (ARFCN) 747. The operator S uses the ARFCN 850 and hopping frequency ARFCN 815. 3.1.1 shows the variation of RTWP. Figure 1.1 Variation of RTWP due to load
The interference in the cell is caused by a load with loose connection. Once the load is touched, the RTWP changes sharply. The RTWP changes as below:
The main and diversity are irrelevant
The RTWP fluctuates sharply
The interference lasts for a period
The RTWP changes irregularly in terms of time
3.1.2 Multi-frequency Intermodulation Due to Improper Connection of Multiple RF The multiple RF connection involves duplexer, feeder, and jumper connector. The site is constructed with indoor distribution system shared by multiple operators. The antenna-feeder structure is complex. Wherein, multiple hybrid couplers, feeders, and jumpers are improperly connected, so the RTWP is as shown in 3.1.2. Figure 1.2 Variation of RTWP due to improper connection of multiple RF
The RTWP changes as below:
The RTWP fluctuates sharply
The interference lasts for a period
The RTWP changes irregularly in terms of time
3.1.3 Single Frequency Intermodulation Due to Improper Connection of Feeder and Jumper The 3G signals and 2G signals are combined. The 2G network uses only one channel number. Intermodulation occurs due to improper connection of feeder and jumper. 3.1.3 shows the antenna-feeder structure.
Figure 1.3 Antenna-feeder structure
3.1.3 shows the variation of RTWP due to improper connection of feeder and jumper. Figure 1.4 Variation of RTWP
The RTWP changes as below:
The main and diversity are irrelevant
The RTWP fluctuates sharply
The interference lasts for a period
The RTWP changes irregularly in terms of time
3.1.4 Multi-frequency Intermodulation Due to Interaction of 2G and 3G signals This is an indoor site, with 2G and 3G signals combined. It is an indoor distributed system shared with other operators. 3.1.4 shows the variation of RTWP due to interaction of 2G and 3G signals.
Figure 1.5 Variation of RTWP due to interaction of 2G and 3G signals
In 3.1.4, the main interference (in red) is caused by intermodulation of DCS signals and 3G signals at a connector. The diversity is not connected to antenna. The external signals near cabinet interferes diversity.
The RTWP changes as below:
The main and diversity are irrelevant
The RTWP fluctuates sharply
The interference lasts for a period
The RTWP changes irregularly in terms of time
3.2 Cases of External Interference 3.2.1 Sites Around Repeaters of Self-excitation Interference 3.2.1 shows the site distribution around the site 501800.
Figure 1.6 Site distribution around the site 12PKP012
In the network as shown in 3.2.1, a 3G NodeB 12PKP012 transmits a self-excitation signal every hour approximately. Therefore the uplink in multiple cells is interfered. The uplink interference varies according to the direction and the distance between the cell and the repeater. However, it is clear that the uplink interference occurs every hour approximately. Figure 1.7 Variation of RTWP in adjacent cells (1)
Figure 1.8 Variation of RTWP in adjacent cells (2)
Figure 1.9 Variation of RTWP in adjacent cells (3)
Figure 1.10 Variation of RTWP in adjacent cells (4)
Figure 1.11 Variation of RTWP in adjacent cells (5)
Figure 1.12 Variation of RTWP in adjacent cells (6)
The site 501800 is an indoor site with a single antenna.
The RTWP changes as below:
The main and diversity are relevant
The interference influences multiple cells that are close to each other
The interference is mutational
The interference changes with a regular internal
3.2.2 Uplink Interference to Host Cell Due to Repeater Self-excitation The NodeB 45680 uses a 3G repeater. The host cell of the repeater is the first cell 54291 of the NodeB 45680. The occurrence time of self-excitation of the repeater is irregular. 3.2.2 shows the RTWP variation of cell 54291.
Figure 1.13 Variation of RTWP
The RTWP changes as below:
The main and diversity are relevant
The interference is mutational
3.2.3 Uplink Interference to Host Cell Due to Improperly Configured Gain of Repeater and Selfexcitement The gain of the repeater is 90 dB. 3.2.3 shows the RTWP variation of cell 45680. Figure 1.14 RTWP variation of cell 45680
After adjustment of the repeater gain to 70 dB, the RTWP becomes normal. The RTWP variations feature the same as that of improperly configured gain of repeater. Namely, the interference is strong and stable.
3.2.4 Uplink Interference to 3G Antenna Due to Close Radiation from 2G Repeater Antenna The 3G antenna is interfered by a 2G repeater antenna another operator. The 3G antenna uses space diversity. As
shown in 3.2.4, the 3G antenna is a diversity antenna and the main antenna is far from 2G antenna. Figure 1.15 Antenna location
Figure 1.16 RTWP variation
The RTWP changes as below:
The main and diversity are relevant
The interference is mutational
3.2.5 RTWP Variation Due to Passing Trains The NodeB is close to the railway with intensive trains passing by. 3.2.5 shows the site location near the railway.
Figure 1.17 Site location
Figure 1.18 RTWP variation of a NodeB near railway
3.2.6 Uplink Interference Due to State Switch of Indoor Air-conditioner Controller 3.2.6 shows the uplink interference fluctuation upon state switch of indoor air-conditioner controller.
Figure 1.19 RTWP variation due to indoor air-conditioner
3.2.7 Uplink Interference Due to Power On or Off of Outdoor Air-conditioner of Other Operator 3.2.7 shows the RTWP variation due to power on or off of outdoor air-conditioner of other operator. Figure 1.20 RTWP variation due to power on or off of outdoor air-conditioner of other operator
3.2.8 Uplink Interference Due to Power On or Off of Indoor Emergency Lights 3.2.8 shows the RTWP variation due to power on or off of indoor emergency lights, marked in red.
Figure 1.21 RTWP variation due to power on or off of indoor emergency lights
3.2.9 Uplink Interference with Period of 200 Seconds This uplink interference is probably due to air-conditioner compressor, but this cannot be confirmed due to property restriction. 3.2.9 shows the long-time RTWP variation. Figure 1.22 Long-time RTWP variation
3.2.9 shows the short-time RTWP variation.
Figure 1.23 Short-time RTWP variation
The RTWP changes as below:
The main and diversity are relevant
The interference is mutational
The interference changes with a regular internal
3.2.10 Interference Caused by the Spectrum Analyzer YBT250 at 1924.3 MHz 3.2.10 shows the interference caused by the spectrum analyzer YBT250 at 1924.3 MHz when the directional antenna approaches the YBT250. Figure 1.24 Frequency spectrum when the directional antenna approaches the YBT250
When locating interference, pay attention to the feature of YBT250.
3.2.11 Uplink Interference Due to Transmission Line 3.2.11 and 3.2.11 show the uplink interference due to transmission line.
Figure 1.25 Uplink interference due to transmission line (1)
Figure 1.26 Uplink interference due to transmission line (2)
3.2.12 Interference Like Self-excitation 3.2.12 shows the long-time RTWP variation of the interference like self-excitation.
Figure 1.27 Long-time RTWP variation of the interference like self-excitation
3.2.12 shows the short-time RTWP variation of the interference like self-excitation. Figure 1.28 Short-time RTWP variation of the interference like self-excitation
3.2.12 shows the frequency spectrum feature. Figure 1.29 Frequency spectrum feature
3.2.12 is a static figure. Its frequency spectrum features as below:
The scanned frequency ranges from 1914 MHz to 1951 MHz
The amplitude of different frequencies is different
The frequency jumps after a scanning period The RTWP changes as below:
The main and diversity are relevant
The interference is mutational.
4
Appendix
4.1 Key Device Parameters 4.1.1 Filter The bandwidth of the bandpass filters used for uplink/downlink electromagnetic interference tests is 60 MHz. For the specifications, see 4.1.1. The bandpass filter allows the signals within the useful frequency band to pass and suppresses interference signals beyond the useful frequency band. 4.1.1 shows a bandpass filter. Generally, the two ends of the bandpass filter can serve as an input or output end. Figure 1.30 Bandpass filter
Table 30.1 Specifications for uplink and downlink bandpass filters Operating Frequency (MHz)
1920 to 1980
2110 to 2170
Insertion Loss
0.7 dB at 1920 MHz
0.9 dB at 2110 MHz
0.4 dB at 1950 MHz
0.3 dB at 2140 MHz
0.7 dB at 1980 MHz
0.7 dB at 2170 MHz
–3-dB Bandwidth (MHz)
1912–1987
2106–2177
Out-of-band suppression
≥ 70 dB (1900–1999 MHz)
≥ 70 dB (2090–2192 MHz)
Operating Frequency (MHz)
1920 to 1980
2110 to 2170
VSWR
< 1.15
< 1.15
Group Delay (ns)
30–50
30–50
4.1.2 Low-Noise Amplifier Although a low-noise amplifier (LNA) is built in the YBT250, the noise figure is so high that it is difficult for the receiver sensitivity to meet the requirement. Therefore, an external LNA is required to improve the receiver sensitivity of the tester. For a cascade network, with a high-gain amplifier installed at the front end of the system, the noise figure of the system depends on the noise figure of the first LNA. Therefore, a ZRL-2400LN low-noise amplifier of Mini-Circuits is selected. The gain is 25 dB and the noise figure is 1.2 dB. However, an extra power supply is required for the LNA. Table 30.2 Specifications for the LNA
4.1.3 Antenna An omni-directional antenna can be used for electromagnetic interference measurement, but it is not favorable for locating interference sources. It is recommended that a directional antenna be used to locate interference sources. The higher the antenna directivity is, the higher the gain and the search capability are. Common directional antennas include plate antennas, Yagi antennas, and log periodic antennas. For convenience, only a directional antenna is used for tests in different directions. A Yagi antenna is used for narrowband and high-gain short-wave communication. A Yagi antenna is simple, light but solid, and convenient to feed, however, the frequency band of such an antenna is narrow and the resistance to interference is low. A log periodic antenna is a broadband antenna or a frequency-independent antenna. Compared with other high-gain antennas, a log periodic antenna has a higher directivity and a larger attenuation to signals in unexpected directions. Therefore, a log periodic antenna is preferred in interference tests. A log periodic antenna (see 4.1.3) made by Shenglu Antenna Co., Ltd. is used. For the major technical specifications, see 4.1.3.
Figure 1.31 Log periodic antenna SL14088A
Table 31.1 Major technical specifications of a log periodic antenna Major Technical Specifications Model
SL14088A
Frequency Range
806 MHz to 960 MHz, 1710 MHz to 2500 MHz
VSWR
< 1.5
Input Impedance
50 Ω
Gain
11 dBi
Front-to-Back Ratio
12 dB
Horizontal Lobe Width
50°±10°
Vertical Lobe Width
40°
Polarization Mode
Vertical or horizontal
Maximum Power
50 W
Grounding Mode
DC grounding
Input Interface
N female connector
If no log periodic antenna is available, a Yagi antenna can be used. 4.1.3 lists the major technical specifications of a Yagi antenna manufactured by Shenglu Antenna Co., Ltd. Table 31.2 Major technical specifications of a Yagi antenna Major Technical Specifications Model
TDJ-1800/2000B14G13
Frequency Range
1710 MHz to 2150 MHz
VSWR
<1.5
Input Impedance
50 Ω
Gain
13 dBi
Front-to-Back Ratio
18 dB
Major Technical Specifications Lobe Width
41°
Polarization Mode
Vertical or horizontal
Maximum Power
150 W
Grounding Mode
DC grounding
Input Interface
N female connector
An omni-directional vehicle mounted antenna (see 4.1.3) manufactured by Shenglu Antenna Co., Ltd. is used. 4.1.3 lists the major technical specifications. Figure 1.32 Omni-directional vehicle mounted antenna SL15209A
Table 32.1 Major technical specifications of an omni-directional vehicle mounted antenna Major Technical specifications Model
SL15209A
Frequency Range
880 MHz to 965 MHz/1710 MHz to 2170 MHz
VSWR
<2.0
Input Impedance
50 Ω
Gain
≥ 3 dBi
Polarization Mode
Vertical
Maximum Power
50 W
Input Interface
SMA-J
4.2 Method and experience in external interference location 4.2.1 Middle location Determine the possible location of interference according to RTWP statistics and environment. Perform bidirectional test around the interference source to approach the source. This is called the middle location. Figure 1.33 Schematic drawing of middle location
4.2.2 Two-point location The precaution for this method is that you must know the approximate interference direction. In the direction, measure the signals to compare the signal strength in two selected spots. Locate the interference by calculating the variation of interference strength. To use the variation of signal strength for interference location, you must know the direction and approximate location of interference. Then move a omnidirectional antenna to the interference and judge the location relationship between the omnidirectional antenna and the interference. Finally fix the specific location of interference near the interference source by using the directional antenna. Figure 1.34 Schematic drawing of two-point location
4.2.3 Method of Locating the Field Strength Change Step 8 Step 9
Determine the interference direction and the rough position. Move an omni-directional antenna towards the interference source and determine the position relationship between the speed of signal change and the interference source.
Step 10
Use a directional antenna nearby the interference source to find out its specific position.
4.2.4 Case Study of Locating an Interference Source In an urban area, the long-distance direction finding is usually accurate. However, after you approach the interference source, it is difficult to determine its position on the ground owing to strong signal reflection and refraction in a short distance. Tests are usually conducted at crossroads where high buildings and large advertisement signs stand and signal reflection and shielding occur. Therefore, when you move towards the direction finding direction, you should pay close attention to the change in the field strength. If the field strength decreases, the direction finding direction is very likely incorrect. Owing to the reflection and refraction caused by some high buildings, the field strength within a small range far away from the interference source increases. This may lead you to the wrong direction and wander there for a long time. In such a case, it is important that you identify the problem and find another direction finding point to locate the interference source. A great error exists in direction finding near railway tracks or high-voltage lines. You should avoid performing direction finding near such places. You can adopt the cross location method and field-approaching method, but avoid climbing up buildings. The probability of locating the interference source is high on a high building; however, it is not recommended that you climb up the building to locate the direction. If you are not familiar with the terrain, you may make mistakes in identifying the point. It is also difficult to contact the occupants of the building before climbing the building. It is advisable to first determine the rough direction of the interference source at the selected place, and then approach the interference source according to the field strength change in a drive test. Owing to the effect of fast fading during movement of the vehicle, the field strength is not steady at times. You should pay attention to the average and maximum field strengths when locating an interference source. At crossroads, especially an overpass (the relative relief is high), you should slow down to reduce the effect of fast fading so that you can accurately locate the direction of interference source. With the cooperation of the YBT250, the log periodic antenna can achieve good direction finding results in a far field or near field. As you can read the signal level during a drive test, you can determine whether you are approaching or moving away from a signal source. The effect is considerably satisfactory. The YBT250 can simultaneously display the signal average and the signal level. During direction finding, you do not need to use it to memorize the levels in different direction, but directly use the level value and the average value to quickly locate the direction. The combination of the YBT2500 with a log periodic antenna does not apply to an intermittent interference source. When you approach the interference source, especially signals in a building, it is satisfactory to use the audio method of the YBT250. You can judge the specific position of a signal source according to the sharpness or flatness of the sound generated by the YBT250. The Okumura's empirical formula for propagation loss is as follows: Lm = 69.55 + 26.16lOg(f) – 13.82log(hb) – a(hm) + [44.9 – 6.55lOg(hb)]lOg(d) In this formula, it can be seen that the receiving level of signals during interference source finding is related to only one variable—distance. The signal level changes significantly near the interference source. Therefore, when the signal level is low, determine the direction of the interference source, drive the vehicle fast and compare the field strengths. When the signal level is high, approach the interference source in the given direction until the level is above –60 dBm. Then, monitor the field strength as you walk in the area until the field strength is –50 dBm. Then, climb up the building to locate the source of interference. It is unnecessary to use an antenna operating in the test band. As direction finding only involves the comparison of signal strength, it is sufficient if the antenna used can receive signals. Thus, the time for replacing antennas can be saved. You need to ensure that the difference between the frequency band in which the antenna operates and the frequency of the signals to be searched should not be too great. When searching for an interference source, you should comprehensively consider, analyze, and determine the direction indication, strength, and surrounding environment. However, a rapid change in the field strength is the most important factor when searching for the interference source in a complex environment. If the field strength change is insignificant even near the interference source, the monitoring device may be in the saturation state. In this case, you can increase the attenuation at the input end of the monitoring device or remove a normal antenna from the device and use a short conductor as an antenna to accurately locate the interference source.
