September 6, 2025, at 许
My base station is experiencing strong, unidentified signals. The signal characteristics are a "crackling" sound with fluctuating frequency and amplitude, affecting the 437-440 MHz range. Repeaters, direct-to-sky antennas, and satellite communication are all severely affected, making amateur radio operation almost impossible, especially my regular UHF DX communications.
The interfering signal fluctuated wildly within the range, causing significant disruption. I was extremely frustrated and furious, and I was determined to find the source of the interference offline.
However, recently I've been very busy with my studies, mostly writing assignments or working on them, and the only time I had free was to make a CJU antenna.
Finally, I couldn't take it anymore. The day before yesterday, I asked my teacher in advance about the homework content, and then completed all four subjects' assignments. Then, I went home to finish the remaining two.
Finally, I have time to investigate the interference!
Surveying equipment
Receiver: RTL-SDR V4 (a clone version, used as is)
Antenna: Homemade standard CJU antenna (center frequency 435 MHz; gain 4 dBi; directional pattern resembling a teardrop, can only be used with the large-diode method)
Terminal: Using a phone to download SDR++ (I originally intended to bring my laptop, but it was too large. I considered using the TinySA Ultra, but its sensitivity and dynamic range are similar).
(P.S.: The main support beam of the antenna, as well as the file cover used for obtaining a new license, are included.)
Next comes what everyone enjoys and finds interesting.
Surveying process
First, the 438 MHz direction-finding measurement I'm currently taking shows that the direction is slightly west of north, as measured from the building rooftop. However, because the signal strength fluctuates constantly, the data quality is questionable. While I obtained some data during a relatively stable period (but further down, I realized this wasn't accurate).
Subsequently, while on my way to have dinner at my grandmother's house, I turned on the direction finder. There were many high-rise buildings nearby, and the distance to potential interference sources was sufficient, resulting in significant reflections and a wide range of scattered frequencies. This made it very difficult to determine the direction accurately, so I relied solely on the changes in amplitude within the measured path to make a simple directional judgment.
Based on the amplitude variations along the westward direction, I have identified the source of interference as being located to the west (approximately the same distance as moving away from the radio).
Subsequently, I realized that relying on its fluctuating frequencies for analysis was unreliable and extremely difficult, so I began to look for signals with a consistent frequency.
After searching, it was finally found in the 450 MHz frequency band. Due to the unusual frequency (this frequency apparently is not a standard domestic data transmission frequency), subsequent spectrograms will be marked.
Disclaimer: This testing activity only detects the signal from this interference source, using SDR equipment without transmitting capabilities and not belonging to amateur radio stations. The interference source exhibits similar signals in the 430 MHz band. The purpose of this activity is to eliminate harmful interference from amateur radio operations and promote orderly operation of amateur radio, all monitoring is for interference detection and analysis purposes, and there are no issues with receiving and transmitting non-amateur radio signals.
Once I knew the correct frequency, it became much easier. I started walking north from my grandmother's house and began measuring the direction at a crossroads.
After eliminating multiple reflections from the building structures, the antenna was finally pointed slightly westward, towards north.
Therefore, we proceeded north along Highway 3, making a bearing measurement at each intersection. At each intersection, the antenna direction gradually moved towards the west.
On a dead-end road, the antenna pointed towards north-northwest, but the road was impassable; I had to return to the previous intersection and proceed west, then continue straight on Jingcheng Road.
As I approached the local farmers' market, the environmental conditions were very poor, with excessive reflections that made it difficult to distinguish between real signals and reflections. Therefore, I chose the direction with the largest positive signal, which was due west.
As I was moving west, I noticed something was wrong – a sudden and dramatic drop in price, which indicated that I had made a mistake.
I stopped in an open area and measured again, pointing the antenna towards northeast.
We drove through and around Zhuang Village, taking a very long detour that eventually led us back to where we started.
However, fortunately, the spacious environment allowed me to obtain more reliable data.
I have combined data from Tianzailu Village and Dantu Road, using the two-line intersection method, to pinpoint the location of the interference source at the intersection of Industrial Road and Construction Road.
If you continue reading further down, you will see that these two sets of data are very reliable and the results are highly accurate.
We arrived at this intersection, and the signal strength was very high. I adjusted the LNA gain to 0, but I still received a signal.
However, I took a detour to avoid the road that was causing interference.
The antenna pointed towards the factory on the northeast side, which we locals call a waste-to-energy plant. However, there were also peaks in other areas, and these are undoubtedly reflections. But this is from my perspective as an observer looking back at the entire process. When I was measuring, I didn't distinguish between these reflected signals. This is because one location had a signal strength that exceeded the direction towards the factory. The trees obscured my view, and I didn't examine it closely enough. In reality, several buildings formed something like a concave mirror (which could also be considered a crude satellite dish), causing the signals to converge and then reflect back to me.
I received a reflected signal from the factory, and the source of the signal was an incorrect conclusion coming from the southwest direction.
I started moving westward, but it didn't seem to be helping at all. When the gain reached zero, moving westward actually increased the gain instead of decreasing it, and also reduced the amplitude of the spectrum.
I found a clear, unobstructed view of the enemy's position. Wow! It was slightly east of north!
This thing really messed with me (actually, it was four factories located at the corners of a street intersection, creating a very complex environment).
I returned to that intersection, and this time I identified the waste-to-energy plant as the source of interference. I drove north and arrived at the entrance of the waste-to-energy plant.
This is incredible! It's exceeding expectations! The mirror images are even appearing!
I'm setting the gains to 0, and the spectrum is turning red!
With a racing heart and trembling hands, I lowered the frequency to 438.500.
Indeed, this interference was sweeping across the 438 frequency band, producing a very loud "clicking" sound, and the spectrum showed a deep red.
You're right, I've been looking for you for so long. It's you! Your repeater is causing interference and making it impossible for me to contact DX stations. The satellite is also affected by the noise. I've mentioned this many times before.
My antenna was firmly pointed at the factory, and I smiled with satisfaction – the mystery was finally solved!
I immediately asked my mother to record evidence, and I also submitted a report to the radio center afterward.
Summary
This is likely interference from a data transmission device at the waste power plant, possibly due to a malfunction. The next step is to attempt coordination or directly report it to the Zhengzhou Radio Center for handling (I'm not sure if reporting it will result in a fine, and this frequency seems to be associated with a specific band).
This was my second attempt at triangulation (the first being the "Black Broadcast" triangulation).
The UHF frequency band typically uses direct propagation, making it easy to aim, but the signals are very susceptible to reflection and convergence, which also poses a challenge for ranging techniques.
I would like to share my experiences with you all here.
First, in terms of positioning method, UHF band initially favored walking-based proximity.
As the name suggests, the key to approaching a target on foot is to get close. As shown in the figure, this is a comparison between the actual direction and the direction measured at the base station, which shows some deviation. This deviation may be caused by reflection or limitations in equipment and operation.
However, when I was targeting the road between Duotou Road and Tianzha Village, as shown above, the results were very accurate. This is because we eliminated more complex factors along the way.
Then, selecting the location for the measurement point.. A clean, unobstructed environment is needed. Of course, it doesn't need to be too strict; leaving a few 70m gaps is sufficient, and even a relatively sparse arrangement will work fine, without causing excessive reflection.
Signal confirmation: We need to confirm whether it's a genuine signal.. When using UHF direction finding, we inevitably receive reflected signals. I have a few methods to distinguish between these and actual signals.
First: Antenna polarization. Walls and other reflections are imperfect, similar to diffuse reflection, so the polarization will change. Rotating the antenna usually doesn't cause a significant change. However, the antenna of a real signal source has a constant polarization, and rotating the antenna significantly degrades the signal quality. But it is possible that the antenna of the opposing device is special. This is just one method, and we don't know what kind of antenna the other side is using. However, most are similar to dipole antennas, which default to vertical polarization, and then make minor adjustments.
Second: Change direction. Point the antenna backward. This approach has two intentions. First, to utilize the back lobe of the antenna's radiation pattern and confirm its presence. Second, to address direct reflections, which are less common and typically only occur in close proximity.
Third: Change the measurement location and use other locations to assist in determining the correct direction. For example, if I find that the initial measurement is incorrect after taking a wrong turn, I will re-measure from a different location.
In summary: When surveying, you need to move around and not just stay in one place. If there is too much reflection, don't linger; instead, look for a better location (an open area or a high vantage point). Don't be afraid to make mistakes when surveying; if you notice a significant drop in signal strength, stop nearby and carefully measure the area. This will help you find a better location. Ultimately, your goal is always to point towards your target. Make good use of the antenna's radiation pattern. Be able to analyze the environment and identify potential reflection points. You don't need the best equipment; an antenna plus an SDR (Software Defined Radio) is sufficient. Most importantly, practice surveying in different locations and gain practical experience. There is no substitute for hands-on experience, and you can't learn everything from reading articles.
For any follow-up regarding interference coordination and resolution results, I will continue to monitor the situation.
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