Build a low-cost radio interference direction-finding device and network.
The purpose of writing this post is to help more amateur radio operators who are suffering from interference and spectrum congestion caused by cranes, security personnel, hotels, and other entities occupying amateur radio frequencies. This interference often directly occupies the uplink and downlink frequencies of local repeaters, seriously infringing on the legitimate rights of amateur radio enthusiasts.
Due to the interference affecting our local赣州峰山 repeater,龙南高山 repeater, and 南康南山 repeater (both uplink and downlink frequencies), we developed this project with the simple intention of contributing a small amount to our amateur radio community.
We need to protect our own legal rights, and when determining the location of interference, we can firstOfflineWe would negotiate with the other party or their representative, as many radio users are unaware that "radio spectrum is a crucial national strategic resource." Providing offline legal education to them is the best way to reach an agreement. As enthusiasts, we understand better than anyone else that the frequencies in this amateur band are likely not entered by radio users themselves, but rather by the manufacturers or sellers of radios. Our goal is simply to eliminate interference from this amateur band; we don't want to eliminate interference for the sake of eliminating interference. If we can negotiate a solution with non-malicious parties, that would be ideal. If negotiation is not possible, we can seek assistance from the relevant radio management authorities.
This post may not be entirely accurate, so please point out any errors to me. I have also made the solution open-source on the Loolu Open Source Community when writing this post.Links to the Lihua Open Source Community
The original price comes from an average price on Taobao, which may fluctuate. Please verify it yourself.
We would like to thank the open-source amateur radio project, "BI4LU attenuator," the "Lutron Open Hardware Platform" attenuator, and the antenna configuration provided by BH2RSJ. We are grateful for their contributions.
Note: Hand assembly requires a soldering iron and solder wire.
First, for direction finding, you can use many receivers that display field strength to determine the direction. Specific information such as frequency and sub-audio frequencies can be obtained using handheld radios like QuanSheng UV-K5 or UV-K6, which have spectrum analysis capabilities. The UV-K5 and similar models are relatively inexpensive, costing less than 100 RMB, or you can use an RTL-SDR with a more expensive TinySA spectrum analyzer.
The overall survey link is:
Surveying instrument (e.g., handheld device) → Attenuator → Surveying antenna → Interference source
The attenuator can be obtained through the "Lichuang Open Source Community" project (thank you to BI4LU for the open-source version), or purchased from "Same Name" (at a price of 15 yuan). If you replicate it yourself, the attenuator's PCB can be made using Lichuang's free prototyping service (FR4, double-sided board, with lead-free tin plating), and all the resistors used can be purchased in bulk from Taobao (starting at 100 pieces for 100 yuan), with a cost of around 12 yuan each. (Metal film resistor, 1/4W, resistance values: 43ohm, 82ohm, 91ohm, 150ohm, 33ohm, 330ohm, 18ohm, 11ohm), but it is possible to manufacture attenuators in bulk. The switch used（SW-TH_SS-22D07）, 10 units at around 2 yuan each on Taobao. The SMA interface (SMA-KE angled offset, external thread, internal hole) costs approximately 0.5 yuan per unit. You can produce several more to share with local enthusiasts and expand the directional measurement network, thereby improving accuracy.
The directional antenna can be constructed using a Yagi antenna configuration. I will manually construct a Yagi antenna using PVC pipes, and this manual construction method is already very mature. There are many established methods to reference. I will not elaborate further on the manual construction of a Yagi antenna here. Alternatively, I have replicated the directional antenna design (inspired by BH2RSJ), and performed simulation using MMANA-GAL_Basic. The parameters and specifications are as follows:
In practice, the directional performance of this antenna is quite good when used with a nulling array. The directional antenna scheme I replicated costs 173 yuan/5 pieces, or 34.6 yuan per piece, plus the SMA interface on the board, which brings the price down to around 38 yuan per piece. As the saying goes, many people make great things happen. Similarly, it can be shared with local enthusiasts to expand the directional network, improve directional speed and accuracy.
These devices need to be connected using SMA coaxial cables. You will need to provide your own cables.
Here, if the surveying equipment has been prepared and you have access to available attenuators and surveying antennas, you can begin to manually identify interference sources.
If you already have a soldering iron and solder wire, here's the Bill of Materials (BOM) and estimated cost:
Decay attenuator PCB – Jia Li Chuang offers free prototyping
Decay resistor (metal film resistor) 1/4W
Values: 43 ohm, 82 ohm, 91 ohm, 150 ohm, 33 ohm, 330 ohm, 18 ohm, 11 ohm Approximately 12 yuan.
Onboard SMA interface 0.5 yuan/piece
Testing PCB antenna – Jia Li Chuang prototype – 173 yuan, 5 pieces
Board-mountable SMA connector (SMA-KWE, right-angle, external thread, internal hole) – 2.9 yuan/piece
SMA coaxial cable (assuming 10 cm per cable, 1 m per cable)
5 yuan / 10 cm, 7 yuan / 1 m
Total: Approximately 205 yuan.
Then, you can obtain five sets of directional finding antennas, which are available to enthusiasts, and start performing triangulation-based directional finding using parameters such as field strength and spectrum.
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