Radio Fence: Unauthorized Signal Localization on Amateur Radio Band

Abstract

Misuse of amateur radio frequencies by non-licensed transmitters leads to interference, problems and security threatsfortheamateurradiocommunicationservices.Inthiswork,weintroduceRadioFencethatisalowcostcompactradio direction finding (RDF) solution that helps locate suspicious radio frequency transmissions in the amateur radio bands. Radio Fenceuses directionalantennas,software-definedradioreceivers,GPSsensor,andsignalangleestimationalgorithms to determine the direction of arrival of RF signals.. With the help of triangulation and trigonometric techniques, the geographical location ofthesignalsourcecanbecalculated.TheprototypeimplementationofRadioFenceincludesrdf(radio direction finder system and host system. Experimental evaluation demonstrates the feasibility of accurate signal localization for educational and regulatory use cases.

Introduction

Amateur radio is widely used in communication and emergencies, but unauthorized signals can cause interference. Professional direction-finding systems are accurate but costly and complex, making them unsuitable for student-level projects. To address this, the project proposes a simpler RDF system that estimates signal direction using signal strength measurement, antenna rotation, and basic triangulation geometry.

Methodology

The system uses Raspberry Pi-based RDF nodes with an RTL-SDR receiver, GPS module, and a rotating parabolic antenna driven by a stepper motor. As the antenna rotates, it measures signal strength at different angles, and the direction with the highest strength is used as the estimated signal source.

Each node sends real-time data (angle of arrival, GPS location, signal strength, frequency, bandwidth) to a central system using UDP communication. Software is built using Python, SDR tools, motor control logic, and a PyQt + CesiumJS interface for visualization.

For localization, triangulation is used, where multiple nodes intersect signal direction lines to estimate transmitter location.

Results

A working prototype was successfully developed that can:

• Detect signal direction in real time
• Perform antenna-based scanning
• Track GPS location of nodes
• Transmit data to a central system
• Visualize signals on a 3D map (CesiumJS)

A parabolic reflector antenna improved direction accuracy by focusing signal strength, making it more suitable than earlier antenna designs. Testing showed better results with multiple RDF nodes compared to a single node, though accuracy still varies due to noise and environmental interference.

Discussion

The system is low-cost, portable, and practical for educational use, but has limitations such as:

• Sensitivity to signal reflections and noise
• Need for antenna calibration
• Reduced accuracy with only one node

Conclusion

In this project, we built a simple system to detect and locate unauthorizedradiosignals.Theideawastomakesomething affordable but still useful. Theresultsshowthatitispossibletoestimatesignaldirection and location using basic components like SDR, GPS and directional antennas. While it is not perfect, it works well enough for learning and small-scale applications.

References

[1] ARRLHandbookforRadio Communications. [2] GNURadioDocumentation. [3] RTL-SDROfficialDocumentation. [4] ITUGuidelinesonRadioDirectionFindingSystems. [5] Amateur Radio Regulatory Framework, India.

Copyright

Copyright © 2026 Arkay Kudali, Sahil Lotlikar, Harikrishna K M, Vinayak Dewoolkar. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.