Today's work involved a detailed RF hardware analysis to understand the detection range limits of the current scanner setup and diagnose an intermodulation interference problem.
The platform monitors P25 uplink frequencies — the transmissions from handheld and vehicle-mounted radios carried by officers in the field, not the high-power downlink transmissions from fixed tower sites. This is a deliberate design choice: uplink monitoring gives us the officer's position, which is the information users actually want. But it comes with a physics penalty. Tower transmitters broadcast at 50–100 watts from elevated antennas. Portable radios transmit at 1–3 watts from ground level. The uplink signal is inherently weaker and detectable at shorter range.
I modeled the detection envelope using measured receiver sensitivity, the narrowband processing gain from the FFT filter, LNA characteristics, and realistic P25 uplink transmit powers. With the current hardware configuration, the estimated detection range is roughly 0.3–1.5 miles for portable radios and 1–4 miles for vehicle-mounted mobile radios, depending on terrain and whether a low-noise amplifier is in the receive chain.
Diagnostic testing revealed an intermodulation problem: strong downlink signals from nearby P25 base stations in the 851–869 MHz band are overloading the SDR's front end, raising the noise floor on the uplink frequencies around 806–824 MHz. The interference is antenna-coupled — it persists even without any amplification in the receive chain — and reduces effective sensitivity by approximately 5 dB. The solution is a bandpass filter that passes the uplink band while attenuating the downlink energy by 50+ dB before it reaches the receiver. Testing confirmed that the LNA hardware was not contributing to the problem, narrowing the fix to a single passive component.