HPR4526: Baofeng and SDR++

Hacker Public Radio

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Goal: send a voice signal on VHF or UHF, receive and record with SDR via RTL dongle

Core Problem and Resolution

An issue may have been a hardware I/Q Imbalance in the RTL-SDR, which suppressed the desired signal, resulting in very low output volume that required excessive manual amplification.

• Initial Problem: Very quiet, barely audible recorded audio, initially on UHF 433 MHz
• Key Discovery: An I/Q imbalance caused a repeating signal "image" in the spectrum,
• Frequency Change: Switched to the VHF 145.500 Mhz band,
• Gain Override: To overcome the defect and the failing AGC, gain stages were manually adjusted:
• Squelch: Squelch (SQL) was set to OFF (0) to prevent the software from muting the quiet, legitimate signal.
• Adjusted the final Sound Output/Recording Gain on the host OS to match the maximized digital output, achieving audible voice.

Summary of Key Radio and Software Settings

Transmitting Radio (Baofeng UV-5R):

• Operation: Simplex (direct radio-to-SDR).
• Mode: Narrowband FM (NFM).

Receiving Software (SDR++):

• AGC: Disabled (Crucial override).
• RF/Tuner Gain: Adjust (Inject best signal power).
• AF Gain: Adjust (Produce loudest digital audio).
• Squelch (SQL): OFF (0) (Prevent muting of quiet signal).
• Filter Bandwidth: 12.5 kHz(Correct setting for NFM).

The ultimate resolution was forcing input and output gain in SDR++ and volume adjustment in Audacity

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