RTL-SDR for VHF Air Band AM on GNU Radio – Telecommunications, Navigation & Electronics

Introduction

In previous posts we looked at various ways to use the RTL-SDR with GNU Radio on either Raspberry Pi4 or Windows. In this post we look at the RTL-SDR for VHF Air Band (108-137MHz) reception on GNU Radio. The latest version of GNU Radio >=3.8xx is used (Ref.1).

RTL-SDR for Air Band Setup

Fig.1 RTL-SDR Version 1 with MCX/SMA/PL259 Connectors

The RTL-SDR is an incredibly versatile receiver and can be used to receive the VHF Air & Marine bands (Ref.2). The receiver comes in various versions and you need to match the antenna connector to the receiver input connector (Ref.3). Figure 1 shows the original RTL version1 that has an MCX input connector. The VHF antenna comes with a UHF PL-259 connector, so two adapters are required: UHF/SMA & SMA/MCX (Ref.4).

GNU Radio Reception

Fig.2 GNU Radio VHF Air Band AM Receiver Schematic

Figure 2 shows the GNU Radio schematic for a VHF Air Band AM receiver. The theory for the AM demodulation is covered in a previous post (Ref.5). The RTL is tuned to the AM carrier frequency, and baseband I & Q samples are demodulated by simply going into a complex/magnitude converter which removes any phase difference between the RTL local oscillator and the actual carrier frequency. The baseband samples are Low Pass Filtered. The RTL block has a variable rf_gain from 0 – 40dB. This is adjusted for minimum required gain to minimize intermod/spurious. The QT Gui Sink is very versatile and has a spectrum, waterfall and scope display. The audio sink block at 48KHz connects to the laptop audio speaker. Figure 6 YouTube video shows the reception of the local YYZ ATC arrival channel on 132.800MHz.