RTL-SDR for VHF Air Band AM on GNU Radio

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
Fig.3 VHF Air Band Receive Spectrum
Fig.4 VHF Air Band Receive Waterfall
Fig.5 VHF Air Band Demod Scope Output

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.

Fig.6 YouTube Video RTL-SDR for VHF Air Band AM on GNU Radio

Please send your comments, questions and suggestions to:
jclark@clarktelecommunications.com

YouTube Channel
YouTube Channel

References

1. – “GNU Radio Installation”
https://wiki.gnuradio.org/index.php/InstallingGR

#2. – “RTL-SDR for VHF Air & Marine Bands”
https://jeremyclark.ca/wp/telecom/rtl-sdr-for-vhf-air-marine-bands/

#3. – “RTL-SDR Antennas & Connectors”
https://jeremyclark.ca/wp/telecom/rtl-sdr-antennas-connectors/

#4. – “RTL-SDR – Versions/Connectors/Adapters/Antennas”
https://www.youtube.com/watch?v=uhqTHLOfIn0

#5. – “RTL-SDR for AM on GNU Radio”
https://jeremyclark.ca/wp/telecom/rtl-sdr-for-am-on-gnu-radio/

By Jeremy Clark

Jeremy Clark is a Senior Telecommunications Engineer and Advanced Amateur Radio Operator VE3PKC. He is the author of E-Books on Telecommunications, Navigation & Electronics.