Carter Dack, 2024.
Software defined radios are highly valuable tools in today’s world. These instruments interact with modern computers and allow for the reception and transmission of analog radio signals via software. Their use is widespread across industries/services and without them our critical infrastructure would be far less sophisticated and reliable.
This article will explore the basic use of a Software Defined Radio (“SDR”) and provide commentary on what value these tools might provide to a hobbyist and a society alike. In order to better understand these tools, it will be useful to gain some background into how traditional (analog) radio systems work.
Background
Analog radio systems have a long history, dating back to the early 20th century, and they leverage the fundamental physical properties of electromagnetic waveforms to transmit information wirelessly. At the core of these systems are two primary components: the transmitter and the receiver. The transmitter generates and radiates oscillating electromagnetic energy, which propagates through the air as electromagnetic waves. These waves can be captured by a receiver, which then detects and amplifies the signal, allowing the transmitted information to be decoded.
The concept behind analog radio transmission is analogous to how sound waves operate. Just as sound is produced by vibrating air molecules at specific frequencies, electromagnetic waves are created by the oscillation of electric and magnetic fields at particular frequencies. These frequencies are carefully chosen to carry information effectively over distances. The transmitter modulates the electromagnetic wave by altering its amplitude, frequency, or phase to encode the desired information—such as voice, music, or data—onto the carrier wave.
Once the modulated wave reaches the receiver, it demodulates the signal to extract the original information. This process is highly dependent on the frequency at which the electromagnetic wave is transmitted, as different frequencies have distinct propagation characteristics. Lower frequencies tend to travel longer distances and penetrate obstacles more effectively, making them suitable for AM radio, while higher frequencies, which can carry more data, are used for FM radio and other forms of communication like television broadcasting.
Figure 1.1 - Basic Radio System Visualization
Enter SDRs
Software-Defined Radios (SDRs) are an advanced form of radio technology where traditional hardware components are replaced with software-based processing. Unlike conventional analog radios, which depend on dedicated hardware for tasks like modulation and filtering, SDRs use software to perform these functions, making them highly adaptable and versatile. The core of an SDR typically includes a radio frequency (RF) front end for analog signal processing and a digital processing unit, such as a general-purpose processor (GPP) or digital signal processor (DSP), that handles the software-driven tasks. This design allows SDRs to support a wide range of frequencies, modulation schemes, and communication standards with just a change in software.
One of the key advantages of SDRs is their ability to be reprogrammed to accommodate new protocols and standards without the need for new hardware. This flexibility makes SDRs ideal for applications in research, military, and commercial communications. Additionally, SDRs support wideband operation, enabling them to monitor and transmit across multiple frequencies simultaneously. They also facilitate advanced signal processing techniques, such as cognitive radio, which dynamically adjusts its parameters to optimize performance based on the surrounding environment. Overall, SDRs represent a significant advancement in radio communication, offering greater flexibility, upgradability, and capability compared to traditional analog systems.
Albeit a crude example, simple hobbyist SDRs are a great way of showcasing this technology and getting a grasp of what the radio spectrum has to offer.
Figure 2.1 - RTL-SDR Consumer SDR
Getting Started
Once an SDR device has been acquired, some manual setup will need to take place. First, the software which will interact with the SDR hardware will need to be installed and configured. In this example, SDR# was leveraged. This software is open source and contains all the necessary software drivers in order to run on Windows.
Figure 3.1 - Downloading SDR#
Once the software package is downloaded and extracted, a driver can be installed for the specific SDR hardware that is being used. In this case, the `install-rtlsdr.bat` script was the correct install script, since the RTL-SDR hardware was in use.
Figure 3.2 - Installing Drivers
Figure 3.3 - Overwriting Drivers
Once the proper drivers have been installed, the primary SDR# software can be run, and the SDR hardware device can be selected to begin receiving transmissions.
Figure 3.4 - Running SDRSharp.exe
In the United States, the radio spectrum is carefully divided into different frequency bands, each allocated for specific purposes to ensure organized and efficient communication. The Federal Communications Commission (FCC) oversees these allocations, assigning bands for commercial broadcasting (such as AM and FM radio), government and military communications, emergency services, and scientific research. For example, certain frequencies are reserved for public services like television broadcasting and cellular networks, while others are designated for amateur radio operators, who are licensed to explore and communicate over specific frequencies. Additionally, bands are allocated for industrial, scientific, and medical (ISM) applications, including those used by microwave ovens and medical devices. This structured allocation prevents interference between different services and maintains the reliability of communication across various sectors.
It is important to note that transmitting on certain frequencies without proper authorization is illegal and can result in significant penalties, including fines and imprisonment. In the United States, the Federal Communications Commission (FCC) strictly regulates who can transmit on specific frequencies and under what conditions. Unauthorized transmissions, including those that interfere with licensed services like emergency communications, government operations, or commercial broadcasts, are prohibited. Additionally, it is illegal to transmit false distress signals, encrypted messages on amateur radio frequencies, or content that violates FCC regulations, such as indecent or obscene material. Anyone operating radio equipment that is setup to transmit must be aware of these restrictions and ensure they comply with all applicable laws and regulations to avoid legal consequences.
Figure 3.5 - FCC’s Segmented Radio Spectrum
To get started listening to some publicly broadcasted radio signals, a National Weather Service broadcast was identified to have a strong enough signal strength to listen in. By tuning the radio to 162.550 MHz, the frequency designated for this service, a voice can be heard relaying weather information.
Figure 3.6 - Finding a Viable Signal
Figure 3.7 - Audio of Weather Broadcast
Technology Applications
SDRs have become highly valuable for critical infrastructure applications due to their flexibility, adaptability, and ability to operate across a wide range of frequencies and communication protocols. In telecommunications, SDRs allow network operators to quickly adapt to new standards, such as 5G, without needing extensive hardware upgrades. This capability is crucial in maintaining robust and reliable communication networks, especially as the demand for higher data rates and more efficient spectrum use increases. SDRs also enable more efficient spectrum management, allowing operators to dynamically allocate bandwidth and optimize network performance in real-time, which is essential for ensuring continuous and high-quality service in both urban and remote areas.
In satellite and military operations, SDRs offer significant strategic advantages. For satellite communications, SDRs provide the ability to switch between different frequency bands and modulation schemes, enabling seamless communication with various satellite systems. This is particularly important for global positioning systems (GPS), weather monitoring, and secure military communications, where reliability and adaptability are paramount. In military operations, SDRs enhance tactical communication by allowing forces to operate across different frequencies and standards, making them resilient to jamming and other forms of electronic warfare. Additionally, the reprogrammable nature of SDRs means they can be updated with new encryption methods and communication protocols on the fly, ensuring that military communications remain secure and effective in ever-changing environments.
However you decide to look at it, SDRs have pushed the physical bounds of traditional radio systems and have enabled highly enhanced operations of a technologically simple system that dates back over 100 years.
References
Glynn, D, et al. “Buy RTL-SDR Dongles (RTL2832U).” Rtl, 22 June 2024, www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/.
“A History of Radio Broadcasting in Georgia Exploring Georgia Radio.” How Radio Works – A History of Radio Broadcasting in Georgia, exhibits.library.gsu.edu/georgia-radio/1920s-pioneers/how-radio-works/. Accessed 27 Aug. 2024.
“Radio.” Wikipedia, Wikimedia Foundation, 19 Aug. 2024, en.wikipedia.org/wiki/Radio.
“SDR# and Airspy Downloads.” AIRSPY, airspy.com/download/. Accessed 27 Aug. 2024.
U.S. Frequency Allocation Chart, www.ntia.doc.gov/files/ntia/publications/2003-allochrt.pdf. Accessed 28 Aug. 2024.
US Department of Commerce, NOAA. “NWR Station Listing.” National Weather Service, NOAA’s National Weather Service, 15 Nov. 2022, www.weather.gov/nwr/station_listing.
