Radio is the transmission and receiving of electromagnetic radiation with wavelengths longer than that of infrared light. You’re forgiven if that definition didn’t make sense, though, and until a few years ago, it wouldn’t have to me either. Radio technology powers almost all wireless connectivity around us: Bluetooth, WiFi, 3G, 4G, and your microwave oven—all these operate on radio principles. The technology is over a hundred years old, and yet I realized that I knew very little about it.
So, I decided to start learning and reached out to my national amateur radio association, the Pakistan Amateur Radio Society (PARS), a member of the International Amateur Radio Union (IARU), which in turn represents amateur radio to the International Telecommunication Union (ITU), the UN agency whose job it is to coordinate telecommunication operations and services worldwide. PARS runs a few radio repeaters across the country, and one of them was in Lahore, where I live.
While radio has used fundamentally the same technology since Guglielmo Marconi first conducted his experiments in 1895, improved circuit design and signal processing techniques have allowed us to transmit much more and much farther out than before. Now, it’s possible for someone with a laptop and less than $30 worth of equipment to receive a wide range of radio frequencies, and we’ll be doing just that.
In this software-defined radio tutorial, I’ll set up a software-defined radio device (SDR) and an antenna, and listen to a conversation between two licensed ham radio operators via the Lahore repeater. I will then use the same equipment to receive an image transmitted from the International Space Station, a spacecraft orbiting the Earth, and use it to claim the ARISS SSTV Award, demonstrating how easy it is to browse the radio spectrum with cheap equipment and be recognized. Through the hardware used in this article, you can only receive radio transmissions and not transmit your own own, but that is fine since you need an amateur radio license before doing so, anyways.
Caution! It is very easy to perform an illegal activity with radio equipment, which is why this article will continuously caution you and cite the law. The author lives in and conducted these experiments legally in Pakistan. While federal radio laws in Pakistan are severely restrictive, your jurisdiction may be more so. In 2019, a United Nations expert was arrested in Tunisia for possessing the same software-defined radio device we will be using. It is your responsibility to ensure that you comply with local laws when performing radio experiments.Please note, though: I am not a lawyer, and this does not constitute legal advice. You must consult your own lawyer for clarification.
If you live in Pakistan, you must obtain a PARS shortwave listening (SWL) membership before you obtain a radio receiver. Pakistan’s Wireless Telegraphy Act, 1933 prohibits possession of wireless telegraphy apparatus; however, SWL members are allowed to possess receivers. Reach out to me for a PARS reference letter, if you’re looking to become a member.Setting Up Our Dipole Antenna and SDR Receiver
“What even is a ‘software-defined’ radio?” I hear you ask!
A software-defined radio, is a radio device where most of the electrical components “emulated” in software. Before the rise of SDRs, you would need a dedicated circuit to perform the task of processing the signals to and from the radio. Things like signal filtering, frequency mixing, radio wave detection, signal amplification, modulation/demodulation, and others were done with dedicated circuits. However, since computers have become faster, we can perform these functions in software instead, making these types of radios software-defined.
A popular (and cheap) SDR receiver is Digital Video Broadcast (DVB-T) receiver with the Realtek RTL2832U controller and tuner integrated circuit. While their original purpose was to receive video, these are now repurposed to receive radio signals, and have come to be known as RTL-SDR devices. I’ll be using the RTL-SDR receiver and dipole from RTL-SDR.com. It currently costs $29.95, ships worldwide, comes with a temperature compensated oscillator (TCXO), and a bias tee, which are great to have, but beyond the scope of this article to discuss. It also comes with an adjustable dipole antenna kit, letting you listen from ~70 MHz to ~1030 MHz signals.Day 2 of ARISS Int'l F2F:David Honess of ESA opens today's sessions proposing excellent ways to engage kids with ISS ham radio projects using Raspberry Pi, RTL-SDR, SSTV mode communications, etc. through an online linkup with our F2F meeting in Montreal. pic.twitter.com/Mp25cljrAH— ARISS (@ARISS_status) June 27, 2019
The RTL-SDR dipole kit I’m using was also recommended by representatives of the European Space Agency at the Amateur Radio in Space (ARISS) International Face-to-face Meeting in June, 2019.
Setting up the antenna is easy. Screw the long prongs of the antenna into the center, mount it on a window with the provided suction cup, and open the dipole arms at exactly 49.65 cm (1 foot 7.55 inches) each. Connect the female end of the provided longer cable to the dipole’s male end, the male end of the longer cable to your SDR, and then mount the antenna vertically as high as possible outside. Preferably on a window, using the provided suction cup mount. Here’s a picture of how it should look:The antenna should be vertically oriented. Note that this is a demonstration, the arms of the antenna should be open much longer, but they wouldn’t fit in the camera frame, so I mounted had to close them and mount it indoors for this photograph.
Finally, screw the dangling end of the long cable into your SDR, and plug your SDR into your computer’s USB port. At this point, you can go with any number of SDR applications, but since I’m on MacOS, where options are limited, I’ll use CubicSDR.brew cask install cubicsdr
Upon opening CubicSDR, you’ll be presented with a dialogue box to select your SDR and its settings. Choose the Generic RTL2832U OEM like I’ve chosen in the image, and change the sample rate to 2.048MHz
When CubicSDR starts, you can start browsing the spectrum immediately. I recommend starting with familiar FM radio broadcasts. Here’s a video of me browsing the local radio stations as they are available to me in Lahore.
Next up, we listen to two amateur radio operators on the Lahore, repeater, but before we start, let’s discuss what amateur radio even is.What Is Amateur Radio?
“Okay, but what is ‘amateur’ radio?” I hear you ask this time!
Amateur radio is the use of the radio spectrum by licensed operators for non-commercial activities. These may include communication, training, experimentation, contesting, or more. Every jurisdiction may have their own legal definition as well. Amateur radio operators are restricted to using the frequencies dedicated to the amateur service.
“Amateur Service” means a radio communication service for the purpose of self-training, intercommunication and technical investigations carried out by amateurs, that is, by persons duly authorized under these Regulations interested in radio technique solely with a personal aim and without pecuniary interest;– Amateur Radio Services Regulations, 2004, Pakistan
With that settled, let’s look at what our local amateur radio frequencies are. They’re published along with the definitions of everything by the Pakistani Frequency Allocation Board, in a document called the Pakistan Table of Frequency Allocations. I maintain a a gist of them all for easy reference, but here are the very high frequency (VHF) bands:UnitFrequency rangeITU – Region 3Pakistan's allocationsMHz144 – 146AMATEURAMATEUR-SATELLITE5.216AMATEURAMATEUR-SATELLITEMHz146 – 148AMATEURFIXEDMOBILE5.217AMATEURFIXEDMOBILE