Below the Noise Floor — Episode 1: What HF Radio Actually Is

High frequency radio has been talking to the other side of the planet for over a hundred years. No servers, no cell towers, no infrastructure - just a transmitter, an antenna, and the ionosphere. This first episode covers what HF radio actually is, how skywave propagation works, the amateur license structure, and how software-defined radio has changed what is possible. Plus: a preview of the AetherSDR series coming next.

Transcript

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There is a layer of the radio spectrum that most people have completely forgotten about. It sits below the frequencies your cell phone uses, below Wi-Fi, below the FM dial. It is older than all of those things by decades, and it does something none of those technologies can do. It talks to the other side of the planet. Welcome to Below the Noise Floor. This show is about learning amateur radio and software-defined radio from the inside out, not from a textbook, not from a classroom. From the antenna up. I am not an expert doing a lecture series. I am someone who got licensed, bought some equipment, and is now genuinely trying to understand what I have gotten myself into. If that sounds like your situation, or something you are curious about, you are in the right place. One more thing before we get into it. These episodes are mostly AI generated. I use AI as a tool to help me learn and organize my thinking, and I found the results good enough that I figured others might enjoy them too. The curiosity is real. The equipment is real. The learning is real. The content just has some help getting from my head into a script. I thought you should know that. Let me start with the thing that pulled me into this in the first place. HF radio, high frequency radio, covers the spectrum from roughly 3 MHz to 30 MHz. That band has a different name in amateur radio circles. We call it the shortwave bands. And the reason it matters, the reason people have been obsessed with it for over a hundred years, comes down to a quirk of physics that is almost hard to believe until you see it working. The Earth has an upper atmosphere called the ionosphere. It is a region starting at about 50 miles up where solar radiation ionizes gas molecules, stripping electrons free and creating layers of charged particles. Those layers act like a mirror for certain radio frequencies. A signal transmitted at the right frequency, aimed at the right angle toward the sky, bounces off the ionosphere and comes back down to Earth hundreds or thousands of miles away. Then it can bounce again off the surface and go back up. And again. A single HF transmission can circle the globe multiple times. This is called skywave propagation. And it is the reason that during World War II, a field radio operator in the Pacific could communicate with a command post on the other side of the ocean. It is why shortwave broadcasting reached listeners behind the Iron Curtain when no other medium could. It is why right now, tonight, there are amateur radio operators making voice contacts and data contacts with strangers on other continents using equipment that fits in a backpack powered by a battery with no infrastructure between them and the other person, except the ionosphere. No servers, no cell towers, no company in the middle, just physics. That is what got me. Now the ionosphere is not static. This is where it gets complicated, and honestly, where a lot of the fascination comes from. The ionosphere changes constantly. It responds to the sun, the 11-year solar cycle, daily sunrise and sunset, solar flares, geomagnetic storms. A band that is dead quiet at noon might be alive with signals after dark. A frequency that reaches Europe from the West Coast might not reach the Midwest. Propagation, the behavior of radio waves through the atmosphere, is something operators spend years learning to predict and work with. There are monitoring networks, real-time maps, decades of accumulated knowledge, and still, there is an element of art to it. You develop a feel for what the bands are doing. Different parts of the HF spectrum behave differently depending on conditions. The lower frequencies, the 80 and 40 meter bands, tend to work better at night when the ionosphere is less disturbed by solar radiation. They are good for regional contacts, a few hundred miles up to a couple thousand. The higher frequencies, 20 meters, 17 meters, 15 meters, 10 meters, those come alive during periods of high solar activity and can support contacts halfway around the world in the middle of the day. Learning which band to use for which kind of contact at which time of day and season is one of the core skills of HF operating. There are also three bands that sit at the edges of or below the traditional HF range that are worth knowing about. 160 meters, 630 meters, and 2200 meters. These are real amateur allocations, and operators do use them, but they require specialized antennas given their long wavelengths, and they see a fraction of the traffic that the main HF bands do. For most newcomers to HF, they are interesting footnotes rather than starting points. To work HF as an amateur, you need a license. In the United States, the FCC licenses amateur radio operators at three levels technician, general, and amateur extra. The technician license is the entry point and gets you access to VHF and UHF frequencies and a small slice of HF. The general license opens up most of the HF spectrum and requires passing a 35-question written exam. The amateur extra license is the top tier, requires another exam, and gives you access to every frequency available to U.S. amateurs. That structure matters because HF is primarily a general and extra class playground. If you want to work the world, you need at least a general. The exams are not trivial, but they are not impossible either. The question pools are publicly available, there are free study tools online, and most people who commit a few weeks to it can pass. There is a second thing you need besides a license. You need a transceiver, a radio that can both transmit and receive on HF frequencies. And here is where the landscape has changed dramatically in the last decade. Software-defined radio has transformed what is possible at almost every price point. Traditional radio hardware used analog circuits to do the work of tuning, filtering, amplifying, and demodulating signals. Software-defined radio moves as much of that processing as possible into software running on a computer. The hardware becomes simpler. In some cases, it is not much more than an antenna interface and an analog to digital converter. The software does the rest. This has two huge consequences. First, cost. Receive only SDR dongles that cover a wide chunk of the spectrum can be purchased for $20 or $30. Serious SDR transceivers capable of transmitting across the full HF spectrum have come down enormously from where they were 10 years ago. Second, flexibility. Because the processing happens in software, what your radio can do is limited more by code than by hardware. You can update firmware, load new modes, visualize the spectrum in ways that analog radios never could. One of the signature tools of SDR operation is the waterfall display. Instead of seeing just the frequency you are tuned to, a waterfall shows you a wide swath of the spectrum as a scrolling color map. Signals appear as bright traces. You can see multiple signals simultaneously. You can watch propagation changing in real time. You can spot a weak signal visually before you can hear it. If you have never looked at an HF waterfall during good band conditions, it is a genuinely remarkable thing. Dozens of signals visible at once, some strong and steady, some faint and drifting, all telling you something about where they came from and how far the signals are traveling. This is what drew me toward a particular piece of software that is going to be the focus of a lot of what we cover on this show going forward. Ether SDR. It is a cross-platform SDR client built specifically for the Flex Radio ecosystem, running on Linux, Windows, and Mac OS. It is being actively developed, it is technically interesting, and it represents a certain philosophy about what radio software should be able to do. We are going to spend several episodes going deep on Ether SDR, what it does, how to configure it, the features that matter for HF operating, and the learning curve that comes with it. That series starts next episode. Before we get there, I want to say one more thing about why this hobby, specifically, and why now. We live in a communications environment that is almost entirely mediated by corporations. Every message you send, every call you make, every piece of data you move across the internet travels through infrastructure owned by someone else, governed by terms of service, subject to outage, dependent on a functional grid and a functioning economy. That infrastructure is remarkable and we should not take it for granted. But it is also fragile in ways that most people do not think about until it fails. Amateur radio exists outside that infrastructure. It is peer-to-peer in the oldest sense of the phrase. When conditions are right, the path between two amateur operators is the ionosphere and nothing else, and the community of people who understand how to use it, who know the bands, who know the modes, who have working stations they built and maintained themselves, is a distributed network of capability that does not depend on anyone else staying online. There is also just the pure pleasure of it. The satisfaction of making contact with someone you have never met on a frequency you predicted would be open, using a signal that bounced off the upper atmosphere to get there. There is feedback in this hobby that is immediate and physical in a way that a lot of modern technical pursuits are not. The antenna wire in the air, the numbers on the tuner, the waterfall lighting up, someone coming back to your call. It is a good thing to learn. That is what this show is about. Next episode, we start with Ether SDR what it is, why it exists, and how to start thinking about the software defined radio approach to HF operating. This is below the noise floor.