The Cloud-Burner: How to Master NVIS for Reliable Local Comms

1,593 words, 8 minutes read time.

If you have just earned your Technician or General class license, you have probably already realized that the radio world is full of “dead zones.” You might be able to talk to a guy in Italy using a massive tower, or a guy across town using a local repeater, but what about the people two counties over? Often, that sixty to two-hundred-mile range is a “skip zone” where your signal just flies right over their heads. This is where Near-Vertical Incidence Skywave, or NVIS, comes in. Think of it as taking your radio signal and pointing it straight up at the sky, using the atmosphere like a giant mirror to bounce that energy right back down into your local region. It is the ultimate tool for keeping your community connected when the internet goes out or the repeaters fail. It doesn’t require a hundred-foot tower or a thousand-dollar antenna; it requires a little bit of wire, a low branch, and the willingness to learn how the air above your head actually works.

Understanding the Ionospheric Mirror

To get a handle on NVIS, you have to understand that the ionosphere isn’t just empty space; it’s a layer of the atmosphere filled with particles that have been “charged up” by the sun. We call this ionization. During the day, the sun is hitting these layers hard, making them thick and reflective. At night, they thin out. For NVIS to work, we need to pick a frequency that is low enough to be reflected back down rather than passing through into space. This is governed by something called the Critical Frequency, or $f_c$. If you try to send a signal straight up at a frequency higher than $f_c$, it’s gone forever. For new hams, the rule of thumb is simple: use the 40-meter band (7 MHz) during the bright part of the day, and move down to the 80-meter band (3.5 MHz) or 160-meter band (1.8 MHz) as the sun goes down.

The goal here is to keep your “angle of incidence” near ninety degrees. Imagine standing in a room with a flashlight and a mirror on the ceiling. If you shine the light at a sharp angle toward the wall, the light bounces off and hits the far corner of the room—that is your standard long-distance “DX” skip. But if you shine that flashlight straight up at the ceiling, the light bounces right back down onto your head. That is NVIS. By “burning the clouds” with your signal, you create a solid umbrella of coverage that fills in all those local gaps. The math behind this is surprisingly straightforward. The Maximum Usable Frequency (MUF) for your local area is roughly equal to that Critical Frequency because the “Secant” of your ninety-degree angle is essentially one:

$$MUF = f_c \cdot \sec(0^\circ) = f_c \cdot 1$$

When you stay below that $f_c$ limit, you ensure your signal doesn’t punch through the atmosphere and disappear. Instead, you get a reliable, high-strength signal that blankets your entire region, regardless of hills, buildings, or trees that might block a standard line-of-sight signal.

The Low-Hanging Wire: Your NVIS Antenna

The most common mistake new hams make with NVIS is trying to get their antenna too high. We are taught that height is king, but in the NVIS world, the ground is actually your friend. To push your signal straight up, you want a horizontal dipole antenna mounted very low—usually only 10 to 15 feet off the ground. When the antenna is this low, the radio waves that hit the ground reflect back up and join with the waves going toward the sky. This creates a massive “lobe” of energy pointing at the zenith. If you put that same antenna 50 feet in the air, the energy starts to focus toward the horizon, which is great for talking to Japan, but terrible for talking to the next town over.

When you build a low antenna, the “impedance” of the wire changes. Impedance, represented by the letter $Z$, is basically how much the antenna resists the flow of electricity from your radio. A standard dipole in free space is about 72 ohms, but when you bring it close to the dirt, that number drops. You might see your SWR (Standing Wave Ratio) jump around because the ground is “soaking up” some of that energy or reflecting it back into the wire. The formula for this total resistance looks like this:

$$Z = R_{rad} + R_{loss}$$

Your goal is to keep $R_{rad}$ (the energy actually leaving the antenna) high and $R_{loss}$ (the energy turning into heat in the dirt) low. You can help this by laying a “reflector wire” on the ground directly underneath your antenna. This acts like a mirror on the floor, bouncing even more energy up toward the sky and away from the dirt. It is a simple, cheap way to make a basic wire antenna perform like a professional military setup. It is about working smarter with the space you have, using the foundation of the earth to amplify your reach.

Operating with Discipline and Purpose

NVIS isn’t just about the gear; it’s about the man behind the mic. Because you are using lower frequencies like 40 and 80 meters, you are going to encounter a lot of noise. These bands are where lightning crashes and electronic interference from house appliances live. To be successful, you have to develop a “radio ear.” You learn to listen through the static for your brothers. You also have to be ready to change bands. If you’re talking on 40 meters and the signals start to fade as the sun sets, don’t just keep cranking the power. That is a waste of electricity and hard on your gear. Instead, understand that the ionosphere is changing. Be the leader who says, “The sun is going down, the critical frequency is dropping—let’s move the net to 80 meters.”

This kind of communication is a responsibility. In an emergency, NVIS is often the only thing that works when the cell towers are down and the repeaters have no power. As a new ham, mastering this technique means you are becoming a valuable asset to your family and your community. You aren’t just playing with a hobby; you are learning the physics of the atmosphere so you can provide a lifeline when it matters most. It takes patience to learn the cycles of the sun and the quirks of your local soil, but that discipline is what separates a true operator from someone who just bought a radio.

Take pride in the “bench time.” Build your own dipoles, experiment with different heights, and don’t be afraid to fail. Every time you tune an antenna or successfully make a contact two towns over during a storm, you are gaining technical mastery. You are learning to provide for those around you by using your mind and your hands. Keep your station clean, keep your character grounded, and remember that the strength of the airwaves comes from the discipline of the men who use them. Whether you are a Technician just starting out or a General looking to expand your skills, NVIS is the gateway to a whole new level of radio capability.

Looking Ahead: The Power of Local Links

The future of radio isn’t just in satellites or high-speed digital networks; it’s in the resilient, local links that we build ourselves. As you grow in this craft, you’ll find that NVIS is a bridge. It connects people across distances that are too far to see but too close for standard skip. It is a testament to the order of the world—that even the very air above us is designed in a way that allows us to reach out to one another. By mastering the “Cloud-Burner” technique, you are stepping into a long tradition of operators who value self-reliance and technical skill.

Continue to study the $SFI$ (Solar Flux Index) and watch how the bands open and close. Treat your fellow hams with respect and kindness, and always be willing to help the next new guy who is trying to figure out why his signal isn’t getting out. We are a community built on shared knowledge and a commitment to the craft. Stand tall, keep your wires taught, and we will see you on the air.

Call to Action

If this story caught your attention, don’t just scroll past. Join the community—men sharing skills, stories, and experiences. Subscribe for more posts like this, drop a comment about your projects or lessons learned, or reach out and tell me what you’re building or experimenting with. Let’s grow together.

D. Bryan King

Sources

• ARRL: Near Vertical Incidence Skywave (NVIS) Overview
• Electronics Notes: Understanding NVIS Propagation
• DX Engineering: Wire Antenna Building Basics
• NOAA: Real-Time Ionograms and Critical Frequency Data
• W8JI: NVIS Antenna Height and Ground Interaction
• QST Magazine: The Secret of NVIS Success
• HamRadio.me: Investigating NVIS Antenna Performance
• Navy-Radio: The AS-2259 NVIS Tactical Antenna
• Viasat: Tactical NVIS Systems and Applications
• Standardics: A Comprehensive Guide to NVIS Comms
• ScienceDirect: Ionospheric Refraction Physics
• Raymond Fowler: Practical NVIS for Amateur Operators
• Nonstop Systems: Extensive NVIS Bibliography and Research
• ITU: Prediction of NVIS Propagation Frequencies
• Ready for Ham Radio: NVIS for Emergency Preparedness

Disclaimer:

The views and opinions expressed in this post are solely those of the author. The information provided is based on personal research, experience, and understanding of the subject matter at the time of writing. Readers should consult relevant experts or authorities for specific guidance related to their unique situations.

Directional-hemispherical reflectance (Climatology 🌍)

Directional-hemispherical reflectance is the reflectance of a surface under direct illumination. Directional-hemispherical reflectance is the integral of the bidirectional reflectance distribution function over all viewing directions. It is sometimes called "black-sky albedo".

https://en.wikipedia.org/wiki/Directional-hemispherical_reflectance

#DirectionalHemisphericalReflectance #Climatology #ElectromagneticRadiation

Effects of Electromagnetic Radiation on Living Beings by Felix A. Farret, 2019

The objective of this book is to show in detail how electromagnetic waves existing in the environment can affect the electrochemical currents present in the brains and bodies of living beings that serve to communicate with their internal organs as well as with other living beings.

@bookstodon
#books
#nonfiction
#ElectromagneticRadiation

The planets can be observed not only in visible light but also in the radio range! The first attempts at radio observations of planets began in 1954 when intense radio bursts from Jupiter were detected. This radiation did not come directly from the planet but from processes in its magnetosphere.

#FunFact: Jupiter's magnetic field is massive, even relative to the size of the planet itself – it extends over 650 million kilometers (to the orbit of Saturn!) and is 4000 times stronger than Earth's magnetic field. 🚀🌍 A pulsar's magnetic field, however, can be up to 100 trillion times stronger than Jupiter's!

Radio waves penetrate deep into planet atmospheres or the surface, providing us with information that cannot be obtained through other methods. For example, phase fluctuations in the radio emissions were detected on the Moon and Mercury. On Mars, high electrical conductivity in the soil due to iron oxides was detected.

#funfact Radio astronomy amateurs assure that Jupiter can also be "received" and "heard" at home. All you need is a radio receiver for radio waves in the 18 to 22 MHz range and an antenna specifically designed for this frequency range. 📡

📷 Radiation belts of the planets, NASA, JPL, NASA/AP | Collage: A. Kazantsev.

#radioastronomy #planetaryresearch #jupiter #moon #mercury #mars #magnetosphere #atmosphere #pulsar #electromagneticradiation

Directional-hemispherical reflectance (Climatology 🌍)

Directional-hemispherical reflectance is the reflectance of a surface under direct illumination. Directional-hemispherical reflectance is the integral of the bidirectional reflectance distribution function over all viewing directions. It is sometimes called "black-sky albedo".

https://en.wikipedia.org/wiki/Directional-hemispherical_reflectance

#DirectionalHemisphericalReflectance #Climatology #ElectromagneticRadiation

🦊FoxiMax #228 6/8
https://foximax.com/

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Identifying Malware by Sniffing its EM Signature

The phrase "extraordinary claims require extraordinary evidence" is most often attributed to Carl Sagan, specifically from his television series Cosmos. Sagan was probably not the first person to put forward such a hypothesis, and the show certainly didn't claim he was. But that's the power of TV for you; the term has since come to be known as the "Sagan Standard" and is a handy aphorism that nicely encapsulates the importance of skepticism and critical thinking when dealing with unproven theories.

It also happens to be the first phrase that came to mind when we heard about Obfuscation Revealed: Leveraging Electromagnetic Signals for Obfuscated Malware Classification, a paper presented during the 2021 Annual Computer Security Applications Conference (ACSAC). As described in the mainstream press, the paper detailed a method by which researchers were able to detect viruses and malware running on an Internet of Things (IoT) device simply by listening to the electromagnetic waves being emanated from it. One needed only to pass a probe over a troubled gadget, and the technique could identify what ailed it with near 100% accuracy.

Those certainly sound like extraordinary claims to us. But what about the evidence? Well, it turns out that digging a bit deeper into the story uncovered plenty of it. Not only has the paper been made available for free thanks to the sponsors of the ACSAC, but the team behind it has released all of code and documentation necessary to recreate their findings on GitHub.

Unfortunately we seem to have temporarily misplaced the $10,000 1 GHz Picoscope 6407 USB oscilloscope that their software is written to support, so we're unable to recreate the experiment in full. If you happen to come across it, please drop us a line. But in the meantime we can still walk through the process and try to separate fact from fiction in classic Sagan style.

Baking a Malware Pi

The best way of understanding what this technique is capable of, and further what it's not capable of, is to examine the team's test rig. In addition to the aforementioned Picoscope 6407, the hardware configuration includes a Langer PA-303 amplifier and a Langer RF-R H-Field probe that's been brought to rest on the BCM2837 processor of a Raspberry Pi 2B. The probe and amplifier were connected to the first channel of the oscilloscope as you might expect, but interestingly, the second channel was connected to GPIO 17 on the Pi to serve as the trigger signal.

As explained in the project's Wiki, the next step was to intentionally install various rootkits, malware, and viruses onto the Raspberry Pi. A wrapper program was then used that would first trigger the Picoscope over the GPIO pin, and then run the specific piece of software under examination for a given duration. This process was repeated until the team had amassed tens of thousands of captures for various pieces of malware including bashlite, mirai, gonnacry, keysniffer, and maK_it. This gave them data on what the electromagnetic (EM) output of the Pi's SoC looked like when its Linux operating system had become infected.

But critically, they also performed the same data acquisition on what they called a "benign" dataset. These captures were made while the Raspberry Pi was operating normally and running tools that would be common for IoT applications. EM signatures were collected for well known programs and commands such as mpg123, wget, tar, more, grep, and dmesg. This data established a baseline for normal operations, and gave the team a control to compare against.

Crunching the Numbers

As explained in section 5.3 of the paper, Data Analysis and Preprocessing , the raw EM captures need to be cleaned up before any useful data can be extracted. As you can imagine, the probe picks up a cacophony of electronic noise at such close proximity. The goal of the preprocessing stage is to filter out as much of the background noise as possible, and identify the telltale frequency fluctuations and peaks that correspond to individual programs running on the processor.

The resulting cleaned up spectrograms were then put through a neural network designed to classify the EM signatures. In much the way a computer vision system is able to classify objects in an image based on its training set, the team's software demonstrated an uncanny ability to pick out what type of software was running on the Pi when presented with a captured EM signature.

When asked to classify a signature as ransomware, rootkit, DDoS, or benign, the neural network had an accuracy of better than 98%. Similar accuracy was achieved when the system was tasked with drilling down and determining the specific type of malware that was running. This meant the system was not only capable of detecting if the Pi was compromised, but could even tell the difference between a gonnacry or bashlite infection.

Accuracy took a considerable hit when attempting to identify the specific binary being executed, but the system still manged a respectable 82.28%. Perhaps most impressively, the team claims an accuracy of 82.70% when attempting to identify between various types of malware even when attempts were made to actively obfuscate their execution, such as running them in a virtualized environment.

Realistic Expectations

While the results of the experiment are certainly compelling, it's important to stress that this all took place under controlled and ideal conditions. At no point in the paper is it claimed that this technique, at least in its current form, could actually be used in the wild to determine if a computer or IoT device has been infected with malware.

At the absolute minimum, data would need to be collected on a much wider array of computing devices before you could even say if this idea has any practical application outside of the lab. For their part, the authors say they chose the Pi 2B as a sort of "boilerplate" device; believing it's 32-bit ARM processor and vanilla Linux operating system provided a reasonable stand-in for a generic IoT gadget. That's a logical enough assumption, but there's still far too many variables at play to say that any of the EM signatures collected on the Pi test rig would be applicable to a random wireless router pulled off the shelf.

Still, it's hard not to come away impressed. While the researchers might not have created the IT equivalent of the Star Trek medical tricorder, a device that you can simply wave over the patient to instantly see what malady of the week they've been struck by, it certainly seems like they're tantalizingly close.

#computerhacks #hackadaycolumns #securityhacks #electromagneticradiation #malware #signalanalysis #spectrogram

How Much is That Shirt in the (Atmospheric) Window?

Summer is fading into a memory now, but as surely as the earth orbits the sun, those hot and sweaty days will return soon enough. And what can you do about it at the level of a single, suffering human being? After all, a person can only remove so much clothing to help cool off. Until someone figures out a way to make those stillsuits from Dune, we need an interim solution in which to drape ourselves.

We've seen the whitest paint possible for cooling buildings, and then we saw a newer, whiter and more award-winning paint a few months later. This paint works by the principle of passive cooling. Because of its color and composition, it reflects most light and absorbs some heat, which gets radiated away into the mid-infrared spectrum. It does this by slipping out Earth's atmospheric window and into space. Now, a team based in China have applied the passive cooling principle to fabric.

Wait, What's the Atmospheric Window?

Technically speaking, there are two atmospheric windows -- one in the infrared spectrum, and another in the radio spectrum. For the purposes of this discussion, we are only concerned with the infrared window. But let's back up a bit.

What gets absorbed. Image via University of Rochester's Department of Physics and Astrology

The energy radiating from the Sun includes much more than just the light we see and the heat we feel. Earth's upper atmosphere absorbs gamma rays, x-rays, and some of the ultraviolet part of the spectrum. Think of the upper atmosphere as a blanket that protects Earth from these harmful rays.

There is a hole in the blanket -- or a window, if you will -- in the low end of the infrared range (roughly 8-14 μm) that lets in visible light and heat because none of the atmospheric gasses absorb that wavelength. That's a good hole; it's a hallmark of a habitable planet. So, this is what is meant by the atmospheric window -- light and heat can go both ways. The window lets in light and heat, but more importantly, it also lets it escape. This way the planet is nice and warm, but not to a deadly degree.

Fabric of the Universe

So, back to the fabric. To reiterate, passive cooling materials work by reflecting almost all of the incoming light, keeping those energetic photons from heating the surface. These materials also absorb heat from whatever they're covering and radiate it out.

Passively cooling something like a building is kind of a set-it-and-forget-it type of thing. Clothing, on the other hand, has to be flexible, breathable, and must stand up to repeated washings. So, how does this fabric work already? Much of its reflectivity comes from titanium dioxide powder, like the stuff in some kinds of sunscreen. These titanium dioxide nanoparticles are embedded in fibers made of PLA, which emit radiation (lose heat) in the mid-infrared spectrum.

Then the fabric is further coated with polyetrafluoroethylene (PTFE), which reflects the part of the UV spectrum that the titanium dioxide doesn't. PTFE is hydrophobic, so it will repel water from the outside and sweat from the inside. The fabric is woven together rather than knit, and has a carefully-calculated pore size. In tests, the fabric reflected more than 92% of sunlight. The team half-covered a vest with their fabric and sat someone in the sunlight while wearing it. They monitored the person's body with infrared cameras and found that the side covered by the fabric measured an average of 3° C cooler than the side without the fabric.

One Shirt, Please -- With a Jolly Wrencher

This stuff seems to tick all the boxes -- it's breathable, washable, and (we assume), comfortable enough to wear all day in the desert. It's also supposed to be biodegradable, but we have to wonder what effect sweat and laundry detergents and double rubs might have on the fibers' ability to passively cool someone on a long enough timeline. Since it's mostly plastic, we do wonder how it feels.

And it comes in any color you want, as long as it's white. The good news is that it can be embroidered. Dyeing it is a no-no because it will lose its function. But if you can embroider it, you can probably make it visually interesting and still viable. A shirt made of this magic fabric will probably cost even more than that Hypercolor shirt we loved so much in the 90s, but at least it won't announce our anxiety to the world in living color. But if you need to keep cool at all costs, an atmospheric window-wear shirt might just be the ticket.

#chemistryhacks #featured #interest #originalart #atmosphericwindow #climatechange #electromagneticradiation #greenhousegases #infrared #passivecooling