Studying Airplane Radio Reflections With SDR

October 11, 2018 by Lewin Day 11 Comments

A property of radio waves is that they tend to reflect off things. Metal surfaces in particular act as good reflectors, and by studying how these reflections work, it’s possible to achieve all manner of interesting feats. [destevez] decided to have some fun with reflections from local air traffic, and was kind enough to share the results.

The project centers around receiving 2.3 GHz signals from a local ham beacon that have been reflected by planes taking off from the Madrid-Barajas airport. The beacon was installed by a local ham, and transmits a CW idenfication and tone at 2 W of power.

In order to try and receive reflections from nearby aircraft, [destevez] put together a simple but ingenious setup.

ADS-B data was plotted on a map and correlated with the received reflections.

A LimeSDR radio was used, connected to a 9 dB planar 2.4 GHz WiFi antenna. This was an intentional choice, as it has a wide radiation pattern which is useful for receiving reflections from odd angles. A car was positioned between the antenna and the beacon to avoid the direct signal overpowering reflected signals from aircraft.

Data was recorded, and then compared with ADS-B data on aircraft position and velocity, allowing recorded reflections to be matched to the flight paths of individual flights after the fact. It’s a great example of smart radio sleuthing using SDR and how to process such data. If you’re thirsty for more, check out this project to receive Russian weather sat images with an SDR.

[Thanks to Adrian for the tip!]

Using AI To Pull Call Signs From SDR-Processed Signals

October 9, 2018 by Steven Dufresne 8 Comments

AI is currently popular, so [Chirs Lam] figured he’d stimulate some interest in amateur radio by using it to pull call signs from radio signals processed using SDR. As you’ll see, the AI did just okay so [Chris] augmented it with an algorithm invented for gene sequencing.

His experiment was simple enough. He picked up a Baofeng handheld radio transceiver to transmit messages containing a call sign and some speech. He then used a 0.5 meter antenna to receive it and a little connecting hardware and a NooElec SDR dongle to get it into his laptop. There he used SDRSharp to process the messages and output a WAV file. He then passed that on to the AI, Google’s Cloud Speech-to-Text service, to convert it to text.

Despite speaking his words one at a time and making an effort to pronounce them clearly, the result wasn’t great. In his example, only the first two words of the call sign and actual message were correct. Perhaps if the AI had been trained on actual off-air conversations with background noise, it would have been done better. It’s not quite the same issue, but we’re reminded of those MIT researchers who fooled Google’s Inception image recognizer into thinking that a turtle was a gun.

Rather than train his own AI, [Chris’s] clever solution was to turn to the Smith-Waterman algorithm. This is the same algorithm used for finding similar nucleic acid sequences when analyzing genes. It allowed him to use a list of correct call signs to find the best match for what the AI did come up with. As you can see in the video below, it got the call signs right.

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Help For High-Frequency Hobbyists

October 5, 2018 by Brian McEvoy 21 Comments

Dead-bug circuit building is not a pretty affair, but hey, function over form. We usually make them because we don’t have a copper circuit board available or the duty of making one at home is not worth the efforts and chemical stains.

[Robert Melville and Alaina G. Levine] bring to light a compromise for high-frequency prototypes which uses the typical FR4 blank circuit board, but no etching chemicals. The problem with high-frequency radio is that building a circuit on a breadboard will not work because there is too much added inductance and capacitance from the wiring that will wreak havoc on the whole circuit. The solution is not new, build your radio module on a circuit board by constructing “lands” over a conductive ground plane, where components can be isolated on the same unetched board.

All right, sometimes dead-bug circuits capture an aesthetic all their own, especially when they look like this and they do allow for a darned small package for one-off designs.

Fully 3D Printed And Metalized Horn Antennas Are Shiny And Chrome

October 4, 2018 by Brian Boucheron 19 Comments

We’ve seen our share of 3D printed antennas before, but none as well documented and professionally tested as [Glenn]’s 3D printed and metalized horn antennas. It certainly helps that [Glenn] is the principal engineer at an antenna testing company, with access to an RF anechoic chamber and other test equipment.

Horn antennas are a fairly simple affair, structurally speaking, with a straight-sided horn-shaped “cone” and a receptacle for standardized waveguide or with an appropriate feed, coaxial adapters. They are moderately directional and can cover a wide range of frequencies. These horns are often used in radar guns and as feedhorns for parabolic dishes or other types of larger antenna. They are also used to discover the cosmic microwave background radiation of our universe and win Nobel Prizes.

[Glenn]’s antennas were modeled in Sketchup Make, and those files plus standard STL files are available for download. To create your own horn, print the appropriate file on a normal consumer-grade fused deposition printer. For antennas that perform well in WiFi frequency ranges you may need to use a large-format printer, as the prints can be “the size of a salad bowl”. Higher frequency horns can easily fit on most print beds.

After printing, [Glenn] settled on a process of solvent smoothing the prints, then metalizing them with commonly available conductive spray paints. The smoothing was found to be necessary to achieve the expected performance. Two different paints were tested, with a silver-based coating being the clear winner.

The full write-up has graphs of test results and more details on the process that led to these cheap, printed antenna that rival the performance of more expensive commercial products.

If you’re interested in other types of 3D printed antenna, we’ve previously covered a helical satcom feed, a large discone antenna, and an aluminum-taped smaller discone antenna.

Antennas That You Install With A Spray-Can

September 27, 2018 by Al Williams 30 Comments

With the explosion in cell phones, WiFi, Bluetooth, and other radio technologies, the demand for antennas is increasing. Everything is getting smaller and even wearable, so traditional antennas are less practical than ever. You’ve probably seen PCB antennas on things like ESP8266s, but Drexel University researchers are now studying using titanium carbide — known as MXene — to build thin, light, and even transparent antennas that outperform copper antennas. Bucking the trend for 3D printing, these antennas are sprayed like ink or paint onto a surface.

A traditional antenna that uses metal carries most of the current at the skin (something we’ve discussed before). For example, at WiFi frequencies, a copper antenna’s skin depth is about 1.33 micrometers. That means that antennas have to be at least thick enough to carry current at that depth from all surfaces –practically 5 micrometers is about the thinnest you can reasonably go. That doesn’t sound like a lot, but when you are trying to make something thin and flexible, it is pretty thick. Using MXene, the researchers made antennas as thin as 100 nanometers thick — that’s 10% of a micrometer and only 2% of a conventional antenna.

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Buy A Baofeng While You Still Can? FCC Scowls At Unauthorized Frequency Transmitters

September 25, 2018 by Jenny List 93 Comments

There was a time when a handheld radio transceiver was an object of wonder, and a significant item for any radio amateur to own. A few hundred dollars secured you an FM walkie-talkie through which you could chat on your local repeater, and mobile radio was a big draw for new hams. Thirty years later FM mobile operation may be a bit less popular, but thanks to Chinese manufacturing the barrier to entry is lower than it has ever been. With extremely basic handheld radios starting at around ten dollars and a capable dual-bander being yours for somewhere over twice that, most licencees will now own a Baofeng UV5 or similar radio.

The FCC though are not entirely happy with these radios, and QRZ Now are reporting that the FCC has issued an advisory prohibiting the import or sale of devices that do not comply with their rules. In particular they are talking about devices that can transmit on unauthorised frequencies, and ones that are capable of transmission bandwidths greater than 12.5 kHz.

We’ve reported before on the shortcomings of some of these radios, but strangely this news doesn’t concern itself with their spurious emissions. We’re guessing that radio amateurs are not the problem here, and the availability of cheap transceivers has meant that the general public are using them for personal communication without a full appreciation of what frequencies they may be using. It’s traditional and normal for radio amateurs to use devices capable of transmitting out-of-band, but with a licence to lose should they do that they are also a lot more careful about their RF emissions.

Read the FCC statement and you’ll learn they are not trying to restrict the sale of ham gear. However, they are insisting that imported radios that can transmit on other frequencies must be certified. Apparently, opponents of these radios claim about 1 million units a year show up in the US, so this is a big business. The Bureau warns that fines can be as high as $19,639 per day for continued marketing and up to $147,290 — we have no idea how they arrive at those odd numbers.

So if you’re an American who hasn’t already got a Baofeng or similar, you might be well advised to pick one up while you still can.

UV5-R image via PE1RQM

Submarine To Plane: Can You Hear Me Now? The Hydrophone Radar Connection

September 23, 2018 by Brian McEvoy 34 Comments

How does a submarine talk to an airplane? It sounds like a bad joke but it’s actually a difficult engineering challenge.

Traditionally the submarine must surface or get shallow enough to deploy a communication buoy. That communication buoy uses the same type of radio technology as planes. But submarines often rely on acoustic transmissions via hydrophones which is fancy-talk for putting speakers and microphones in the water as transmitters and receivers. This is because water is no friend to radio signals, especially high frequencies. MIT is developing a system which bridges this watery gap and it relies on acoustic transmissions pointed at the water’s surface (PDF warning) and an airplane with high-precision radar which detects the oscillations of the water.

The complexity of the described setup is mind-boggling. Right now the proof of concept is over short distances and was tested in a water tank and a swimming pool but not in open water. The first thing that comes to mind is the interference caused by waves and by aerosols from wind/wave interactions. Those challenges are already in the minds of the research team. The system has been tested to work with waves of 8 cm (16 cm measured peak to trough) caused by swimmers in the pool. That may not sound like much, but it’s about 100,000 times the surface variations being measured by the millimeter wave radar in order to detect the hydrophone transmissions. Add to that the effects of Doppler shift from the movement of the plane and the sub and you have a signal processing challenge just waiting to be solved.

This setup is very interesting when pitched as a tool for researching aquatic life. The video below envisions that transmitters on the backs of sea turtles could send communications to aircraft overhead. We love seeing these kinds of forward-thinking ocean research projects, like our 2017 Hackaday prize winner which is an open source underwater glider. Oceanic studies over long distances have been very difficult but we’re beginning to see a lot of projects chipping away at that inaccessibility.

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