Radio Hacks

1467 Articles

Passive Radar Explained

April 12, 2026 by 31 Comments

It is an old trope in submarine movies. A sonar operator strains to hear things in the ocean but dares not “ping” for fear of giving away the boat’s location. Radar has a similar problem. If you want to find an airplane, for example, you typically send a signal out and wait for it to bounce off the airplane. The downside is that the airplane now knows exactly where your antenna is and, these days, may be carrying missiles to home in on it. In a recent post, [Jehan] explains how radar, like sonar, can be passive.

Even if you aren’t worried about a radar-homing missile taking out your antenna, passive radar has other advantages. You don’t need an expensive transmitter or antenna, a simple SDR can pull it off. You don’t need a license for the frequencies you want to use, either. You are just listening.

Continue reading “Passive Radar Explained” →

934 MHz: When The Government Really Doesn’t Want You To Have CB

April 7, 2026 by 39 Comments

In the mid 1970s there were a spate of movies depicting the romance and lifestyle of truck drivers in the southern half of the United States. Over on the other side of the Atlantic these were naturally received not as works of drama but as documentaries, and thus began a craze for British drivers to do up their Ford Capri so in the right light and with your eyes nearly closed, it almost looked like Burt Reynolds’ Pontiac Trans Am from Smokey and the Bandit.

Such a fine automobile was of course incomplete without a CB radio, highly illegal at the time, which led to an underground CB craze and its eventual legalization in 1981. [Ringway Manchester] is here with a tale from that era, of 934 MHz CB, an odd and underused allocation that was eventually phased out for commercial services.

When UK CB was eventually legalized by the government, it was very obvious that they really didn’t want to. Brits got 27 MHz as FM only with meager power and a weird set of frequencies that nobody else had, and a second band way up in the UHF range, at 934 MHz. We remember they originally tried to make a UHF band the only allocation on purpose because it was nearly useless for mobile operation, and Brits only got 27 MHz by fighting back in the political lobbying space.

The video below tells the story of the band, with relatively scarce and expensive equipment leading to it being an exclusive band more similar to the amateur bands, with little resemblance to its raucous 27 MHz counterpart. How much activity there was depended very much on where in the country you were, which of course wasn’t where your Hackaday scribe was as a teenager even if it had been affordable. Eventually the government saw the little flashing pound signs and grabbed it back for a mobile radio service that never materialized, and now the frequencies are part of the mobile phone spectrum.

Have a watch for an odd bit of UK radio nostalgia and some 2020s illegal CB’ers, and if you want more it’s a subject we’ve touched on before.

Continue reading “934 MHz: When The Government Really Doesn’t Want You To Have CB” →

An overlay is shown on a topographical map. High points are highlighted in blue. The letters "A" and "B" are shown in red text at two points.

Using A Scientific Satellite For Passive Radar

March 30, 2026 by 5 Comments

The basic principle of radar systems is simple enough: send a radio signal out, and measure the time it takes for a reflection to return. Given the abundant sources of RF signals – television signals, radio stations, cellular carriers, even Wi-Fi – that surround most of us, it’s not even necessary to transmit your own signal. This is the premise of passive radar, which uses passive RF illumination to form an image. The RF signal doesn’t even need to come from a terrestrial source, as [Jean Michel Friedt] demonstrated with a passive radar illuminated by the NISAR radar-imaging satellite (pre-print paper).

NISAR is a synthetic-aperture radar satellite jointly built by NASA and ISRO, and it completes a pass over the world every twelve days. It uses an L-band chirp radar signal, which can be picked up with GNSS antennas. One antenna points up towards the satellite, and has a ground plane blocking the signal from directly reaching the second antenna, which picks up reflections from the landscape under observation. Since the satellite would illuminate the scene for less than a minute, [Jean-Michel] had to predict the moment of peak intensity, and achieved an accuracy of about three seconds.

The signals themselves were recorded with an SDR and a Raspberry Pi. High-end, high-resolution SDRs such as the Ettus B210 gave the best results, but an inexpensive homebuilt MAX2771-based SDR also produced recognizable images. This setup won’t be providing any particularly detailed images, but it did accurately show the contours of the local geography – quite a good result for such a simple setup.

If you’re more interested in tracking aircraft than surveying landscapes, check out this ADS-B-synchronized passive radar system. Although passive radar doesn’t require a transmitter license, that doesn’t mean it’s free from legal issues, as the KrakenSDR team can testify.

Stream Deck Radio Controller Built With Cheap Yellow Display

March 23, 2026 by 4 Comments

Stream decks are pretty useful in all kinds of contexts, but commercial models can feel a bit pricy for what is effectively a bunch of buttons. [WhiskeyTangoHotel] has whipped up one of their own on the cheap using some readily available parts.

The build came about due to the use of Stream Decks as a common way to control the Flex-6400 software-defined radio. [WhiskeyTangoHotel] figured that using a full-priced Elgato Stream Deck was overkill for this purpose, and that a cheaper interface could be put together for less. Enter the Cheap Yellow Display—a combination of the ESP32 microcontroller with a 2.8-inch touchscreen LCD. It was simple enough to code the device such that it had four big touch buttons to control RIT-, RIT+, XIT-, and XIT+ on the Flex-6400. Plus, with the ESP32 having WiFi onboard, it’s able to control the radio wirelessly—you just need to feed the unit 5 volts, and you’re up and running.

Continue reading “Stream Deck Radio Controller Built With Cheap Yellow Display” →

An oscilloscope display is seen in lower left corner. In the rest of the image, two purple circuit boards are connected by SMA RF cables. A wire antenna is connected to one board.

Building A $50 SDR With 20 MHz Bandwidth

March 22, 2026 by 9 Comments

Although the RTL-SDR is cheap, accessible, and capable enough for many projects, it does have some important limitations. In particular, its bandwidth is limited to about 3.2 MHz, and the price of SDRs tends to scale rapidly with bandwidth. [Anders Nielsen], however, is building a modular SDR with a target price of $50 USD, and has already reached a bandwidth of almost 20 MHz.

If this project looks familiar, it’s because we’ve covered an earlier iteration. At the time, [Anders] had built the PhaseLoom, which filters an incoming signal, mixes it down to baseband, and converts it to I/Q signals. The next stage is the PhaseLatch, a board housing a 20-MHz, 10-bit ADC, which samples the in-phase and quadrature signals and passes them on to a Cypress FX2LP microcontroller development board. [Anders] had previously connected the ADC to a 6502 microprocessor instead of the FX2LP, but this makes it a practical SDR. The FX2LP was a particularly good choice for this project because of its USB 2.0 interface, large buffers for streaming data, and parallel interface. It simply reads the data from the SDR and dumps it to the computer.

Continue reading “Building A $50 SDR With 20 MHz Bandwidth” →

Analog Video From An 8-Bit Microcontroller

March 21, 2026 by 24 Comments

Although the CRT has largely disappeared from our everyday lives, there was a decades-long timeframe when this was effectively the only display available. It’s an analog display for an analog world, and now that almost everything electronic is digital, these amazing pieces of technology are largely relegated to retro gaming and a few other niche uses. [Maurycy] has a unique CRT that’s small enough to fit in a handheld television, but since there aren’t analog TV stations anymore, he decided to build his own with nothing but an 8-bit microcontroller and a few other small parts.

Continue reading “Analog Video From An 8-Bit Microcontroller” →

Open Source Radar Has Up To 20 KM Range

March 12, 2026 by 48 Comments

Phased-array radars are great for all sorts of things, whether you’re doing advanced radio research or piloting a fifth-generation combat aircraft. They’re also typically very expensive. [Nawfal] hopes to make the technology more affordable with an open-source radar design of their own.

The design is called the AERIS-10, and is available in two versions. Operating at 10.5 GHz, it can be built to operate at ranges between 3 or 20 kilometers depending on the desired spec. The former uses an 8 x 16 patch antenna array, while the latter extends this to a 32 x 16 array. Either way, each design is capable of fully-electronic beam steering in azimuth and can be hacked to enable elevation too—one of the most attractive features of phased array radars. The hardware is based around an STM32 microcontroller, an FPGA, and a bunch of specialist clock generators, frequency synthesizers, phase shifters, and ADCs to do all the heavy lifting involved in radar.

Radar is something you probably don’t spend a lot of time thinking about unless you’re involved in maritime, air defence, or weather fields. All of which seem to be very much in the news lately! Still, we feature a good few projects on the topic around these parts. If you’ve got your own radar hacks brewing up in the lab, don’t hesitate to let us know.