AM radios were easy to understand. The strength of the signal goes up and down, and the audio follows. FM radio is a little more difficult. [AllAmericanFiveRadio] has an old tube FM set and takes us on a tour of how the FM discriminator works. You can see the video below.
The first step is to look at the IF signal on the scope. It is hard to see, but the frequency is changing, and that’s the basis of modulation that the discriminator has to resolve.
As far as timekeeping goes, there’s nothing more accurate and precise than an atomic clock. Unfortunately, we can’t all have blocks of cesium in our basements, so various agencies around the world have maintained radio stations which, combined with an on-site atomic clock, send out timekeeping signals over the air. In the United States, this is the WWVB station located in Colorado which is generally receivable anywhere in the US but can be hard to hear on the East Coast. That’s why [JonMackey], who lives in northern New Hampshire, built this WWVB simulator.
Normally, clocks built to synchronize with the WWVB station include a small radio antenna to receive the 60 kHz signal and the 1-bit-per-second data transmission which is then decoded and used to update the time shown on the clock. Most of these clocks have internal (but much less precise) timekeeping circuitry to keep themselves going if they lose this signal, but [JonMackey] can go several days without his clocks hearing it. To make up for that he built a small transmitter that generates the proper timekeeping code for his clocks. The system is based on an STM32 which receives its time from GPS and broadcasts it on the correct frequency so that these clocks can get updates.
The small radio transmitter is built using one of the pins on the STM32 using PWM to get its frequency exactly at 60 kHz, which then can have the data modulated onto it. The radiating area is much less than a meter, so this isn’t likely to upset any neighbors, NIST, or the FCC, and the clocks need to be right beside it to update. Part of the reason why range is so limited is that very low frequency (VLF) radios typically require enormous antennas to be useful, so if you want to listen to more than timekeeping standards you’ll need a little bit of gear.
Crosley was a famous name in radio for more than one reason. The National VOA Museum of Broadcasting has a video telling [Powel Crosley Jr.’s] story, and the story behind the 500 kW WLW transmitter. WLW was an AM broadcast station often called the nation’s channel since its signal covered most of the United States. The first Crosley station was identified at 8CR, running 20 watts from [Crosley’s] living room. Quite a modest start! By 1922, he had moved to his family business location along with 500 watts of output. Over the years, WLW got more powerful until it was finally a 500 kW giant.
There are plenty of hobbies around with huge price tags, and ham radio can certainly be one of them. Experienced hams might have radios that cost thousands of dollars, with huge, steerable antennas on masts that can be similarly priced. But there’s also a side to the hobby that throws all of this out of the window in favor of the simplest, lowest-cost radios and antennas that still can get the job done. Software-defined radio (SDR) turned this practice up to 11 as well, and this radio module uses almost nothing more than a microcontroller to get on the air.
The design uses the capabilities of the Raspberry Pi Pico to handle almost all of the radio’s capabilities. The RF oscillator is driven by one of the Pico’s programmable I/O (PIO) pins, which takes some load off of the processor. For AM and SSB, where amplitude needs to be controlled as well, a PWM signal is generated on another PIO which is then mixed with the RF oscillator using an analog multiplexer. The design also includes a microphone with a preamplifier which can be fed into a third PIO; alternatively it can receive audio from a computer via the USB interface. More processor resources are needed when generating phase-modulated signals like RF, but the Pico is still quite capable of doing all of these tasks without jitter larger than a clock cycle.
Of course this only outputs a signal with a few milliwatts of power, so for making any useful radio contacts with this circuit an amplifier is almost certainly needed. With the heavy lifting done by the Pico, though, the amplifier doesn’t need to be complicated or expensive. While the design is simple and low-cost, it’s not the simplest radio possible. This transmitter sends out radio waves using only a single transistor but you will be limited to Morse code only.
The FM broadcast band has been with us since the middle of the 20th century, and despite many tries to unseat it, remains a decent quality way to pick up your local stations. It used to be that building an FM broadcast receiver required a bit of RF know-how, but the arrival of all-in-one receiver chips has made that part a simple enough case of including a part. That’s not to say that building a good quality FM broadcast receiver in 2024 doesn’t involve some kind of challenge though, and it’s one that [Stefan Wagner] has risen to admirably with his little unit.
Doing the RF part is an RDA5807MP single chip radio, but we’d say the center of this is the CH32V003 RISC-V microcontroller and its software. Twiddling the dial is a thing of the past, with a color display and all the computerized features you’d expect. Rounding it off in the 3D printed case is a small speaker and a Li-Po pouch cell with associated circuitry. This really is the equal of any commercially produced portable radio, and better than many.
Even with the all-in-one chips, there’s still fun in experimenting with FM the old way.
These days, affordable software defined radios (SDRs) have made huge swaths of the spectrum available to hobbyists. Whether you’re looking to sniff the data from that 433 MHz thermometer you’ve got in the backyard or pick up transmissions from satellites, the same little USB-connected box can make it happen.
But even the best SDR is constrained by the antenna it’s connected to, and that’s where it can still get a little tricky for new players. Luckily, there’s a new option for those who want to pick up signals from space without breaking the bank: the Discovery Dish by KrakenRF. After reaching 105% of its funding goal on December 20th, the handy little 65-cm aluminum reflector looks like it’s on track to ship out this summer.
The Discovery Dish was designed from the ground up to enable hobbyists to receive real-time weather data from satellites transmitting in the L band (GOES, NOAA, Meteor, etc.) and experiment with hydrogen line radio astronomy. Neither of which are anything new, of course. But having a pre-built dish and feed takes a lot of the hassle out of picking up these distant signals.
Although the current prototype has a one-piece reflector, the final Discovery Dish will break down into three “petals” to make storage and transport easier. If you don’t want to take it all the way apart, you can simply remove the feed to make it a bit more compact. Speaking of which, KrakenRF is also offering three different feeds depending on what signals you’re after: L band, Inmarsat, or hydrogen line.
You still have options if you’ve got to keep your radio hacking on a tighter budget. As we saw recently, you can actually pull an ET and pick up weather satellites using a foil-lined umbrella. Or spend a little at the big box hardware store and grab some aluminum flashing.
Continue reading “Discovery Dish Lets You Pick Up The Final Frontier”
Microwave imaging is similar to CT imaging, but instead of X-rays, the microwaves are used to probe the structure and composition of an object. To facilitate experimentation with microwave imaging, [Zehao Li] and [Kapil Gangwar] developed a system based on the RP2040 to control the height and rotation of a test object.
Their control system has a refreshingly physical user interface—a keypad. The keypad is used to configure the object’s position and the scanning step size, while user menus and the sample position are displayed in a clean and uncluttered interface over VGA. The RP2040 runs a multi-threaded program to handle user input, VGA display, and precise driving of two stepper motors for sample positioning.
The microwave imaging was performed by measuring the RF transmission over 2.5-8 GHz between two Vivaldi antennas on either side of the sample at a variety of angles. 2D cross-sections of the test object were reconstructed in Matlab using filtered back-projection. In this proof-of-concept demonstration, a commercial vector network analyzer was used to collect the data, but one could imagine migrating to a software defined radio (SDR) in the future.
A video demonstrating the system is embedded below the break. If you’re interested in DIY radio imaging, you might be interested in this guide to building your own synthetic aperture radar setup, or this analysis of an automotive radar chip.
