Hearing The Unhearable

August 7, 2021 by Elliot Williams 51 Comments

My wife was watching a crime drama, and one of the plot twists involved a witness’ hearing aid malfunctioning so that he could hear electromagnetic waves around him. It’s not so implausible, if you think about it. Many hearing aids have a t-coil, which is essentially an inductor that’s designed to couple with the speaker in a telephone. If that went haywire, maybe you could hear all the changing magnetic fields around you, and if you could escape the constant hum of the mains power line, it might even be interesting.

So of course, she turns to me and says “we need to make one!” It shouldn’t be hard at all — a big inductor and an amplifier should do the trick. In fact, it’ll probably be easy enough that it’ll make a good introduction-to-electronics project for my son. But there are also enough unknowns here that it’ll be interesting. How big a coil? How close? How sensitive? What about that mains frequency bit? Ferrite core or not?

None of this is rocket science, for sure, but it will probably be full of kludges, discoveries, and straight-up exploration. In short, the perfect weekend project. And in the end, it’ll expose something that’s normally invisible, and that’s where the fun lies.

This must be the same urge that drove Faraday and Marconi, Volta and Maxwell. There’s something amazing about directly sensing, seeing, hearing, and understanding some of the stuff that’s outside of our limited hearing and eyesight, and yet is all around us. I can write down the equations that describe it — I learned them in school after all — but there’s no substitute for poking around in your own home. Who knows, maybe in a few more weekends we’ll build ourselves an all-band receiver.

What’s your favorite super power?

SBITX: Hackable HF SDR For The Raspberry Pi

August 6, 2021 by Danie Conradie 32 Comments

Cheap, easy to use SDR dongles are an immensely powerful tool for learning about radio technology. However, building your own SDR is not something too many hackers are confident to tackle. [Ashhar Farhan, VU2ESE] hopes to change this with the sBITX, a hackable HF SDR transceiver designed around the Raspberry Pi.

[Ashhar] introduced the project in talk at the virtual “Four Days In May” annual conference of the QRP Amateur Radio Club International. Watch the full talk in the video after the break. He first goes over the available open source SDR radios, and then delves into his design decisions for the sBITX. One of the primary goals of the project was to lower the barrier of entry. To do this, he chose the Raspberry Pi as base, and wrote C code that that anyone who has done a bit of Arduino programming should be able to understand and modify. The hardware is designed to be as simple as possible. On the receive side, a simple superheterodyne architecture is used to feed a 25 kHz wide slice of RF spectrum to an audio codec, which send the digitized audio to the Raspberry Pi. The signal is then demodulated in software using FFT. For transmit, the signal is generated in software, and then upconverted to the desired RF frequency. [Ashhar] also created a GUI for the 7″ Raspberry Pi screen.

At the moment the sBITX is still in the development stage, information is spread between the video after the break, it’s accompanying PDF, the GitHub repo, and a thread on the BITX20 group.

[Ashar Farhan] is well known in the ham radio community for low cost radio designs like the BITX, and it’s successor, the μBITX. He also created the Antuino, an Arduino based antenna tester. Continue reading “SBITX: Hackable HF SDR For The Raspberry Pi” →

Phase Coherent Beamforming SDR

August 4, 2021 by Al Williams 16 Comments

The days when software defined radio techniques were exotic are long gone, and we don’t miss them one bit. A case in point: [Laakso Mikko’s] research group has built a multichannel receiver using 21 cheap RTL-SDR dongles to create a phase coherent array. This is useful for everything from direction finding and passive radar or beam forming. The code is also available on GitHub.

The phase coherence does require the dongle’s tuner can turn off dithering. That means the code only works with dongles that use the R820T/2. The project modifies the dongles to use a common clock and a switchable reference noise generator.

Continue reading “Phase Coherent Beamforming SDR” →

Fail Of The Week: Flipped Cable Leads To Fried Radio

August 3, 2021 by Donald Papp 18 Comments

[Doug]’s newly-installed Yaesu FT-891 mobile transceiver failed to power up despite a careful installation, and it turns out to have ultimately been caused by a reversed cable. There’s a happy ending, however. Since the only real casualties were a blown resettable fuse and a badly-burned resistor that damaged the PCB, [Doug] was able to effect a repair. Things could have been worse, but they also could have been better. Damage could have been prevented entirely with some better design, which [Doug] explains during his analysis of what went wrong.

The destroyed SMT resistor and pads were easily replaced with a through-hole version, thanks to the schematics.

The main problem was that the generic RJ12 cable that [Doug] used to connect radio components had its connections reversed. This would not be a problem if it was used to connect a landline telephone to the wall, but it was a big problem when used to connect the radio components together. According to the radio schematics, the two center wires carry +13 V and GND, which meant that a reversed cable delivered power with reversed polarity; never an optimal outcome.

Once the reversed power arrived at the other end, [Doug] discovered something else. Diodes whose job would be to protect against reverse polarity were marked DO NOT INSTALL, probably to shave a few cents off the bill of materials. As a result, the full 13 V was soaked up by a 1/8 W surface mount resistor which smoldered and burned until a fuse eventually blew, but not before the resistor and pads were destroyed. Thankfully, things cleaned up well and after replacing the necessary parts and swapping for a correct cable, things powered up normally and the mobile radio was good to go.

Curious for a bit more details about mobile radio installations? Check out our own Dan Maloney’s rundown on installing a discontinued (but perfectly serviceable) Yaesu FT-8900R.

Hacking A PIC To Redefine A Microphone’s Transmit Frequency

August 1, 2021 by Al Williams 1 Comment

Software defined radio and widespread software-controlled PLL synthesis for RF has been a game changer. Things like the RTL-SDR can be any kind of radio you like on almost any frequency you like. But not every SDR or PLL system opens the configuration doors to you, the end user. That was the problem [vgnotepad] faced when trying to connect a Sennheiser wireless microphone to some receivers. They didn’t use the same frequencies, even though the transmitter was programmable. The solution to that is obvious — hack the transmitter!

The post is only part one of several parts and if you read to the end, you’ll learn a lot about what’s inside the device and how to crack it. Luckily, the device uses a PIC processor, so getting to the software wasn’t a big issue.

Continue reading “Hacking A PIC To Redefine A Microphone’s Transmit Frequency” →

A Satellite Upconverter Need Not Be Impossible To Make

July 31, 2021 by Jenny List 8 Comments

Those readers whose interests don’t lie in the world of amateur radio might have missed one of its firsts, for the last year or two amateurs have had their own geostationary satellite transponder. Called Es’hail-2 / AMSAT Phase 4-A / Qatar-OSCAR 100, it lies in the geostationary orbit at 25.9° East and has a transponder with a 2.4 GHz uplink and a 10.489 GHz downlink. Receiving the downlink is possible with an LNB designed for satellite TV, but for many hams the uplink presents a problem. Along comes [PY1SAN] from Brazil with a practical and surprisingly simple solution using a mixture of odd the shelf modules and a few hand-soldered parts.

An upconverter follows a simple enough principle, the radio signal is created at a lower frequency (in this case by a 435 MHz transmitter) and mixed with a signal from a local oscillator. A filter then picks out the mixer product — the sum of the two — and amplifies it for transmission. [PY1SAN]’s upconverter takes the output from the transmitter and feeds it through an attenuator to a MiniCircuits mixer module which takes its local oscillator via an amplifier from a signal generator module. The mixer output goes through a PCB stripline filter through another amplifier module to a power amplifier brick, and thence via a co-ax feeder to a dish-mounted helical antenna.

The whole thing is a series of modules joined by short SMA cables, and could probably be largely sourced from a single AliExpress order without too much in the way of expenditure. It’s by no means easy to get on air via Es’hail-2, but at least now it need not be impossibly expensive. Even the antenna can be made without breaking the bank.

We covered Es’hail-2 when it first appeared. May it long provide radio amateurs with the chance to operate worldwide with homebrew microwave equipment!

Continue reading “A Satellite Upconverter Need Not Be Impossible To Make” →

Rare Radio Receiver Teardown

July 30, 2021 by Al Williams 15 Comments

We’ll admit we haven’t heard of the AGS-38, it reminds us of the shortwave receivers of our youth, and it looks like many that were made “white label” by more established (and often Japanese) companies. [Jeff] found a nice example of this Canadian radio and takes it apart for our viewing pleasure. He also found it was very similar to a Layfayette receiver, also made in Japan, confirming our suspicions.

The radio looks very similar to an Eico of the same era — around the 1960s. With seven tubes, radios like this would soon be replaced by transistorized versions.

Continue reading “Rare Radio Receiver Teardown” →