Radio Hacks

1339 Articles

A DIY Step Attenuator, By Gluing Together Two Smaller Ones

June 4, 2019 by 4 Comments

In the RF world, attenuators are a useful test and measurement tool. Variable units that can apply different levels of attenuation in discrete steps are even better. [DuWayne] made a 63 dB step attenuator by putting two smaller units in series, with an Arduino Nano in control of them. With a 3D printed enclosure and OLED for feedback, the device is easily adjusted with a single rotary encoder. There was even room to add a micro USB plug for recharging the power supply.

The two smaller digital attenuators [DuWayne] used are essentially breakout boards for the PE4302 digital RF attenuator, and cheaply available from the usual overseas sources. They are capable of up to 31.5 dB of attenuation in 0.5 dB steps, and by using two in series (and controlling them in parallel) [DuWayne] gets a range of up to 63 dB. The design files can be downloaded from a Dropbox share for the project, should you wish to try any of it for yourself.

Are you interested in RF and maybe software defined radio (SDR)? We’ve covered all the stuff you’ll need to get started with an inexpensive RTL-SDR, and sooner or later you may find yourself in need of [Dan Maloney]’s info on cheap and effective dummy loads.

Your Table Is Ready, Courtesy Of HackRF

June 4, 2019 by 28 Comments

Have you ever found yourself in a crowded restaurant on a Saturday night, holding onto one of those little gadgets that blinks and vibrates when it’s your turn to be seated? Next time, bust out the HackRF and follow along with [Tony Tiger] as he shows how it can be used to easily fire them off. Of course, there won’t actually be a table ready when you triumphantly show your blinking pager to the staff; but there’s only so much an SDR can do.

Even if you aren’t looking to jump the line at your favorite dining establishment, the video that [Tony] has put together serves as an excellent practical example of using software defined radio (SDR) to examine and ultimately replicate a wireless communications protocol. The same techniques demonstrated here could be applied to any number of devices out in the wild with little to no modification. Granted these “restaurant pagers” aren’t exactly high security devices to begin with, but you’d be horrified surprised how many other devices out there take a similarly cavalier attitude towards security.

[Tony] starts by using inspectrum to examine the Frequency-shift keying (FSK) modulation used by the 467.750 Mhz devices, and from there, uses Universal Radio Hacker to capture the actual binary data being sent over the air. Between studying the transmissions and the information he found online, he was eventually able to piece together the packet structure used by the restaurant’s base station.

Finally, he wrote a Python script which generates packets based on which pager he wants to set off. If he’s feeling particularly mischievous, he can even set them all off at once. The script outputs a binary file which is then loaded into GNU Radio for transmission via the HackRF. [Tony] says he’s not quite ready to release his script yet, but he gives enough information in the video that the intrepid hacker could probably get their own version up and running by the time he gets it posted up to GitHub anyway.

We saw some very similar techniques demonstrated at the recent WOPR Summit security conference, so once you’re done hacking the local restaurants, you can take these same lessons and apply them to the rest of the Internet of Things. If you’re wondering, it’s even easier to eavesdrop on the non-restaurant pagers.

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A Doppler Radar Module From First Principles

May 31, 2019 by 5 Comments

If you’ve ever cast your eyes towards experimenting with microwave frequencies it’s likely that one of your first ports of call was a cheaply-available Doppler radar module. These devices usually operate in the 10 GHz band, and the older ones used a pair of die-cast waveguide cavities while the newer ones use a dielectric resonator and oscillator on a PCB. If you have made your own then you are part of a very select group indeed, as is [Reed Foster] and his two friends who made a Doppler radar module their final project for MIT’s 6.013 Applications of Electromagnetics course.

Their module runs at 2.4 GHz and makes extensive use of the notoriously dark art of PCB striplines, and their write-up offers a fascinating glimpse into the world of this type of design. We see their coupler and mixer prototypes before they combined all parts of the system into a single PCB, and we follow their minor disasters as their original aim of a frequency modulated CW radar is downgraded to a Doppler design. If you’ve never worked with this type of circuitry before than it makes for an interesting read.

We’ve shown you a variety of commercial Doppler modules over the years, of which this teardown is a representative example.

Using A Cheap Handheld Radio As A Morse Transceiver

May 20, 2019 by 38 Comments

Both grizzled hams and potential future amateur radio operators are well-served by the market these days. Powerful and capable UHF and VHF handheld transceivers can now be had for well under $100, something unimaginable as recently as 20 years ago. Of course, a major part of the amateur radio scene used to be Morse code. Not to worry though, you can do that with a handheld, too!

The setup is simple but effective. A Morse code training unit generates tones in response to input from a Morse keyer. This audio is passed into the headset port of a Baofeng handheld transmitter. A toggle switch is wired up to the Push-To-Transmit circuit of the Baofeng to trigger transmission when required.

It’s a little different from the more typical constant-wave transmission methods that are so seldom used nowadays, but it gets the job done. Morse code has always been appreciated in situations where voice transmission is difficult due to low bandwidth or interference, and now it’s easy for new hams to give it a try.

Morse code can be a trial to learn, but spare a thought for the folks who had to pick it up back in 1939. Video after the break.

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The $50 Ham: Dummy Loads, Part 2

May 17, 2019 by 21 Comments

In the last installment of “The $50 Ham” I built a common tool used by amateur radio operators who are doing any kind of tuning or testing of transmitters: a dummy load. That build resulted in “L’il Dummy”, a small dummy load intended for testing typical VHF-UHF handy talkie (HT) transceivers, screwing directly into the antenna jack on the radio.

As mentioned in the comments by some readers, L’il Dummy has little real utility. There’s actually not much call for a dummy load that screws right into an HT, and it was pointed out that a proper dummy load is commercially available on the cheap. I think the latter observation is missing the point of homebrewing specifically and the Hackaday ethos in general, but I will concede the former point. That’s why at the same time I was building L’il Dummy, I was building the bigger, somewhat more capable version described here: Big Dummy.

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Pluto (SDR) Goes Ethernet

May 10, 2019 by 14 Comments

Pluto may no longer be a planet, but it is still a fun software defined radio (SDR) set up from Analog Devices. The inexpensive radio uses a USB connector and looks somewhat like a network connection to your PC. But what if you want to really use it with a network? [SignalsEverywhere] shows you how to do it using a USB network adapter and a USB connection adapter.

Just plugging a USB dongle into the box isn’t sufficient, an extra power supply is required as well as a minor bit of configuration. The IP address will be static so you might want to use an IP that your DHCP server won’t hand out, or reserve the IP on your local network.

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Clock Radio Receives Upgrade

May 5, 2019 by 8 Comments

Before everyone had a cell phone alarm to wake them up in the mornings, most of us used clock radios that would faithfully sit by our beds for years. You could have either a blaring alarm to wake you up, or be gently roused from slumber by one of your local radio stations. These devices aren’t as commonly used anymore, so if you have one sitting in your parts drawer you can make some small changes and use it to receive radio stations from a little further away than you’d expect.

This Panasonic clock radio from [Ryan Flowers] has several upgrades compared to the old clock radio hardware. For one, it now can receive signals on the 7 and 14 MHz bands (40 and 20 meters). It does this by using separate bandpass filters for each frequency range, controlled by a QRP Labs VFO kit which can switch between the two filters automatically once programmed. The whole thing is powered by 8 AA batteries, true to form with a clock radio from the ’90s.

[Ryan] notes that his first iteration was a little quiet but he’s now able to receive radio stations from as far away from Japan with this receiver. Even without a license, you can make these changes and listen in to stations from all around the world, as long as you don’t start transmitting. If you want to make a small upgrade from this clock radio though, it’s not that hard to get into.

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