WiFi networking is one of those things that is reasonably simple to use, but has a lot of complex hidden features (dare we say, hacks) that make it work, or work better. For example, consider the Distributed Coordination Function (DCF) specified in the standard. Before a station can send, it has to listen for a certain time period. If the channel is clear, the station sends. If not, it has to delay a random amount of time before trying again. This is a form of Carrier Sense Multiple Access (CSMA) channel management.
Unfortunately, listening time is dead time when–at least potentially–there is no data transmitted on the network. DCF allows you to use various handshaking packets to do virtual carrier detection and ready/clear to send, but these are also less efficient use of bandwidth. There are other optional coordination functions available in the WiFi standard, but they all have their drawbacks.
[Aleksandar Kuzmanovic] at Northwestern University and two of his students have recently published a paper with a new way to coordinate multiple unrelated wireless networks using ubiquitous FM broadcast radio signals called WiFM. Instead of trying to synchronize to the WiFi data channel, this new scheme selects a strong FM radio station that broadcasts Radio Data Service (RDS) data (the data that populates the song titles and other information on modern radios).
What do you get if you cross a software defined radio (SDR) and an iconic children’s drawing toy that we are sure is a trademarked name? If you are [devnulling], you wind up with the Etch-A-SDR. The box uses an Odroid C1, a Teensy, and the ubiquitous RTL-SDR.
The knobs work well as control knobs (as you can see in the video below). When you are bored listening to the radio, you can reset the box and go into Etch-a… um, drawing mode. The knobs work like you’d expect and you can even erase the screen with a vigorous shake.
Construction crews tearing up the street to lay new internet fiber optic cable created a unique opportunity for [Bastian Bloessl]. The workers brought two mobile traffic lights to help keep the road safe while they worked. [Bastian] had heard that these lights use the 2 meter band radios, so he grabbed his RTL-SDR USB stick and started hacking. Mobile traffic lights are becoming more common in Europe. They can be controlled by a clock, traffic volume via an on-board camera, wire or radio. They also transmit status data, which is what [Bastian] was hoping to receive.
A quick scan with GQRX revealed a strong signal on 170.760 MHz. Using baudline and audacity, [Bastian] was able to determine that Audio Frequency Shift Keying was used to modulate the data. He created a simple receiver chain in GNU radio, and was greeted with a solid data stream from the lights. By watching the lights and looking at the data frames, [Bastian] was able to determine which bits contained the current light status. A quickly knocked up web interface allowed him to display the traffic light status in real-time.
It’s a bit scary that the data was sent in plaintext, however this is just status data. We hope that any command data is sent encrypted through a more secure channel.
[Veghead] recently went to a surplus warehouse filled with VHS editing studios, IBM keyboards, electronic paraphernalia from 40 years ago, and a lot of useless crap. His haul included a wooden keypad from an old alarm system that exuded 1980s futurism, and he figured it would be cool to hook this up to an alarm system from 2015. How did he do that? With software defined radio.
After pulling apart the alarm panel, [Veghead] found only a single-sided board with a 9V battery connector. There were no screw terminals for an alarm loop, meaning this entire system was wireless – an impressive achievement for the mid-80s hardware. A quick search of the FCC website showed this alarm panel was registered to two bands, 319MHz and 340MHz, well within the range of an RTL-SDR USB TV tuner dongle.
After capturing some of the raw data and playing it back in Audacity, [Veghead] found a simple OOK protocol that sends two identical binary patterns for each key. A simple program takes the raw bit patterns for each key press and codes them into a map for each of the twelve buttons.
Although the radio still works, [Veghead] found the waveforms captured by his RTL-SDR were an abomination to RF. All the components in this security system are more than 30 years old at this point, and surely some of the components must be out of spec by now. Still, [Veghead] was able to get the thing working again, a testament to the usefulness of a $20 USB TV tuner.
[Carl] just found a yet another use for the RTL-SDR. He’s been decoding Inmarsat STD-C EGC messages with it. Inmarsat is a British satellite telecommunications company. They provide communications all over the world to places that do not have a reliable terrestrial communications network. STD-C is a text message communications channel used mostly by maritime operators. This channel contains Enhanced Group Call (EGC) messages which include information such as search and rescue, coast guard, weather, and more.
Not much equipment is required for this, just the RTL-SDR dongle, an antenna, a computer, and the cables to hook them all up together. Once all of the gear was collected, [Carl] used an Android app called Satellite AR to locate his nearest Inmarsat satellite. Since these satellites are geostationary, he won’t have to move his antenna once it’s pointed in the right direction.
As far as antennas go, [Carl] recommends a dish or helix antenna. If you don’t want to fork over the money for something that fancy, he also explains how you can modify a $10 GPS antenna to work for this purpose. He admits that it’s not the best antenna for this, but it will get the job done. A typical GPS antenna will be tuned for 1575 MHz and will contain a band pass filter that prevents the antenna from picking up signals 1-2MHz away from that frequency.
To remove the filter, the plastic case must first be removed. Then a metal reflector needs to be removed from the bottom of the antenna using a soldering iron. The actual antenna circuit is hiding under the reflector. The filter is typically the largest component on the board. After desoldering, the IN and OUT pads are bridged together. The whole thing can then be put back together for use with this project.
Once everything was hooked up and the antenna was pointed in the right place, the audio output from the dongle was piped into the SDR# tuner software. After tuning to the correct frequency and setting all of the audio parameters, the audio was then decoded with another program called tdma-demo.exe. If everything is tuned just right, the software will be able to decode the audio signal and it will start to display messages. [Carl] posted some interesting examples including a couple of pirate warnings.
[Elia] was experimenting with LNAs and RTL-SDR dongles. If you’re receiving very weak signals with one of these software defined radio dongles, you generally need an LNA to boost the signal. You can power an LNA though one of these dongles. You’ll need to remove a few diodes, and that means no ESD protection, and you might push the current consumption above the 500mA a USB port provides. It does, however, work.
We’ve seen people open up ICs with nitric acid, and look inside them with x-rays. How about a simpler approach? [steelcityelectronics] opened up a big power transistor with nothing but a file. The die is actually very small – just 1.8×1.8mm, and the emitter bond wire doesn’t even look like it’ll handle 10A.
Gigantic Connect Four. That’s what the Lansing Makers Network built for a Ann Arbor Maker Faire this year. It’s your standard Connect Four game, scaled up to eight feet tall and eight feet wide. The disks are foam insulation with magnets; an extension rod (with a magnet at the end) allows anyone to push the disks down the slots.
Once again the ubiquitous USB TV tuner dongle has proved itself more than capable of doing far more than just receiving broadcast TV. Over on the RTL-SDR blog, there’s a tutorial covering the measurement of filter characteristics using a cheap eBay noise source and an RTL-SDR dongle.
For this tutorial, the key piece of equipment is a BG7TBL noise source, acquired from the usual online retailers. With a few connectors, a filter can be plugged in between this noise source and the RTL-SDR dongle. With the hardware out of the way, the only thing remaining is the software. That’s just rtl_power and this wonderful GUI. The tutorial is using a cheap FM filter, and the resulting plot shows a clear dip between 50 and 150 MHz. Of course this isn’t very accurate; there’s no comparison to the noise source and dongle without any attenuation. That’s just a simple matter of saving some scans as .csv files and plugging some numbers in Excel.
The same hardware can be used to determine the VSWR of an antenna, replacing the filter with a directional coupler; just put the coupler between the noise source and the dongle measure the attenuation through the range of the dongle. Repeat with the antenna connected, and jump back into Excel.