Measuring Filters and VSWR With RTL-SDR

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.

Using librtlsdr Over TCP

[Texane] built a low-cost software defined radio rig which could be remotely controlled. This allows the hardware to be placed outside for better reception, while being controlled from any PC that can connect over TCP. To do this, he created a fork of librtlsdr, the library used to turn cheap TV tuners into software defined radios.

The official release of rtl-sdr includes the rtl_tcp utility, which is meant for this purpose. Unfortunately, not all of the SDR tools for Linux support this. By modifying the library itself, remote devices interact with software in the same way as local devices. This means that any software that supports librtlsdr should work.

The outdoor rig contains a BeagleBone Black and the SDR hardware, sealed up in a weather-resistant box. This connects to [Texane]’s home network over ethernet, and allows SDR utilities to be run elsewhere.

This feature is quite experimental, but the source for the fork is provided for those who want to build the code and try it out.

Over-engineering Ding Dong Ditch

One day, [Samy]’s best friend [Matt] mentioned he had a wireless doorbell. Astonishing. Even more amazing is the fact that anyone can buy a software defined radio for $20, a small radio module from eBay for $4, and a GSM breakout board for $40. Connect these pieces together, and you have a device that can ring [Matt]’s doorbell from anywhere on the planet. Yes, it’s the ultimate over-engineered ding dong ditch, and a great example of how far you can take practical jokes if you know which end of a soldering iron to pick up.

Simply knowing [Matt] has a wireless doorbell is not enough; [Samy] needed to know the frequency, the modulation scheme, and what the doorbell was sending. Some of this information can be found by looking up the FCC ID, but [Samy] found a better way. When [Matt] was out of his house, [Samy] simply rang the doorbell a bunch of times while looking at the waterfall plot with an RTL-SDR TV tuner. There are a few common frequencies tiny, cheap remote controls will commonly use – 315 MHz, 433 MHz, and 900 MHz. Eventually, [Samy] found the frequency the doorbell was transmitting at – 433.8 MHz.

After capturing the radio signal from the doorbell, [Samy] looked at the audio waveform in Audacity. It looked like this doorbell used On-Off Keying, or just turning the radio on for a binary ‘1’ and off for a binary ‘0’. In Audacity, everything the doorbell transmits becomes crystal clear, and with a $4 434 MHz transmitter from SparkFun, [Samy] can replicate the output of the doorbell.

For the rest of the build, [Samy] is using a mini GSM cellular breakout board from Adafruit. This module listens for any text message containing the word ‘doorbell’ and sends a signal to an Arduino. The Arduino then sends out the doorbell code with the transmitter. It’s evil, and extraordinarily over-engineered.

Right now, the ding dong ditch project is set up somewhere across the street from [Matt]’s house. The device reportedly works great, and hopefully hasn’t been abused too much. Video below.

Continue reading “Over-engineering Ding Dong Ditch”

RTL SDR As A Spectrum Analyzer

RTL-SDR, the USB TV tuner turned software-defined radio is an amazing device, capable of listening to nearly anything from 25MHz to 1750MHz, fits in your pocket, and costs about $20. Even more astonishing is that it’s also a kinda-okay spectrum analyzer. [Kerry D. Wong] tested out one of these USB TV tuner, and the results are exactly what you would expect: it lacks a little precision, and sampling bandwidth is only a tiny bit terrible, but it does work.

A stock USB TV tuner doesn’t come with a connector that would normally be used for spectrum analysis. A BNC connector can be easily attached, as can a terminator to match the 75Ω impedance of the SDR. This isn’t really necessary; the frequencies being measured are low enough that you can get away without one.

As far as software goes, [Kerry] first pulled out the usual suspects of the SDR world; rtlsdr-scanner distorted the measured spectrum, as did a lot of other SDR receivers. Gqrx SDR was the first one that worked well, but the king of this repurposing of USB TV tuners was OSMOCOM. There’s a huge number of tools for spectrum analysis right out of the box with this package.

How did the RTL SDR fare as a spectrum analyzer? Feeding some stuff in from a signal generator, [Kerry] discovered the LO in the RTL SDR was off by a hair. Also, OSMOCOM only measures amplitude in dB, not the dBm found in every other spectrum analyzer ever made. By measuring a 0 dBm signal whatever value displayed can be shifted up or down.

So, does it work? Yes, it does. If, for some reason, you need a spectrum analyzer now, can you use this? Yep. Pretty cool.

SDR: Satellite Death Receiver

Halloween may be over, but [happysat] has found a way to listen to the dead. Satellites, that is, specifically those in the 136-138 MHz and 150-400 MHz ranges. He’s using an RTL-SDR dongle and a QFH antenna to detect the death throes of decommissioned navigation and space research satellites.

[happysat] was listening to NOAA/Meteor on the 137MHz band when he made this discovery. When a satellite is near end of life, the last bit of fuel is used to push it into graveyard orbit. This doesn’t always work, however, and when the light is just right, a chemical reaction makes the long-dead batteries conduct and these satellites in purgatory transmit once more.

They’re not sending out anything proprietary useful, just unmodulated carrier that sometimes interferes with currently operational satellites on the 136-138 MHz band. [happysat] captured some audio from two of the oldest satellites that are still broadcasting, and links to a TLE set of dead satellites he created. Check out his frequency database for SDR# as well. Don’t have a weather satellite-capable antenna? Build one!

[via /r/RTLSDR]

Hacklet 19 – Ham Radio


Amateur, or ham radio operators have always been hackers. For much of the early 1900’s, buying a radio was expensive or impossible. Hams would build their own rigs, learning electronics and radio theory along the way. Time moves on, but hams keep hacking. Today we’re highlighting some of the best ham radio projects on!


We start with [DainBramage1991] and his very practical RTL-SDR With Upconverter and Case. [DainBramage1991] fell in love with his low-cost RTL software defined radio dongle. He even added a Ham-It-Up upconverter to cover HF bands. The only problem was RF noise. the Realtek USB sticks tend to have little or no filtering, which means they are very susceptible to noise. [DainBramage1991] used the time-honored technique of insulating with copper clad board. Bits of PCB hold the RTL-SDR and upconverter in place. More PCB separates the two boards. Everything goes into a steel enclosure which keeps that unwanted RF at bay.

foxhunt-attenNext up is [Ryan Miller’s aka KG7HZQ]’s  ham radio fox hunt attenuator. Ham radio fox hunt’s don’t involve baying dogs or horses. In this case a fox hunt is a contest to find hidden low power transmitters. If you’ve never tried one, it’s a heck of a lot of fun. One of the challenges with a fox hunt is to find the direction to the transmitter when you’re very close. Even with directional antennas, reflections and swamped receivers make it hard to figure out just where the transmitter is. The solution is an attenuator, which simply reduces the signal to a more reasonable value. [Ryan] also used copper clad PCB for his circuit. Since the attenuator parts are soldered directly to the PCB, this is more of a Manhattan style design. Two ceramic 1k pots help him achieve his goal of near perfect linear attenuation. We’re betting this attenuator will help [Ryan] win some contests!

psdrWho says amateur radio won’t take you places? It may well be taking [Michael R Colton] to space! [Michael’s] project PortableSDR is one of the five finalists in The Hackaday Prize. We covered Michael earlier in the contest. PortableSDR started as a ham radio project: a radio system which would be easy for hams to take with them on backpacking trips. It’s grown into so much more now, with software defined radio reception and transmission, vector network analysis, antenna analysis, GPS, and a host of other features. We seriously love how [Michael] optimized a small LCD for waterfall display, tuning, and bandpass filter adjustment.

e2ra[W5VO] is working on an Ethernet to Radio Adapter. Every foot of coax in a radio system loses signal. Connections are even worse. It can all add up to several dB loss. [W5VO] wants to put an SDR at the antenna feed-point. With the signal path minimized, more watts make it out when transmitting, and more signal gets back to the receiver when listening. The interface between the SDR and host computer will be all digital; Ethernet to be precise. [W5VO] isn’t the first person to do something like this, microwave systems have had the transmitter and LNB at the antenna for years. That doesn’t take away from [W5VO’s] design at all  He’s been quiet for a while, but we’re hoping he continues on his design!

Where is everyone else? We’re a bit light on projects this week, but we have a good reason. There just aren’t enough ham radio projects on! We’re hoping to change that though. Are you an amateur radio enthusiast? Document your project on the site. Get input from other hams and push the envelope! You might even find yourself on the Ham Radio List!

That’s all for this episode of The Hacklet. As always, QRX is next week. Same hack time, same hack channel, bringing you the best of! 73’s!

PiAware, Automated Airliner Tracking On The Raspberry Pi


For the sufficiently geeky aviation nerd there’s FlightAware, a website that tracks just about every airliner and most private planes currently in flight. The folks at FlightAware compile all the information with the help of a few thousand volunteers around the world that have a bit of hardware to listen to ADS-B transmissions and relay them to the FlightAware servers. Now you can do this with a Raspberry Pi, and as a nice little bonus FlightAware is giving away free enterprise accounts to anyone who does.

Listening in on ADS-B transponders is something Raspberry Pis have been doing for a while, but doing anything useful with the altitude, speed, heading, and registry numbers of various planes flying overhead is pretty much FlightAware’s only reason for existing, and the reason they’ve developed an easy to use software package for the Pi.

Setting everything up requires getting dump1090 running on the Pi, the only hardware required being an RTL-SDR USB TV tuner, a GPS module, and an antenna for 1090 MHz. From there, just send all the data to FlightAware and you get a free enterprise account with them. Not a bad deal for the aviation nerds out there.