I had my first real taste of radio decoding back in the 90’s. Things were a little different then though. For starters you needed a scanner or HF radio (or both), a demodulator (from Pervisell), leads to connect the radio audio output to a computer soundcard and of course software (or just plain old DOS) and a PC to process everything. At the time I had a subscription to Shortwave Magazine (sadly no longer in print) and the decode section was a godsend.
Each decoding practice had different protocols and often things didn’t work out as planned. Many hours (or days) were spent tinkering with leads, interface settings (bloody COM ports!) and antenna locations. If you were unlucky the Windows blue screen of death would make an appearance every now and again!
Successes included Slow Scan TV (SSTV), a system used by radio amateurs to send images around the world. This was exciting stuff! Waiting for the illusive signal, syncing and then watching the image slowly unfold line by line. DTMF was another area that worked quite well, weather FAX imagery was always a crowd pleaser and Aircraft Communications Addressing and Reporting System (ACARS) likewise. An area that failed to deliver was pager messages known as Post Office Code Standardization Advisory Group (POCSAG). Those torturous signals never did seem to communicate with me. All in all very interesting.
With the advent of more powerful computers, plug and play USB connection methods and innovative enthusiast groups things improved. That said, I pretty much stopped decoding and returned to regular monitoring. I often hooked up my Icom IC-R20 to Ham Radio Deluxe (back when it was still freeware) and used the powerful computer controlled features to operate the rig. My decoding interest had waned – frankly it was too much hassle.
Digital slowly gathered pace replacing numerous services that were once enjoyed on a regular radio. Emergency services, TV, even PMR has started to make the switch. Funnily enough decoding weird and wonderful signals hasn’t changed a lot. There are still plenty of enthusiasts using the old school approach. Well that is changing. SDR is where its at now.
SDR stands for Software Defined Radio. The “Radio” itself is no longer a big box covered in knobs and dials or full of filters, modulators, demodulators and amplifiers, oh no. Its just a small USB dongle not much bigger than a memory stick. The “Software Defined” bit is where the radio gets its teeth. All the traditional components are replaced by software enabling the user to exploit more features and modes than any conventional receiver could dream of including Digital Signal Processing (DSP). In addition software can be updated and manipulated unlike a hard coded system.
Software defined radios aren’t exactly new though. The Icom IC-PCR1500 was popular a few years ago and while it was not exactly an SDR in the truest sense, it was similar – a predominantly computer controlled and operated receiver for the most part (it did have a conventional plug in LCD display for mobile use). The IC-PCR1500 was also very expensive.
No, what we are talking about is a digitally inspired low cost SDR and when I say low cost I mean a fully functional receiver for £10 ($15) or less. Originally designed for EU DAB/DVB-T coverage it was discovered that by using the appropriate WinUSB driver the extensive frequency range could be opened up and tuned manually with software.
The Realtek RTL2832U+R820T is just one such SDR and this model is the number one choice for enthusiasts, primarily because of its sensitivity around 1090.000 MHz (the frequency used for ADS-B or Virtual Radar). In addition its frequency range is phenomenal 20MHz-1.7GHz (no gaps) with RF Gain control from 0-49.6 dB. Barring the lower HF section (0-20MHz) that is pretty much high end scanner territory.
When it comes to purchasing one of these USB SDR’s just make sure it has the Realtek RTL2832U+R820T chipset. There are numerous DVB-T dongles around, but not all will work as an SDR, double check the spec! I purchased mine from Amazon and read through the reviews to make sure it was exactly what I was after.
These are commonly sold in a blister pack (see photo above) and generally unbranded (I noticed recently Amazon is now selling a “Keedox” branded dongle). Inside is the dongle itself, a small magnetic base antenna with 1m cable terminating in a micro coax MCX connector (push fit into side of dongle), a CD for BlazeHD digital TV/Radio software/drivers and a IR remote control (again for use with the TV/Radio software).
Quality is good. The dongle itself is plastic and has a rubber non slip coating. A bright blue LED is always on (when plugged in) and the USB end has a cap to protect the connector when not in use. Despite having air vents the dongle does get quite hot during operation. Also if you are use it with a USB extension lead make sure the connection is tight (or secured with insulation tape) as a slight bit of movement will terminate functionality.
The 120mm stock antenna is adequate for test purposes and works surprisingly well on 1090.000 MHz (ADS-B). The magentic base (best attached to a tin lid to act as a groundplane) has a tiny screw thread and fortunately these are interchangeable with similar DVB antennas. A quick fix improvement came via an August DTA207 – this model has a 175-585mm telescopic antenna and while the cable terminates in a regular TV RF socket – not useful as the dongle is MCX, the screw thread is the same. So I duly swapped them out – huge improvement and this enabled me to use the SDR across a much wider range of frequencies without spending a fortune (the DTA207 is about £5!). Of course making a dedicated dipole or collinear antenna is easy and cheap enough, but for what I use it for (ADS-B and AIS) the telescopic works brilliantly.
First things first.
Don’t install the software or drivers that came with the unit! To use the dongle as an SDR you’ll need to install a specific WinUSB driver. Installing the CD drivers does confuse things somewhat and conflictions are common place. For the sake of the review I did install them on a spare PC (to see what would happen!) and while it didn’t cause a huge problem – the drivers are easily overwritten with Zadig, see below – it did throw up a few issues when I tried to uninstall them! Don’t bother unless you really want to watch digital TV!
What is required is a program called Zadig – a Windows driver installer library for USB devices. Make sure you download the correct version for your system (either Windows XP, 7 or 8). I set mine up on a Windows XP system and thankfully didn’t have any issues.
Installation involves selecting the dongle and installing the WinUSB (v6.1.7600.16385) driver. There are lots of user guides online (an excellent source is here). Make sure Zadig.exe is running, insert the dongle into a spare USB port and from the Options menu select List All Devices. You’ll notice that all USB devices are displayed but the one we are after is the one named Bulk in, Interface (Interface 0). Select that and make sure the WinUSB driver is the one selected for installation. Click install driver and after a short while (up to but not exceeding 5 minutes as Zadig will abort after that) you should be good to go. A dialog will pop-up and if all has gone to plan the driver will be installed and ready to use. In some instances a reboot is required. The driver needs to be installed every time a new USB port is used – so remember if things suddenly stop working it could be that. Occasionally the system seems to forget the driver is installed too – if that happens, you guessed it, simply reinstall the driver. There is no need to run Zadig every time you run a SDR program unless you are using a USB port for the first time.
Now we need to test the SDR! Check that out here.