iPhone/iPad/iPod app to decode GMDSS transmissions
You may have heard of the latest SDR craze to hit the radio hobby – the RTL based USB dongle TV tuners. These were originally made to receive and decode the European standard digital television broadcasts. An enterprising hobbyist discovered that they can be tuned throughout the VHF and UHF range, and that you can get at the raw sampled data from the onboard A/D converter (only 8 bit, however). This allows them to be used as a very inexpensive Software Defined Radio (SDR) for VHF and UHF. How inexpensive? Mine was $17 shipped, although you can find them for even less, if you’re willing to get them direct from China and wait a few weeks for delivery.
There’s the dongle itself, as well as the small (about 4″) antenna.
It’s interested to note that the enclosure actually says SDR on it, the word has apparently gotten out about the SDR applications for this dongle, and someone is private branding them.
The USB connector is on the left, the MCX style RF connector is on the upper right.
There are control programs available for Windows, Mac OS X, and Linux. For software, first I decided to try rtl-sdr I copied the libraries to the specified locations, restarted, and was greeted with:
>:rtlsdr_osx cps$ rtl_test -t
Found 1 device(s):
0: ezcap USB 2.0 DVB-T/DAB/FM dongle
Using device 0: ezcap USB 2.0 DVB-T/DAB/FM dongle
Failed to open rtlsdr device #0.
It’s possible this is an older version of the rtl-sdr package, that expects the E4000 tuner chip. (Although a less cryptic error message would sure be helpful)
Then I tried Cocoa Radio. It crashes on launch. So far open sores is zero for two.
So next I tried the Mac OS X port of gqrx. Much better! It came right up, and within a minute I was receiving FM broadcast stations. I have noticed that if I make a change to the sample rate, I need to quit and re-start the app before putting it into run mode, or it crashes.
The sensitivity is not bad, I was able to pick up stations about 50 or 60 miles away using the included tiny 4″ antenna, laying on my desk.
I was also able to pick up 2m packet radio transmissions on 144.39 MHz, and one of the NOAA weather radio stations, on 162.525 MHz.
There are many varieties of these TV tuner dongles out there, mostly the difference is the RF tuner chip used. Previously the E4000 tuner was the preferred one for SDR applications, as it had the widest tuning range, although with a gap in the middle. It apparently is no longer made and is difficult to find tuners that use it. Currently the R820T tuner chip seems to be the preferred one for SDR use, the tuning range is slightly less, but there is no gap. Some eBay vendors identify the chip used, many do not, but there are lists online of the various USB dongles by brand name and model number, with the tuner chip specified, such as here.
My next project was mounting the dongle in a small metal enclosure, with a different RF connector, so I can easily connect one of my existing outdoor antennas to it. Read all about it here.