Friday, November 15, 2019

Reception of Digital Radio on the RTL SDR Receiver

Scanner receivers and are often limited in the areas they can receive. The availability of cheapish Baofeng radios also opened up access to areas of spectrum but again, coverage was limited. In many cases commercial users have either migrated to digital systems or have moved to the UHF spectrum and employed digital modes of operation in that area.
With this in mind, the RTL2832U + R820T2SDR was purchased as it covers a spectrum from 25 MHz to 1.7 GHz and covers 100 KHz to 30MHz. A package shown below cost only £20.00. The Hack RF SDR would be considered at a later date if initial work on the RTL SDR proved favourable.

The RTL SDR radio is wholly dependant on Software to make it function and there are many variations. There are many programs available, but SDR Sharp was the first program tried. Nowadays, the programs are almost ready to go following installation. It is important to install the USB Driver for he SDR dongle before it will function with the software. There is plenty of documentation on YouTube to assist with the set up so installation was simple. 

The Broadcast band was the first area to test. The miniature antenna was sure to pick up a good range of signals to test the reception and quality. All of the parameters of the receiver are software selectable so bandwidth and gain are important for reception of wide band FM signals.

SDR Sharp was tried initially as it was easy to set up and gave good results straight away with the miniature antenna supplied. This is an excellent program for everyday use on HF, VHF and UHF. It was easy to install and once the driver program Zadig had been run, it required very little to set up.

The next area to test was the Amateur bands. The likelyhood or receiving an amateur transmission on VHF or UHF can be pretty slim in Galway By removing the antenna from the SDR receiver, it was possible to receive low power transmissions from the Handheld on both VHF and UHF. Kerchunking the local repeater produced some brief signals.
HF reception was possible but the miniature antenna was really not sufficient to make any impact. By connecting a longer length of wire, signals started to appear. An arbitrary length of wire with 9:1 transformer would produce better results. SSB and AM signals were clear and of reasonable quality. Data Modes such as RTTY, PSK were easily received but there are probably software packages to resolve them from a SDR radio. Not really a priority.
Digital Radio signals were of more importance but SDR Sharp requires plug ins and virtual audio cable and an additional plug in DSD. When these plugins are added in it is possible to decode a wide range of digital voice modes.

Another program, SDRangel was more complex with all of the facilities to decode digital voice signals. Provided that the USB driver has been installed, SDR Angel will work immediately after installation.

To start up, it is necessary to select the device connected, in this case the RTL SDR, and the reception of Digital Voice transmissions is done via the DSD Demodulator.
For the purpose of initial tests, the SDR radio was set up to receive the output of the hotspot. The hotspot was able to receive DMR, D-Star and C4FM on their respective active channels. 

The Screen shots for the various modes are shown below:
DMR operation

In the DMR mode it may be necessary to tune slightly offset from the centre frequency to decode DMR transmissions. The audio was amazingly clear and the DMR ID appeared on the line just above the constellation graphic. It may be necessary to adjust the tuning very slightly but the waterfall display and the constellation graphic will assist. 

D-Star Operation

D-Star signals were easy enough to tune. Note the Shape on the constellation graph. If centred as shown, the D-Star signal will become audible. It may be necessary to fine tune the frequency offset very slightly to ensure that the tuning is centred. Again the signals are very clear.

C4FM Operation

C4FM signals were easy to tune and fine tuning was facilitated by observation of the constellation graphic. Once the signal is centred, the audio became clear.

The next test will be to hook in a more resonant VHF/UHF colinear to the SDR receiver. 

The SDR receiver has many uses:

Full coverage from 100KHz to 1.7GHZ with no gaps.
Broadcast Radio - Reception on HF and VHF 
Decoding RDS signals
DAB Radio Reception
Scanner Receiver 
Air Traffic
Marine Traffic
Public Services  
Decoding Unencrypted Digital Voice - DMR, D-Star, C4FM, P25
Decoding Pager POCSAG/FLEX traffic

Reception and decoding of GPS Signals

Reception of Automatic Dependant Surveillance - Broadcast Mode -S Transponders which broadcast altitude and location information to air traffic controllers. With the appropriate software, these signals can be can be used to produce a "radar" like display on the computer. ADS-B receiver systems can retail for over £200.00. 

Plane plotter is an inexpensive commercially produced application that can be used with an RTL SDR Dongle as shown below

AIS Reception of Marine traffic similar to the ADS-B

Monitoring Meteor Scatter
Radio Astronomy

Spectrum analyser 

Panadaptor to sweep the receiver I.F. showing Waterfall display of adjacent channels. Ideal for spotting activity on a quiet band. One could spend fortunes on a unit manufactured for a specific radio but despite all of the frills and facilities, the basis of the unit is an SDR receiver module.

Triangulation of transmitter locations

and so much more ........

All of the above can be performed for just £20.00. However, it is planned to get a more expensive Hack RF system which has the capability to transmit 30mW throughout its coverage up to 2 GHz. The RTL is one of the less sophisticated SDR systems available but is an excellent starting point for experimentation. 

SDRangel is a little more complex with the plugins as an integral part of the program. It is not really designed for the beginner, however, there are plenty of YouTube videos describing the set up and uses of the program. It would be advisable to watch them first. Using one of the simpler SDR programs will give a good insight into SDR Radio before moving on to SDRangel.

If Digital Radio is not the primary interest, try SDR Sharp or Windows Consol V 3.0. There are many more on offer and all basically perform the same task so the choice really should be the one that suits the task. SDR Sharp is a useful program for Panadaptor use when monitoring the I.F. of a transceiver.