Web-based SDRs explained

With tens of thousands of radio stations streaming their content on the web, the world is your oyster. Whether you’re into Cuban salsa or small-town American music, you’ll find no shortage of stations that cater to your tastes.

This might suggest that all you’ll ever need is an Internet radio portal, such as www.reciva.com, but that would be ignoring one important fact – the airwaves contain much more than just broadcast stations on FM.
In the past, if you wanted to discover the more unusual signals on the radio bands, you’d have needed to buy a shortwave radio and a huge aerial, or share an online radio in 30-second windows. Now there’s an alternative: web-based software-defined radios, or SDRs.

Online SDR receivers

Modern but conventional communications receivers have digital inputs, so they can be controlled from a PC. This led the way for the first generation of online receivers, which enabled remote users to select a frequency via a web interface and listen to a stream of audio representing the station on that frequency.

The snag here was that an ordinary receiver could only be tuned to one frequency at a time, meaning that these web radios were essentially single-user affairs. An SDR (or software-defined radio) is a radio receiver that uses software to achieve most of what a conventional receiver does use electronic circuits.

If you were to buy an SDR as a black box, the processing would be carried out on the receiver’s internal processor, but it’s also possible to do that processing on a PC.

Web-based SDRs

Although you can buy the hardware necessary to turn your PC into an SDR, our theme here is how to listen to the airwaves without a receiver instead. Web-based SDRs have sprung up around the world, offering the second generation of online receivers.

The important point is that there’s no concept of the hardware being tuned to a particular frequency because it’s the software that’s responsible for selecting individual stations from a wideband collected by a high-frequency sound card. The upshot of this is that these online receivers can be used by lots of people at once, each listening to a different frequency.

To get a feel for a web-based SDR, we’re going to use the one at http://websdr.ewi.utwente.nl:8901. This is located at the University of Twente in the Netherlands and offers a good selection of bands. If you get bored and want to listen to the radio bands from a different part of the globe, you can find links to plenty of other Web SDRs at www.websdr.org.


WEBSDR: www.websdr.org provides links to online SDRs worldwide. You’ll need to enable JavaScript in your browser and have Java installed to access them. All these use the same software, which requires that you have JavaScript enabled on your browser and that you also have Java installed – you can obtain the most recent version from www.java.com. Some appear to work in just about any browser, but others are fussier. We suggest you use the latest version of Internet Explorer or Firefox.

With the Twente SDR onscreen, you’ll notice that it shows a waterfall display for each of the bands it covers. This provides a graphical view of a band, changing with time, so you can see where the signals are. This SDR covers several shortwave amateur bands, plus part of the medium wave broadcast bands and the bottom end of the radio spectrum to 165kHz.

We’ll begin by listening to something familiar – a medium wave broadcast station. Select ‘AM’ from the bandwidth controls below the waterfall displays. This stands for amplitude modulation and is the type of modulation used by broadcast stations in the LW, MW, and SW bands.

Now, in the medium wave band – that’s the third waterfall display down, which covers 462-563kHz – and you’ll probably see that the most prominent station is centered on 459kHz. Click in the black area below the waterfall display to select this frequency, and you should hear a German radio station. This is Deutschlandfunk, which is located at Nordkirchen in Northrhine-Westphalia and is the loudest station in this frequency range from this part of Holland.

Ham radio

Now we’ll try something you’re probably less familiar with – we’re going to listen in on one of the radio bands used by amateur radio enthusiasts worldwide. This web SDR covers the 160m, 80m, 40m, 30m, 20m, and 15m bands, and what you hear depends on atmospheric conditions and the time of day.

Generally, the 160m (1,800-1,892kHz), 80m (3,500- 38,00kHz) and 40m (7,000- 72,00kHz) bands will be the most consistent. If you do manage to hear stations on the other bands, they will often be more distant.
Whichever brand you choose, pick stations in the upper half (higher frequency) portion. If you’re listening on the 160m, 80m, or 40m bands you should choose LSB (lower sideband) as the modulation mode in the bandwidth controls, and if you’re in the other bands you should select USB (upper sideband).

These occupy a narrower band of frequencies than AM, so the tuning is much more critical and until you get it spot-on, voices will have a characteristic Donald Duck sound.

The yellow graphic in the black area below each waterfall display shows both the nominal frequency and the selected bandwidth, and you can alter either of these by clicking on and dragging the respective lines. Dragging the nominal frequency – the vertical yellow line – is a good way of fine-tuning. Practice makes perfect, and you should soon be listening in on amateur radio contacts.

Although both stations normally transmit on the same frequency, you may only hear one side of the conversation if the other station isn’t audible from Twente. If you look at the waterfall display for an amateur band, you’ll notice that the stations in the upper-frequency portion tend to be about 3kHz wide (these are voice stations), but those at the bottom end of a band are much narrower.

Try clicking on one of the narrow signals and you’ll probably find that it’s Morse code. If you still have LSB or USB selected then you might hear more than one signal at once, so select ‘CWwide’ or ‘CW-narrow’ from the bandwidth controls (CW stands for ‘continuous wave’). This will reduce the receiver bandwidth so that you hear fewer stations.

Deciphering Morse

You should now be listening to an amateur Morse code signal, but it will probably be totally meaningless to you. Fortunately, the software is available to decode Morse code, and here we’ll see how to use it to decode signals from a web-based SDR.

This is a rather hit-and-miss process since there’s no way of knowing what you’ll find on the bands at any one time. The software we’ll be using for decoding Morse is called MultiPSK. We’ll provide brief details on how to use it here but you might like to refer to our article in PC Plus 279, where we used it with a real (as opposed to SDR) radio receiver.


MULTIPSK: MultiPSK can decode Morse code and a lot more, including radioteletype and slow-scan television, which are used to transmit words and images.

You’ll need to route the audio output from the web SDR (from your web browser) into MultiPSK – see ‘Loopback your audio’ above for details of how to do that. Now start MultiPSK and select ‘RX/TX screen’. You’ll notice that it has a waterfall display, like the ones on the online SDR, but rather than covering a whole band, it covers a much narrower band of frequencies as selected by the SDR.

This is a Morse signal, so click on the ‘CW’ button (yellow) from the block of controls at the top right. Also, click on the most obvious signal in the waterfall display to select the station you’re listening to more accurately.

With a bit of luck, the text display at the bottom will show something like the English language. Take a look at ‘Spotlight on… Understanding amateur Morse first, because the dialogue will be almost unintelligible to the uninitiated.

SDR on your PC

You can also run SDR software on your own PC. To do this live, you’d need some external hardware to amplify the signal and do some preliminary processing but you can use SDR software to extract signals from a recording of a broad swathe of the radio spectrum.

If you struggled to interpret live Morse signals from an online SDR, this is a sure-fire way to see MultiPSK in action. The SDR software we’re using is called Winrad. You’ll also need a recording of a section of an amateur Morse code signal. Although this is a WAV file, it’ll sound odd if you listen to it normally because it’s a broadband recording.


WINRAD: Turn your own PC into an SDR using Winrad. You’ll need extra hardware if you want to do this live, but you can extract signals from a recording using software alone. You’ll find an example at www.dk3qn.com/wfSDRwav

Start Winrad and open the file from ‘Show Options | Select Input | WAV file’, before clicking on the ‘Start’ icon (the right-pointing triangle towards the right of the screen). You’ll see the now-familiar waterfall display, but because Winrad is using your soundcard rather than dedicated hardware, the frequency range is smaller than with the online SDRs.

You’ll soon find your way around Winrad’s interface. Virtually all the signals in this part of the band are Morse, which will give you plenty of opportunities to use MultiPSK.