The idea behind the article is to introduce people to SDR from a shortwave listening point of view, so high performance HF SDRs like the Airspy HF+, Elad FDM-S2 and WinRadio Excalibur are discussed. Thomas notes that these SDRs can perform as well as traditional DX-grade receivers that can cost two to three times more. He also explains what advantages SDR's bring to the shortwave radio listening hobby. This may be a good article to show those still using older hardware radios that haven't yet converted to the SDR world.
The article is currently part one of a three part series, with parts two and three to be released in October and November.
Broadcast shortwave radio is not always archived for long at the station, so finding sound bites from interesting historical events can be difficult. We know that songs are of course recorded, but talk back radio, discussions in between music, news readings, weather updates, ads and pirate radio are all lost over time. Although these things may seem mundane now, future historians may be interested in listening in on this little slice of life.
Thomas' idea is to create a database of shortwave radio IQ recordings so that they can be archived for historical purposes. The project is called "The Radio Spectrum Archive" and has a website set up at spectrumarchive.org. To do this modern software defined radios like the RTL-SDR can be used to record a large bandwidth, however the problem is with data storage as IQ recordings can take up extremely large amounts of disk space.
Interestingly, it turns out that people have actually been making IQ recordings since the 1980's by connecting their shortwave radios to VCR tape recorders. In the modern day these VCR recordings can be digitized into an IQ file, and played back in software like HDSDR. In the video below Thomas demonstrates the playback of a digitized VCR radio recording from May 1 1986. You can hear some interesting news tidbits on the soviet cover-up of Chernobyl, the Challenger disaster and the launch of a new hurricane tracking satellite. If recording was more popular it would have been interesting to hear soviet radio during this time too.
In addition to archiving IQ files, Thomas has been releasing a podcast of curated historical audio recordings from VCR tapes, as well as modern recordings that may be of interest over at shortwavearchive.com.
We envision a future where one day these recordings could be automatically turned into text logs via advanced speech to text software, so they could easily be searched through.
Over on the swling.com blog admin Thomas has been exploring various indoor antenna options for pairing with an HF capable software defined radio. He notes that unless you happen to live in isolation, you're highly likely to experience RFI problems with standard wire antennas. Instead he recommends looking into magnetic loop antennas which are significantly more resistant to urban electric field based RFI noise, and they can also be rotated to null out any other local noise sources. Thomas then goes on to highlight some of the best commercial magnetic loop options for sale. There is also some good advice in the comments section.
We note that magnetic loop antenna seem to work fairly well with the RTL-SDR in V3 in direct sampling mode, but you may need to filter out the broadcast AM band to avoid overload if the loop doesn't do this already.
Over on his blog London Shortwave writes how difficult it can be trying to listen to shortwave radio stations when you’re indoors and in a big city filled with RF noise. His solution is a portable lightweight shortwave travel kit that he can take to the park. The kit that he recommends using includes an Airspy SDR with SpyVerter upconverter, a Toshiba Encore 8″ Tablet and an OTG USB adapter. His antenna is a portable dipole made from two pieces of 6m copper wire connected to a balun, then connected to the SDR with 3m of coax. The whole kit easily fits into a small metal brief case.
For the software London Shortwave uses SDR# and he enjoys capturing large chunks of the HF spectrum for replay later using the base band recorder and file player plugins for SDR#. In his post he also shows how he runs the Airspy in debug mode to restrict it to 6 MHz which is the maximum bandwidth that his tablet’s CPU can handle.
His post shows various example videos of his setup receiving some nice shortwave signals.
Over on the Reddit discussion boards user gat3way has posted about his newly released software project called swscan. Swscan is a Linux console based program that can be used to scan and listen to shortwave broadcast stations. It has a built in database of shortwave station frequencies as well as their broadcast schedules and it will even show you the stations power level and distance you are from the transmitter. Swscan is based on GNU Radio 3.7, so you will need to have that installed first.
As shortwave stations exist at frequencies below the normal tuning range of the RTL-SDR, you will need an upconverter or be using the latest R820T experimental driver which can tune down to around 1 MHz.
His results show that the RTL-SDR and portable receiver are comparable in terms of performance, with a slight edge to the RTL-SDR. He adds that software tweaks available in SDR# can improve the voice quality for the RTL-SDR. However his final recommendation for general shortwave listening is that the portable is still the better option due to it’s ease of use.
YouTube user mutezone has uploaded a video showing a comparison between several portable shortwave radios and the RTL-SDR. His results show that the portable radios performed better than the RTL-SDR dongle at shortwave-dxing, however we are unsure of what method he used (direct sampling or upconverter) to get access to the shortwave bands on the RTL-SDR.
Shortwave Performance test: Portable Radios vs RTL-SDR