Ultra-portable shortwave spectrum capture with Belka-DX and Zoom H1

Sunday, December 06, 2020

This is a quick "how-to" post on making spectrum recordings with the latest model of the ultra-portable Belka shortwave receiver, the Belka-DX. A number of positive reviews of the previous model of this receiver have already been posted online. The new version boasts greater sensitivity, extended shortwave coverage and the ability to monitor I/Q – or radio spectrum – data in real time.

A separate 3.5mm stereo port is used for outputting I/Q data in analogue format. This seems an unusual design choice: nowadays, most SDRs send I/Q data digitally over USB (e.g. see FunCube Dongle Pro+).To get SDR applications to work with this data, Belka's I/Q output needs to be connected to the line input of a regular PC sound card. The digital —> analogue —> digital conversion chain will inevitably result in the addition of small amounts of noise to the final spectrum output. However, one advantage of this design choice is that it is possible to capture this data using a portable audio recorder.

Belka DX I/Q output port

While Belka's I/Q sample rate is reportedly 192 kHz, few recorders are capable of capturing audio at this rate (and none of them are cheap or particularly compact), and the 96 kHz option is far more widely available. In practice, recording at this rate means that roughly 48 khz on each side of the centre frequency become truncated, but on shortwave, 96 khz can still pack several broadcast stations (or, alternatively, a few dozen ham radio transmissions). Additionally, DSP tools in SDR applications such as SDR# can be used to clean up I/Q recordings in ways that are far superior to what's possible to achieve with regular post-processing of audio recordings. 

I made several field spectrum recordings with my Zoom H1, connecting Belka's I/Q output to the recorder's line input and using the "WAV @ 96 kHz / 16 bit" setting. I found that keeping the input level between -24dB and -12dB results in sufficient gain for later analysis without overloading the recorder.

It's worth noting that the gain and the slight offset from the tuned centre frequency seem to change depending on both the frequency and the chosen demodulation mode.


First up is the Voice of America recording I made on 06/11/20 in a London park. To assess the quality of my I/Q capture method I recorded the audio output in parallel using my Sony ICD-PX333. I set Belka's demodulation mode to "AM2", which is its pseduo-synchronous AM detection setting.  Below are the two recordings, with the I/Q data demodulated in SDR#:

Belka's own demodulation is a little on the distorted side compared with SDR#'s I/Q demodulation. This is probably due to the specifics of its "AM2" setting.

Note that I'm using SDR#'s handy "invert spectrum" and "correct IQ" options, and that I've modified the spectrum recording file names so that the centre frequency is displayed correctly.

Below are two more examples of this I/Q capture method and their audio recording counterparts. For the latter, I set Belka's mode to LSB with 50 Hz lower and 4 kHz upper frequency cutoffs. I also used Youssef's excellent noise reduction plugin when demodulating I/Q.

Radio Rebelde on 12/11/20 at 07:01 UTC

Radio Rebelde was barely registering on Belka's SNR meter but the audio is perfectly intelligible, while SDR#'s noise reduction takes this intelligebility to an entirely new level.

Radio New Zealand International on 12/11/20 at 08:00

Very clear reception, with SDR#'s I/Q noise reduction providing a marked improvement over Belka's own audio demodulation.

CQ contest on 40m on 21/11/20 at 15:33

Finally, here is a full I/Q capture of a CQ constest on 40m on 21/11/20, recorded in the same park location (click here to download the original WAV file). An Over The Horizon Radar signal makes an unfortunate appearance during the first 20 minutes of the recording, causing severe interference. You will also notice a few hams ignoring the contest altogether and happily rag chewing!

Overall, this is by far my most portable shortwave spectrum capture combo: it is handheld and absolutely no antenna set-up is required – just plug in the supplied telescopic whip and you are good to go. The reduced bandwidth compared to my AirSpy SDRs is a significant limitation, but it was only a few years ago that I was making recordings with a FunCube Dongle Pro+ that only provided double the above sample rate. It would be amazing if Belka's next model included a microSD card slot and the ability to record the I/Q data directly to it, but for now I am very happy with this way of making spectrum recordings on the move.


Mystery 21 Radio

Monday, November 30, 2020
This morning, while casually exploring the shortwave bands from home using Twente WebSDR, I stumbled across an AM transmission on 4860 kHz. The station was playing some great Euro Pop so I immediately hit "record" and switched over to my local shortwave monitoring set-up (AirSpy HF+ Discovery / YouLoop antenna mounted on the balcony). It turned out that I could pick up their signal quite nicely over here too, albeit with somewhat greater noise levels compared to the WebSDR copy, owing to local QRM:

The station ID was "Mystery 21" and their Facebook page suggests that they are located in Germany. Below is the full WebSDR recording for your listening pleasure:



2020 BBC Antarctic Midwinter Broadcast

Monday, June 22, 2020
Antarctic night sky. Source: Chris Wilson, Australian Antarctic Division
Every year, the BBC World Service makes this special programme for just 40 listeners: the team of scientists and support staff isolated at British research stations in the Antarctic midwinter. The Antarctic Midwinter Broadcast is unlike anything else on the BBC World Service. Presented by Cerys Matthews, it features messages from family and friends at home as well as music requests from Antarctica. For decades it has been part of the traditional midwinter celebrations. 

To listen to the 21:30 BST broadcast on 21st June on shortwave, please tune to:

5790 kHz from Woofferton UK 
7360 kHz from Woofferton UK
9580 kHz from Ascension Islands
Below is my recording of the 5790 kHz transmission, broadcast out of Woofferton, UK on 21/06/2020 at 2130 UTC (not BST as stated on the BBC website). The recording was made using GPD Win, AirSpy Discovery Dual Port and a Sony AN-LP1 active loop antenna, positioned on the balcony of my indoor QTH in London. Many listeners from further afield have reported difficult reception conditions, so being close to one of the transmitters felt a bit like cheating, but it was nice to catch this transmission all the same.


Voice of Turkey and Voice of the Islamic Republic of Iran in English

Sunday, June 21, 2020
Satellite view of Voice of the Islamic Republic of Iran transmitter site in Sirjan
This is a quick post with no particular theme to it, apart from the fact that I recorded two overseas English language broadcasts that are not the usual suspects for my urban indoor location (such as Radio Romania International or China Radio International). One of the countries, Iran, has made substantial improvements to its shortwave transmissions directed towards Europe:
Below are recordings of the Voice of Turkey and Voice of the Islamic Republic of Iran, made on 19/06/2020 using an AirSpy HF+ Discovery and a YouLoop antenna, the latter positioned on the balcony of my indoor listening post in London. SDR# noise reduction and wide filter bandwidth were applied to Iran's signal, resulting in near-FM audio quality.

The programmes focus on current affairs and international relations. Curiously, both countries dedicate more time to the impacts of COVID-19 outside of their own borders.


DXpedition retrospective: Singapore 2019

Wednesday, June 17, 2020
Marina Bay Sands, Singapore. © 2019 London Shortwave
In August 2019 I visited Singapore, where I was able to test a pre-release sample of the AirSpy HF+ Discovery SDR, kindly sent to me by Youssef. Due to a very busy schedule I was only able to make two short outdoor radio recording trips, amounting to three hours in total. Needless to say, the advantages of portable shortwave spectrum capture manifested themselves fully in these hurried circumstances. I made these two trips on August 27th and 29th and I shall start with the latter outing as that was when I made my most prized recording to date.

East Coast Park, 29/08/2019

Spectrum capture QTH on 29/08/2019
East Coast Park is Singapore's largest and occupies 15km of the island's coastline. I chose this location because it was easily accessible and not particularly crowded on the day:

My recording set-up consisted of the GPDWin 5" mini-computer running Windows 10 and SDR# 1631, AirSpy HF+ Discovery and a long wire antenna:

I arrived at 1723 local time (0923 UTC) and after experimenting with reception on different bands for about an hour I settled on the 49 meter band and started recording it just before 1100 UTC (after grabbing a coffee from the large Starbucks situated right behind me). An hour later, as the beach became dark and windy, I packed up my equipment and went back to the hotel. Later that night I reviewed the spectrum capture file and the very first station I stumbled upon was one that had eluded me since I returned to the shortwave listening hobby in 2013: Myanmar Radio.

Myanmar Radio QSL card. Image from Pavel Zhuravlev
Below is a one hour recording of this station extracted from the spectrum capture. It contains a mixture of talk, traditional songs and national rock music. The 50kW non-directional signal on 5915 kHz  is listed as "Myanma Radio Naypyidaw Service in Minority Languages", although I have not been able to establish the exact language of this broadcast. Some time ago one Shortwave Radio Audio Archive contributor said that "this may be one of the most exotic countries still left on shortwave". I agree!

Radio New Zealand International (RNZ Pacific)

I have a special fondness for RNZI as it was my first bit of long-distance DX back in 2013 when I was staying in the Russian countryside. Receiving this station in Singapore would always have been a lot less challenging but it's still nice to hear this old friend from the Pacific on the airwaves, especially as it has been a harder catch in Europe in recent times. Below is a one hour recording RNZI's signal extracted from the spectrum capture file.

KCBS Pyongyang

This is the domestic arm of the North Korean radio service, which regularly makes it into Europe on 9665 and 11680 kHz (almost always faintly) but far less often on this 6400 kHz frequency. From NorthKoreaTech:
The Korean Central Broadcasting Station (KCBS) (Korean: 조선중앙방송, Chinese: 朝鲜中央放送, Japanese: 朝鮮中央放送) is the main domestic radio network in the DPRK. It sits under the Central Broadcasting Committee of the DPRK (called the Radio and Television Committee of the DPRK until 2009). 
KCBS broadcasts from 5am to 3am local time via a network of medium wave and shortwave transmitters that cover the nation. The powerful transmissions can easily be heard in neighbouring countries, including South Korea where some of its frequencies are jammed. 
A central program is broadcast from Pyongyang on most transmitters through the entire broadcast day, but some are reported to carry regional programming between 2pm and 3pm.
Below is a one hour recording of this signal. Note the militant style of the radio presenter and the rousing patriotic music.  There were lots of other North Korean transmissions available during both of my outings but I will avoid repetition here.

Echo of Hope

South Korea's clandestine counter-broadcast on shortwave, beamed into North Korea from Hwaseong but picked up very clearly in Singapore. There isn't much information available on this broadcaster online, although apparently a few listeners managed to obtain the station's QSL cards via South Korea's international broadcaster, KBS World Radio. Below is a one hour recording of their signal.

VMW Marine Weather Station, Wiluna, Australia

Australia Marine Radio Broadcast Areas. Source: The Australian Bureau of Meteorology
The Australian Bureau of Meteorology broadcasts marine forecasts and warnings to mariners over shortwave from Charleville (VMC) for eastern waters and Wiluna (VMW) for western waters. The transmitter specifications for the VMW broadcast state that it is powered at 1 kW (presumably non-directionally, based on the map above) and according to the schedule the forecast recorded below was supposed to be for Western Australia (Northern Zones: NT-WA Border to North West Cape) and the Northern Territory. This signal occasionally makes it into Europe in winter but I've not yet managed to pick it up in London.

Radio Nikkei 1

Radio Tampa QSL card from the late 70s. Source: kusanagi1965
Radio Nikkei 1 (Radio Tampa until 2004) is a nationwide commercial shortwave radio station in Japan, operating from Chiba-Nagara at 50 kW since 1954, and was another first catch for me. Wikipedia's description makes it sound like a fairly unique station in today's shortwave world:
The station features the following four genres as the core of its programming: finance, JRA horse racing (weekends), health-medical, and culture.
Below is a one hour recording of the station's 6055 kHz signal extracted from the spectrum capture.

Voice of Vietnam National Channel 4

This is one of Vietnam's many domestic shortwave services, broadcasting non-directionally at 20 kW from Đắk Lắk and yet another first for me. Below is a one hour recording of this station extracted from the spectrum capture file.

BBC World Service transmitting from Kranji, Singapore

Kranji transmitting station. Source: Google StreetView
Amidst all of this DXing fun it would perhaps be easy to forget that Singapore is home to the Kranji transmitter site, which several international broadcasters use to reach listeners across South-East Asia. My QTH in East Coast Park was merely 20km away from Kranji and the video below clearly shows how strong the local 125 kW signal of the BBC World Service was on that day. The most surprising part was just how well AirSpy coped with having that monster signal next to all the weak transmissions I extracted above: at no point did it look like the ADC was anywhere near its saturation point. If I had more time I would have tried to visit Kranji and check out the transmitters; I hope to do so during one of my future trips to this country.

Pasir Ris Park, 27/08/2019

Spectrum capture QTH on 27/08/2019
Two days earlier I visited Pasir Ris Park on the other side of the island. The park is located right next to Singapore's Changi Airport. It's a great location for DXpeditions because overnight camping is allowed, albeit only with a permit obtained via the National Parks Board's website (something that I would also like to do during my next visit).

I arrived at the recording spot at around 1830 Singapore time and started capturing the 25 meter band 90 minutes later (at 1200 UTC). In retrospect this wasn't the most interesting band to record as it was mostly populated with services from China Radio International and China National Radio, which you can hear pretty much anywhere in the world. However, I did manage to extract a few interesting transmissions:

View from the recording location towards Serangoon Island. © 2019 London Shortwave

View towards Changi Airport approach. © 2019 London Shortwave

Voice of Vietnam's English language service to South East Asia and the Pacific

I have made many recordings of the Voice of Vietnam's English language broadcasts, which can be heard in Europe on 7280 and 9730 kHz at 1600, 1900 and 2130 UTC. What I didn't realise before catching this particular transmission is that the content of the international news bulletin depends on the region to which it is being broadcast. The European transmissions I pick up in London are typically focused on Vietnam-EU and Vietnam-US relations, while in the recording below, the news is Asia-Pacific-centric. To me, this shows that the Vietnamese government still take their shortwave operations seriously.

NHK World Radio Japan

NHK World is the international arm of the Japanese state broadcaster, NHK. The one-hour transmission below was beamed south-west in Japanese out of Ibaragi-Koga-Yamata, at a mighty 300 kW. Perhaps this was the next easiest signal to catch after the BBC World Service out of Kranji, but the reason I include this recording here is that by coincidence I stumbled upon the relay of ABC-Z's weekly program on NHK Radio 1. ABC-Z are a popular Japanese boy band who have been active in under various guises since 2001. You can catch their songs at 18:40, 36:26 and 50:48 in the recording below.

Members of A.B.C-Z

Republic of Yemen Radio (clandestine / Saudi Arabia)

Distance calculation by Google
Republic of Yemen Radio is a clandestine radio station broadcasting out of Saudi Arabia. It delivers programming reflecting the position of the Saudi-led coalition that participates in the ongoing Yemeni civil war. The precise location and power rating of the transmitter are not officially disclosed but are believed to be Jeddah and non-directional 50 kW, respectively. Given the 7,331 km distance to the transmitter site and the non-directional nature of the broadcast, this DX is still somewhat noteworthy.

Overall, I couldn't be happier with what I managed to capture with my portable SDR set-up in the limited time available. Although I wish I had spent more time on the lower bands, as that is where more of the local and exotic stations of the region reside, there's always the next time, and yet another reason to come back.


World music marathon on the Voice of Greece

Thursday, May 28, 2020

The Voice of Greece are well-known for their multi-hour sessions of phenomenally diverse music from around the world. So, without much further commentary, here is a recording of their signal made yesterday at 1904 UTC using AirSpy HF+ Discovery and the YouLoop passive loop antenna, mounted onto a tripod on the balcony of my QTH. The first hour contains tracks from Indonesia, the Middle East, Latin America, and beyond, while the second hour has a selection of traditional Greek music.


All India Radio can be heard in English on shortwave once again

Thursday, May 21, 2020
All India Radio office in New Delhi. Photo by Sanjeev Verma
When I returned to the shortwave bands two weeks ago I was sad to learn that All India Radio had suspended its overseas shortwave transmissions at the end of March, owing to the COVID-19 lockdown imposed by the Indian government. It was also being reported that AIR National Channel – a more difficult catch in Europe compared to the External Services Division (ESD) broadcasts – was running on reduced power. However, it seems that we can now hear AIR's English language news bulletins once again, on the same frequency where a limited number of non-English language ESD broadcasts were announced to be broadcasting two weeks earlier. The transmission takes place daily at 1500 UTC on 11560 kHz and is only 30 minutes long. The mixture of English and Hindi suggests that it is a high-power relay of AIR National Channel programming. Below is a recording of this transmission made on May 21st, 2020 with a PocketCHIP handheld Linux computer, AirSpy HF+ Discovery and a YouLoop passive loop antenna. The antenna was positioned on the balcony of my current QTH in London.


A series of retrospective radio archive posts from under lockdown

Wednesday, May 06, 2020

Hello, dear blog readers and Twitter followers. I hope you are all well and keeping safe in these extraordinary times. I'm sorry I have been out of action on the radio front for a long time and that many of your emails and DMs had gone unanswered until recently. Like many of you, I have been readjusting to life under the COVID-19 lockdown and this has been consuming a lot of my time and energy. My ability to venture outdoors to hunt for faint and exotic radio signals has so far been quite limited. Consequently, I decided to use this blog as a space to explore my previous recordings – spectrum and otherwise  – to take us back to the times when life was normal and to remind us that it will be so again. I will also occasionally post recordings that I am able to capture from my current residential location, but expect fewer of those.

To kick things off, below is a recording of Radio New Zealand International ringing in the Year of 2020, captured outdoors in a rural location in the Russian Central Region, using a Tecsun PL-680 and a long-wire antenna. Listening to it, one gets the sense of a very different outlook on the year ahead compared to what then followed.