FM-DX: How to Identify very short openings – a few examples

42 seconds in a recording of 12 hours length – a “blob” in V3’s “Analyser” module reveals a DX chance.

In the last weeks, I had used Sporadic-E conditions to stroll a bit in the FM broadcast band in search for DX. Elad’s FDM-S3 covers the whole 20 MHz wide band, and Simon Brown’s SDRC V3 software again provides an unique and most valuable tool to dig out DX. Antenna is an active Dressler ARA-200 (R.I.P.).

This blog entry shows how to make use of short openings of only some (ten) seconds.

First step is to record the whole FM broadcast band for hours on external HD. Then you make up so-called “spectrograms” by V3’s Analyser module. This provides you with a picture of activity (signal strengths color-coded) over time and frequency – see screenshot at top of this blog.

Scrolling through this spectrogram, you can make out even the shortest openings. Just click onto one of them, and the software instantaneously tunes into it. The sensitive RDS decoder of V3 is doing the last step – showing its RDS identification.

The short video below gives one example from a recording of June 26, 2020. On 91.8 MHz, I received semi-local transmitter NDR 1 NDS at Visselhövede (5kW@67 km distance), with “Stand by me”. From the spectrogram, I saw a “blob” (see screenshot at the top of this blog), stretching over around 40 seconds. It turned out to be Algerian’s Akfadou transmitter with Chaine 2 programme, 70 kW ERP@1’810km distance! RDS did tell me. Just have a look at the short video below which was made with V3’s video recorder …

First “Stand by me” by a local transmitter, then CHAINE 2 drops in for about 40 seconds, after the local transmitter takes over again.

V3 software provides also an a-symmetrical tuning of bandwidth, even at wide FM/BFM. This is important to identify some stations “in the clear” – if they are prone to some spillover from a local/regional station right on an adjacent channel. The following example spots Radio Marca/Mallorca from Spain on 91.6 MHz, suffering not only from a a strong local just 100 kHz below, but also from a very short appearance “out of the blue”, to where it disappeared again after less than 30 seconds. The latter is shown in the spectrogram, made by the Analyser, where I magnified the small/short signal of Radio Marca over 1’541 km. The video at the bottom shows how to evade the interference from the channel below to get the RDS code “B002 R.MARCA” correct.

On an empty channel, a stations pops up for 28 seconds. Pinpointed by V3’s “Analyser”, decoded as RDS “B002” Radio Marca from Na Burguessa/Mallorca in Spain by the RDS decoder.
With a strong local just 100 kHz below, a-symmetrically tuning the bandwidth helps to identifythe very short pop-up of Radio Marca/Mallorca on 91.6 MHz by RDS.

Sometimes propagtion is too short for any identifcation, neither RDS, nor by announcement. Take the next screenshots as example: The spectrogram shows some very short openings revealing similar pattern which cropping the recording (Crop – > Apply) confirms. It turns out to be an English-speaking stations for a maximum of ten seconds. Parallel listening reveals the same programme on the following eight frequencies: 88.3MHz, 88.4MHz, 88.5MHz, 88,7MHz, 88.9MHz, 89.1MHz, 89.7MHz and 89.8MHz. The only intersection turns out to be Raidió Teilifís Éireann from different locations with their Radio 1 programme.

Up to ten seconds in a ten hours’ recording, marked by a rectangle in the spectrogramme: whodunit?
All nine stations carried a broadcast in English, eight of them in parallel, only that on 89.6MHz remaining unidentified. The solution is easy …

RTE transmitter usually do have RDS onboard, but here the time with a modest signal was too short to raise the alarm. On the other hand, there are stations with RDS, but not programmed or even without RDS at all. Take Radio Tisnath/Algeria in Tamazight, a Berber language, as an example for the first and Radio Blagovestiye/Russia as an example for the latter:

Radio Tisnath from Djebel Tissalah/Algeria [2’091km] should carry RDS-ID of “2202 CHAINE2”, which didn’t pop up – in spite of the good signal. At least a station identification was heard, alas, a bit distorted after a programme in Tamazight in the clear
As with all OIRT band stations, also Radio Blagovestiye from Voronezh/G-Tsa Brono in Russia [1’984km) doesn’t carry any RDS information. You have to wait for the announcement – as here – or to collect some other information to identify such a station.

2 comments

  • Always something to admire in Nils’ posts.

  • Very interesting topic. I have found many Q-DXstations with the same technique you posted here to identify stattions that are 80-150 km far from my house , from the neighrour countries using the asymmetrical filter of SDRC. They are signal of around -90and less dbm

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