Nils Schiffhauer – DK8OK

When HF collects new power for the autumn season, it’s time to thrown an ear onto VHF. I did so in May 2020, and the spectrogram at the top should give you an impression of the power and glory with with Sporadic-E fills the ORT band and the FM broadcast band. Having published some audio examples from both bands in the two recent posts, I now want to make some comments on this exciting propagation mode.

One signature of Sporadic-E is a highly selective propagation in respect to geographical regions – resembling a laser pointer. Please find below an example of just 90 seconds on 93.9 MHz on 29May2020, around 18:00 UTC (the vertical interruption at the last quarter of the audio spectrogram. It starts with a powerful-modulated, yet unidentified, Italien station with pop music. Then, Radio Constanine from Kef El Akhal (per RDS 2425) takes over with a distinctive (lower) sound, gevoerning the channel for a time, until the Italian comes through again. If play the audio clip and following its spectrogram in parallel, you surely will see what I mean.

Parallel listening on two channels is an essential tool to identify the same station on different channels. Place one (e.g. the left) audio channel on one frequency, and the other audio channel with a 2nd RX on the other (in this case: right channel). This can be done easily with SDRC V3 software. There will always be a time lag between both channels for at least two reasons: the main reason is that two frequencies are usually coming from two different locations, fed over different lines from the studio which causes a slight time difference. The other reason: different RXs in SDRC V3 software ore not phase-synchronized, so there is a small difference. Another difference might occur from different propagation time from two locations (300 km = 1ms!).

The screenshot below shows a typical setup: on 87.7 MHz a station in Arabic was heard. It was set onto RX1/left audio channel. To identify this station, RX2 was set to the right audio channel. Leaving RX1 on its frequency as the reference channel, RX2 was tuned to other channels with Arabic programme. The first hit was made on 87,6 MHz, second on 90,8 MHz, third on 92.5 MHz – all carrying the same programme. Hence, it was very likely SNRT Al Idaa Al-Watania from Morocco from four different locations at distances from 2312 to 2613 km.

The Radio Data System RDS provides an identifcation and often some additional information. First, you see the “Programm Identification Code – RDS-PI”. This is used to support an automatically scan of the receiver to tune always the best channel of this programme. Hence, it designates a programme which is used throughout a chaine of programmes. The “Programme Service Code – PDS-PS” needs a bit longer and gives, if programmed, additional information. As the RDS signal sits on edge of the channel and is quite slow, it is a bit delicate to receive from weak stations under interference. The first example below gives a full-blown RDS signal from a local broadcaster, followed by some RDS signals from DX stations.

Fading can be nerve-wrecking. Stations come and go. And they reliably go at top of the hour when the listener expects a station identification … Below I compared the signal from a still near-regional transmitter at a distance of 83 km with that of Radio Constatine/Algeria, 1802 km away. The grey block marks the limit, whre Radio Constanine can be demodulated.