Fade-in: Voice of Broad Masses, Asmara/Eritrea

Figure 1: Signal strength of VoBM on 7140kHz from s/on around 14:06 UTC to s/off around 18:30 UTC.

The evening transmission of the Voice of Broad Masses from Asmara-Selae Daro in Eritrea signs on around 14:06 UTC and signing off around 18:30 UTC. Figure 1 shows the signal levels with a resolution of one second, marked by red points, and the smoothed level, yellow line, with a moving average of 601 points, or 10 ten minutes. Smoothed levels span a range from -106 dBm/Hz to -80 dBm/Hz.

There occur considerable peaks around 14:30 UTC, 16:15 UTC and 17:30 UTC. Raytracing the signal, transmitted by a Quadrant antenna HQ1/.25, will help to reveal some mechanics behind the curve.

Figure 2: At s/on, we have a four-hop propagation via the F1 layer, carrying the main signal.

Figure 2 shows a four-hop propagation via F1 layer at 140-160km with a relative steep elevation of about 22°. The much shorter hops, reflected at the E-layer at a height of about 100km, are of less to no importance. The signal gets through, but very weak. The path itself still is in full sunshine, see Figure 3.

Figure 3: At s/on just after 14:00 UTC, the path between Asmara and DK8OK still is in full daylight.

There is a very short, but distinctive peak at 14:30 UTC. This coincides with a similar short time of three-hop propagation (Figure 4) from a very low azimuth of 3°. Of course, the full path still is in daylight.

Figure 4: Around 14:30 UTC, signal improved a bit as a three-hop propagation (still via F1) comes into play.

Just after 16:30 UTC and near sunset at the transmitter (16:37 UTC), there is reached the bottom of kind of a “Hillary Step” before the last run to the peak. The way to a (quite short) plateau starts around 17:00 UTC. There we have a textbook-like two-hop propagation (Figure 5) with the greyline covering just more than half of the great circle path (Figure 6). There, an elevation of under 5° is needed.

Figure 5: From around 17:00 UTC, a time of good reception starts. Reason is the textbook-like two-hop propagation.
Figure 6: With more than half of the great circle path in darkness, VoBM puts a fine signal into Germany.

Receiver: Winradio’s Excalibur Sigma
Antenna: active vertical dipole (2 x 5 m) MD300DX
Software: V3 by Simon Brown, G4ELI, QtiPlot, PropLab

2 comments

  • As always your analysis is awesome
    Unfotrunately this signal is very poor here in Greece ,most times QRMed by radio amateurs

  • … thanks, Zacharais – and there are many other stations around on other frequencies to try this out. Have a look into the 60 m broadcast band, e.g. 73 Nils, DK8OK

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