Monday, November 10, 2008

160m: A good example of an asymmetrical path

Often on the top-band, it is not easy to contact a DX station that you hear quite well. Before, accusing the DX station to be deaf, have a look at this example. I think that it is quite a good example of an asymmetrical transatlantic path.This plot shows the 160m reported levels by K1JT and mylsef using K1JT's WSPR mode. You can see that K1JT decoded me several times around his sunset, while I could not decode him. On the other hand, near my sunrise (06:28z), I could copy him better than he could. Btw, you may note that the K9AY provides a 4dB better SNR than the vertical (I was using 2 receivers), which is conform to its RDF-factor vs the vertical one (see W8JI receiving pages, k7tjr tables or ON4UN's bible). Note that we were both using 20W, which explains why the plotted levels @1W go below the WSPR decoding threshold (-30dB in 2500Hz BW)

The explanation is provided by the following plot of the noise at my location (VOACAP). You can see a 12dB variation of the noise floor, with a minimum in the middle of the day and a maximum in the middle of the night. At Joe's sunset my noise floor is very high, while it is still low at his location. At my sunrise, the noise floor is lower at my location and maximum at Joe's location. CQFD !
As pointed out by W8JI in his receiving pages, the "man made noise" is not only local noise, but also distant "man made noise" propagated through the ionosphere. In fact, based on the above plot, it seems that even at my location, the local noise is not dominant on 160m (except in the middle of the day, or in case of rain).

This is the 2MHz noise plot for my location (45.8N, 06E). A "man made" noise level" of -155/dBW/Hz @3Mhz is assumed (CCIR quiet). Of course for a higher "man-made noise" level the variation is less important (especially if the dominant source of noise is local).

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