Congrats Bob and All,

Nice work on TX and RX signals, and capture.

The four contiguous callsigns are a thing of beauty, especially at the near-threshold SNR level which says something about the stability of the D-layer in addition to the great work on TX and RX.

The differences between results at QTH’s of Mike, Victor, and Hartmut are also interesting:

At 75kHz at night and starting from Bob’s QTH there should be about 1dB loss over the water between Mikes’ QTH and the QTH’s of Victor and Hartmut;

another 1.0dB loss overland to Victor, and a total of 2dB overland to Hartmut’s QTH.

A total of 2dB additional loss on a path from Bob to Victor (compared to the path between Bob and Mike), and a total of 3dB additional loss on a path from Bob to Hartmut (compared to the path between Bob and Mike).

If I recall correctly, of signals captured on September 29^th, Hartmut’s had the higher SNR level. Although the above comprises a small sample, it still makes me wonder if modal peaks and nulls with distance may be a contributing factor between signal quality at QTH’s of Mike, Victor, and Hartmut, for a signal originating at Bob’s QTH, as opposed to just noise differences between the receive locations.

Bob mentioned: “Will have to start earlier as looks like there was propagation very early on.  Was not fully dark yet when I fired up… looked at 137 later on and 500 [472] but saw nothing going on with "TA" prop on either of those bands…. on this low freq, does it sometimes open when higher frequencies don't?”

I had some thoughts on differences between (a) 75kHz band openings at night and (b) higher-frequency band openings at night:

I think that there are a number of reasons why 75kHz could (at times) open before dark when higher frequency bands don’t.

One example: in the summer, the day/night disadvantage of signals at 137kHz is about 8dB greater than for signals at 75kHz. This leaves room for near-threshold signals at 75kHz to appear near nightfall when signals at 137 with less link margin don’t appear until later. Definitely not always the case, but a significant possibility

Another example: (a) at sunspot maximum, daytime signals at 75kHz improve by ~9dB compared to signals at sunspot minimum; (b) at sunspot maximum, daytime signals at 137kHz don’t change compared to signals at sunspot minimum; and (c) daytime signals at 472kHz worsen by ~9dB compared to signals at sunspot minimum. Since night-time signals are much less affected by sunspot activity at those frequencies, the day/night disadvantage of signals at 137kHz may be roughly 10dB – 11dB  greater than for signals at 75kHz, near sunspot maximum (right around now). This leaves even more room for near-threshold signals at 75kHz to appear near nightfall when signals at 137kHz with less link margin don’t appear until later. Again, definitely not always the case, but a significant possibility.

Even at 5000-6000km path lengths, modal differences (TEx/TMx) between 75kHz and 137kHz (for example) could yield advantages and disadvantages of a few dB to the 75kHz signal (compared to the 137kHz signal) as nightfall and night progress.

There are other interesting examples, and accordingly in cases where +/- 10dB day/night uncertainties matter, each doubling of frequency (roughly speaking) between 1kHz and 500kHz could be considered as a potentially different case. 75kHz is a comparatively less-studied band, and I think that the efforts of Bob and the group, especially under these conditions near the threshold of detectability, are likely to add significantly to the understanding of propagation in this band. I think it would be great if Bob switched on earlier, which is easy for me to say because Bob and the group are doing the hard work, but I think that Bob’s TX efforts and those of the four stations capturing signals are and will be greatly appreciated.

Congrats again to Bob and all for a great effort and great results.

Regards,

Jim

AA5BW