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First observed two days ago, there seems to be a new (to us HF listeners, anyway) network of HF ditter CW transmissions. The purpose of this network, as well as who is operating it, is unknown. It is possible they are for propagation monitoring. Based on observations of listeners and propagation characteristics, it would appear that at least some of the transmissions are coming from North America, possibly the Central US.

The transmissions do not occur at the same time on all frequencies. It appears that each transmitter steps through the frequencies. The following image shows the received signal on three of the frequencies (click on the image to view it as a larger size):

As you can see, thea transmission on each frequency begins right after the transmission on the previous frequency ends. This data was obtained by running a netSDR receiver in 500 kHz wide I/Q capture mode. The resulting recording file was then demodulated at each frequency of interest.

You can also see the second (weaker) dit on the 11150 tranmsission, that occurs shortly before the stronger main dit. (It is less obvious before the second dit, but you can see it, if you squint)

Each pulse (dit) is 130 milliseconds long, and they repeat every 6 seconds.

Next, the demodulated signal for a ditter transmission on each of the above frequencies is shown magnified, to see the exact times of each transmission.

How to find these transmissions:

I find that using an SDR is the easiest way, as you can observe a large portion of the spectrum at once. I use a 500 kHz wide view, and step through HF, looking for the periodic dits. But you can certainly use any radio. Note that the frequencies are all multiples of 25 kHz. They also sometimes occur in groups of three relatively associated frequencies. There are likely additional frequencies that have not yet been discovered.

If you’re hearing any of these transmissions, or have discovered possible additional frequencies, please let us know with a comment!

Transmissions on the following frequencies have been observed (all in kHz):
5450
5575
6225
6550
6750
7700
8000
8275
8775
8825
8900
8975
9050
9225
10050
10450
10575
10900
11025
11150
11225
11300
12450
13100
13250
13325
13875
14400
15100
15400
15625
16000
16350
16550
16725
17475
17650
17950
17975
18050
18100
18200
18450
18625
19300
19650
20100
20175
20250
22050
24050