coastal HF Radars' embroideries

Interesting and in some way curious recording sent me by my friend "Sigurd from Netherlands". By varying the db level of the spectrum it is possible to see that actually the "embroidery" is composed of 3 distinct sweepers signals distinguishable by their different strengths, at least as they were received at my friend's site (figure 1).

 

Fig. 1 - spectral shapes at different db levels

Measurements of the main parameters, carried out separately on each sweep, return the following values:

A)
bandwidth (sweep width): 100 KHz
sweep repetition time (PRF): 2 Hz, slope: 200 KHz/s (500 msec duration)
mode: FMCW - Frequency Modulated Continuous Wave, down-chirped

B)
bandwidth: 50 KHz
sweep repetition time (PRF): 2 Hz,  slope: 100 KHz/s (500 msec duration)
mode: FMCW, down-chirped

C)
bandwidth: 50 KHz
sweep repetition time (PRF): 2 Hz,  slope: 100 KHz/s (500 msec duration)
mode: FMCW, up-chirped

Fig. 2

Note that although the 3 sweeps have same rate, B and C are characterized by lower slopes: that's is due to their scanning width that is lower than the one of sweep A (50/100 KHz). 

Other than the sweep rate, the signals share the same center frequency value: probably they are in some relationship and belong to the same system/organization. The different signal strengths lead to think of different transmitting sites or the use of phased antenna sets, so that the effective radiation pattern of a certain signal is reinforced in the direction of the used receiver (I also add that I have only 15 seconds of recording available so it is not possible for me to know if and how the signals may vary). Since the observed working frequency (around 13 MHz) has been allocated by the International Telecommunication Union to support the use of coastal High-Frequency Radar [1],  those emissions are most likely sourced by some WERA (WavE RAdar) systems located in North Europe: this type of OTH-SW (Over-The-Horizon Surface Wave) radar can pick up back-scattered signals from ranges of up to 200 km [2].

WERA HF coastal radar system (https://www.researchgate.net/figure/WERA-HF-costal-radar-system_fig1_251860940)

https://disk.yandex.com/d/ENoqrSNtbQ03KQ 

[1] https://www.itu.int/en/ITU-R/terrestrial/fmd/Documents/Res%20612.pdf (ITU Resolution 612 of the 2012 World Radio communication Conference)
[2] https://www.radartutorial.eu/19.kartei/10.weather/karte012.en.html