Rogowski coil CWTUM03 (30 MHz) gives a 50 ohm resistance when I measured between its center pin and outer surface of BNC connector. When I measure any signal through a coax with 50 ohm selected on scope as input resistance, signal is attenuated to 50% which could only mean that there is a series resistance in rogowski probe as well. I could understand the parallel 50 ohm resistance at the output of rogowski before coax cable as a seperate terimation irrespective of what coax user connect, but why the series resistance??
The 50Ω resistance you measured IS the series resistance (source resistance) of the coil. Why would you think otherwise?
It is there to provide a good match to the cable, eliminating potential reflections that would confuse high-speed measurements.
You use the 50Ω termination on the scope for the same reason.
This combination of source impedance and load impedance creates a voltage divider that always results in a 50% reduction in voltage when compared to the open-circuit voltage. Sometimes test equipment takes this into account when displaying the data; sometimes it doesn't.