Voltage signal significantly drops when BNC cable is connected
I think my theory in the comment is right.

I've replaced R3 with one of 51 ohm and got the following:



And when I connect RG58, I got the following:



The amplitude has been reduced by 1/2 or something...

It seems that R3 and RG58 are connected in parallel, so the total impedance is halved. And so is voltage readings.

Now the problem is, there are lots of noise so it looks quite ugly. So what I want to do and have to do is to put a larger resistance and to make the current flow only through R3...

I will search for it.