I am trying to understand the impact of unconnected wires in a cable for EMC conducted emissions tests.
My product has a shared unshielded cable for supply and communication (I know it's bad practice...). It consists of 9 untwisted wires (see picture below). The 12VDC supply is on the red and black wires. The communication (RS-485) is on the brown, pink and green wires (not optimal I confess). All the rest is unused (white, yellow, grey and blue).
The test setup is classical (see diagram below).
When all unused wires are left unconnected on both sides, the RS-485 coupling on the supply is tremendous (>1Vpp on the LISN side for a 15m cable). I can literally read the frames directly on the supply.
However, when I tie together the blue and white wires together on both ends, the coupling vanishes (>1Vpp to something barely noticeable on the spectrum analyzer). I must insist on the fact that the two wires are connected to nothing else.
I understand the capacitive coupling when the wires are left unconnected. But I just don't get why these two wires stop the coupling when connected like that.
Did you have any similar issue or understand the physics behind?