This sounds like a contradiction; if the (coaxial) cable is really waterproof and water tight including the connectors you should not see any difference between "in air"or "in water". Or its not a coax cable (maybe two wire line?

Anyway this is related to the classical level measurement technique in industrial tanks or containers using a wire pair...maybe you can do a proper TDR measurement on this cable either with a scope or synthetic pulse from VNA with time domain option.

Sandeep Kumar Kalva If the cable is a coaxial cable, this result indicates the presence of unbalanced currents propagating on the outer metallic faces of the cable. Such currents are caused by poor balancing in the devices that the cable is attached to; since it's apparently an oscilloscope in this case, it means that the scope has poor RF performance.

The cable may still work well when attached to a more appropriately-designed device. Alternatively, one could place ferrite beads around the outside of a coaxial cable to help suppress these currents.

Fritz Caspers Stuart Barth Thank you for the quick answers. The cable I am using is twisted pair cable but not coaxial cable. And it has 3 pairs of twisted wires inside it.

Hi Sandeep,

ok I see ..then when the cable is in water , although the twisted pair wires are well insulated, of course you do get a change in characteristic impedance from the water and thats what you see in the time domain trace..

Sandeep Kumar Kalva Yes, to add to Fritz Caspers ' responses, you need to design the cable to be impedance-matched when it is inside the water. Unless it is de-ionized water, the losses will also be significant -- I would recommend changing to something like a coaxial cable if possible.

Thank you Fritz Caspers and Stuart Barth . We are using De-ionized water. The screenshot shown in the above is with de-ionized water. We have used twisted pair wires to reduce magnetically induced interference. Would you please recommend how to match the impedance between the current twisted pair cable and the water?

Its a fairly tricky task (but possible) to design this matching in water since the wires first have some plastic insulation and then their relative position may not to well be defined. What is the strong argument and objective to use a twisted pair cable at all in this case ?Do you want to measure the water level in some vessel this way?

My advice after many years of "wet testing" : do a proper shielding and grounding of each twisted pair plus the whole assembly, and you will be safe! Without shielding and grounding, there is just no way to transmit signals with twisted pairs across changing media (air, water..) without alterations..

Be careful, grounding is an art (single point star-design to avoid grounding loops which are also a common source of spurious noise)

good luck! And next time use a coaxial multi-pairs cables with all the shielding properly done, it is on the market!

Hi Subhamoy, Thank you very much for the nice explanation. Will try to find the triple coated coaxial cable that has wax coating.