I need somebody to explain properly how the ground loop works, especially if this happens when is only a DC circuit like a power supply for a motor and a coaxial cable which carries a PAL signal. Because at start I thought that my signal will be very affected but nothing really happend. To be more precise let's say that I have 200m of cable which contains 2 conductor with 1.8 \$ \Omega \$ resistance at 200m and a coaxial cable with inner and outer(shield) conductor. If I tie the shield of coaxial at both ends will my ground loop appears and influence my video signal.
A ground loop occurs where there are two paths to ground over a section of cabling. This can occur, for example in your case, where you have a ground shield on a coax cable. If that ground shield were connected to a common ground between the two pieces of equipment (such as earth ground or some other shared ground at both ends of the cable), then there would be a potential ground loop due to multiple return paths from one end of the cable to the other and the resistance of the two paths is not the same. There is also a potential for a ground loop problem if there is induced AC on the shared ground (this is often the source of hum on audio lines).
However, if the coax ground is not connected at the far end, or the equipment at the far end is not connected to a common ground point shared by both ends, then there is only one ground return path and no ground loop exists. (However it is then possible to have a situation where you have no ground between the two pieces of equipment -- both paths are broken -- and your signal doesn't work as intended.)
Ground loop- more than one return path for current. For example, there may be a motr taking high current and due to bad wiring design the current returns on encoder's cable, which is not rated for high current and will just burn.
But that is a good case. One can find this problem quickly. Worst case - nothing burns, but electromagnetic interference appears where you less expect it, because certain harmony of current pulse found its way on unintended wire.
In most exotic situations a standing wave may appear on the loop, causing more EMI interfere.
In any case, just avoid it. For each current carefully design the return path. Use star co nection.