How exactly do inground electric dog fences work? I need help making a square perimeter with one side that dose not trigger the collar
This type of "fence" is in common use with robot lawnmowers and a typical problem is how to isolate a flower bed and prevent the mower entering that area.



Figure 1. Fencing a flower bed. Note that the bridge between the boundary and the flower bed are in very close proximity and so the current travelling in opposite directions cancels out and the mower doesn't see it. Image source: My Robot Mower.

Unfortunately there isn't a simple way to do this in a manner to leave one side of the loop open. To close the loop each side has to have an odd number of wire runs (1, 3, etc.) and so cancellation can't occur.

One thing I can think of is to use earth rods at each side of the gap to close the loop on the open side. The idea here is that the ground current would be so disperse that it wouldn't be detected by the dog collar. This might work where I live as the ground is continuously damp.

Another possibility is to use the coax solution as you have already considered. I would suspect that any screened cable should do the trick. Alternatively run the standard wire through an earthed metal tube.