From my research, this is how it works. I replied in another forum and apol ogize here for the basic electronics, but figured other people might want t o know how it works who aren't EE's. ;)
We should start by saying that any wire with a current flowing through it i s an inductor by means of the fact that if a current flows through the wire , a magnetic field is "induced" around the wire and if a magnetic field mov es across the wire (or visa versa) a current is induced in the wire. To ima gine the polarity of the magnetic field around a wire, you can use the "rig ht hand rule". Wrap your right hand around the wire with your thumb pointin g parallel to the wire. If conventional (positive charge) current is flowin g in the direction of your thumb, then the magnetic field is polarized in t he direction your fingers are curling. The polarity of the magnetic field g enerated by a current flowing through wire is opposite on each side of the wire. If the wire were laying on the ground, you can think of the magnetic field going "down" on one side and "up" on the other. The patent for how th e yardmax polarity detection and system works is described in patent 8,047,
151, but is uses the technology of the 5,794,569 patent from another compan y.
The collar is very sophisticated. It uses an 3-axis accelerometer to determ ine the position of the collar and 3 mutually orthagonal antennas connected to a reciever to get a strong signal no matter the orientation of the coll ar (not for direction finding in this case). In other words, each antenna p oints to one of the 3 dimensions of space and the accelerometer knows the o rientation of the collar with respect to the ground by way of gravity.
The transmitter sends each first cycle of a carrier signal with a higher cu rrent to the wire using an amplitude modulated sine wave signal so that the "wake up" can be detected by the receiver and to provide a reference for t he positive part of the signal since the polarity of the magnetic field fro m the wire (which is an antenna) actually changes back and forth as the cur rent flowing through it changes direction and we need to know when the sign al is positive. The first carrier cycle of each part of the signal is alway s positive, so the receiver in the collar now can tell the polarity of magn etic field from the wire at that point in time. It does this by using the a ccelerometer to determine "down" and then using the antenna most perpendicu lar to the ground to receive the signal and know which direction the curren t is flowing in the antenna. It then compares that to a table in the microp rocessor in the collar that knows what polarity should be inside the bounda ry vs. outside. Now the collar processor can read the following data train of carrier bursts (8 bits worth) to get information sent by the transmitter such as which collar gets which instructions (like how strong a correction ), etc. The collar stores whether it is inside or outside the boundary and can handle all the clever tasks it needs to in yardmax mode, like continuin g the correction to a point until the pet returns inside the boundary and n ot correcting when outside the boundary and trying to get back in.