E-plane

http://www.edaboard.com/thread305746.html

 If you have a square or round half wave resonating patch (without any slots or other features on it), the position of the feed determines how the standing wave develops.

Assuming that you look from positive z-direction towards the XY plane:

When you feed from left (negative x) or right (positive x) direction, the voltage maximums are at the left and right side of the patch, like in a half wave dipole that is on the x-axis. The charge to enable the high voltage on the left AND right side causes the patch current to oscillate from left to right. This is the same as in the half wave dipole. The mechanical equivalent is a square sized cup filled with water. When you move it from left to right only (that is along the x-axis), you can see the standing wave and most of the horizontal movement of the water is in the middle, like in the half wave resonating dipole.

When you excite from the top (positive y) or bottom (negative y), the current oscillates in Y direction and the high voltage edges are the upper and low edge. It is like moving the cup filled with water along the y-axis.

So whether the E-plane is the XZ or YZ plane depends on whether your feed is on the x-axis or y-axis. Somewhat OT, when you feed from both axis, but 90 degrees out of phase, you have circular polarization.

When the patch shape isn't symmetrical (for example by adding a slot under 45 degrees, you may get current oscillating in y-direction also when feeding from x-direction. This is used to get circular polarization from a single feed.

Just see the half wave resonating patch as a very wide electrically half wave dipole above a ground plane.