The field will not change. The impedence will change upwards from the 75 ohms at the center to about 300 ohms if you feed it about the 1/3 point (so-called a "windom"). You may also experience radiation from the feeder as the 2 legs are not symmetrical (or balanced). This can be reduced with a choke at the feed point.
Check some of the ham radio literature - that's where I learned it!
