Quixotron has been working in a similar field, so perhaps he can post with some help. In the meantime, can you describe exactly how you are feeding power from the transmitter to the loop? You mention a trace but you do not mention the length of the trace, how it connects to the loop, and how the transmitter attachs to this trace. It would be useful to know these things in order to help predict the transmission line characteristics.
If you want a theoretical background on a pcb loop, then go ahead and study the electrically small wire loop as it will be quite similar. The impedance of the loop will depend partly on how you are feeding it. Can you describe the geometry of the copper traces at the feedpoint (or better still, provide a picture of some sort).