"At the high end of the electromagnetic spectrum, signals travel over a band of 10 million trillion Hz (that is, 10^22Hz). This end of the spectrum has phenomenal bandwidth, but it has its own set of problems. The wave forms are so miniscule that they're highly distorted by any type of interference, particularly environmental interference such as precipitation. Furthermore, higher-frequency wave forms such as x-rays, gamma rays, and cosmic rays are not very good to human physiology and therefore aren't available for us to use for communication at this point."

Not the best analysis but I'm working. Basically like any other metric, as the frequency increases the space between peaks and valleys decreases and it becomes harder to determine/separate from others 30hz to 230hz is much easier to tell the difference than 15khz to 15.2khz if you want to listen to audio tones. Once you get to microwaves this becomes of course much more difficult.

http://www.informit.com/articles/article.aspx?p=24687&seqNum...