The wavelength of UHF frequencies is approximately 300 mm (12"). At these frequencies the radio waves do not pass freely through materials. In particular, moisture, water and ice cause problems by absorbing the signals. Metals also create problems with reflections. As a result of using the electric field, the ideal antenna is a dipole, as opposed to the loop favored by LF and HF transponders. This means that tags tend to be long and thin. Attempts to shorten the length of a tag usually result in reduced range. Furthermore, the performance of a tag will change depending on the material it is applied to such as a metal can, a container with liquid, or a dense material. The advantage of UHF frequencies is the ability to read a large numbers of tags in a very short time (>200 tags /second). The electric field also doesn't drop off as quickly as the magnetic field allowing extended operating ranges of greater than 5 meters. However, defining the reading zone is problematic because of reflections.