lar bands frequencies in handset-size user equipment. In general, the upper frequency bands (>= 1710 MHz) don't create particular difficulties for the antenna design. The two main challenges are: - Bandwidth: The antenna must have enough bandwidth to cover the additional part of the spectrum, and this is difficult to achieve in the 700-MHz bands as the size of the device small compared to the wavelength - MIMO (multiple-input and multiple-output): Multiple antennas, at least 2, must be integrated in the device to support the MIMO functionality. Below 1 GHz isolation becomes an issue and the correlation between the radiation pattern is high, limiting the MIMO performance. The maximum theoretical bandwidth available for a small antenna is proportional to the volume, measured in wavelengths, of the sphere enclosing the antenna. A typical antenna for mobile devices either PIFA or monopole, excites RF currents on its counterpoise, typically the main PCB of the device or a metal frame. The PCB/frame of the device therefore becomes part of the 'antenna', and the maximum linear dimension of the device should be used as the diameter of the sphere. As a consequence, in the low-frequency bands the bandwidth of the antenna is proportional to the third power of the size of the device using the antenna, and as the frequency is reduced 25 Our thanks to Antenova www.antenova.com for help and advice in providing inputs and background to this section. Any inaccuracies or mistakes will almost certainly be our fault not theirs. Making Telecoms Work the bandwidth reduces quickly. More bandwidth can be traded off at the expense of antenna efficiency. For a typical handset with length around 100 mm, it can be already difficult to cover the legacy 3GPP bands 5+ 8 (824-894 MHz and 880-960 MHz), SO adding support for an additional 100 MHz (698-798 MHz) at an even lower frequency is a major challenge for small devices; for larger devices, for example tablets, the problem is reduced. Although a tuneable/switchable antenna can greatly help i