Tuning the band-stop filter effect in mobile phone antennas

A slot placed between the feed and the ground connections of a single shorted microstrip antenna (PIFA) in a two antenna system can produce a band-stop filter effect in one of the frequency bands where both antennas radiate at the same frequency, but have very high isolation. The slot forms a band-stop filter, creating a large group delay at certain frequencies at a particular anti-resonance frequency. This paper analyzes methods to tune the band-stop filter effect by adjusting the slot width and additional ground connections. The antenna geometry analyzed is similar to a two antenna system inside a mobile device where the first antenna with slots has both lower band and high band resonances (quad-band GSM or dual-band LTE) and the second antenna is a wide-band single frequency antenna at LTE/3G/WiMAX.