Method of Decoupling and Independently Tuning the Second Mode of a Microstrip-Fed Slot Antenna Using Series Inductive Loading

In this letter, a systematic method of decoupling and tuning the second mode of a half-wavelength microstrip-fed slot antenna is presented. This is achieved by introducing a series lumped inductive load at the center of the top edge of a printed slot. At this location, the first (λ/2) mode has a current null, therefore the addition of the loading inductor does not affect the mode. However, the electrical length of the second (λ) mode is increased, allowing independent tuning of this mode over a wide frequency range, simply by changing the value of the inductance. The flexibility of the approach is validated by integrating an inductively loaded, dual-band slot into a compact, microstrip-fed inverted-F antenna to target common cellular and wireless bands (GSM-900, PCS, UMTS-2100, WLAN-2.4 GHz). The multiband antenna is found by measurement to exhibit better than -6 dB | S11| from 880-985 and 1810-2510 MHz, with radiation efficiencies between 84%-96%. The antenna hence demonstrates the suitability of the loading method for achieving multiband support in compact antennas that incorporate a slot in the ground plane.