In this article, an elegant approach is presented for designing multi-band multi-mode rectangular microstrip antennas (MSAs) with high isolation between the bands. The coupling properties of a gap (an extended slot), which is introduced in a rectangular MSA operating in the TM_{010} and TM_{100} modes, are first qualitatively explained through field equivalence theory and image theory. It is shown that even though the slot appears 'transparent' (i.e., strongly couples the two sections of the MSA) to the TM_{010} mode, it effectively decouples the two sections of the MSA in the TM_{100} mode. As a proof-of-concept, the design technique is applied to a conventional patch to achieve dual-band operation with 39~dB of measured isolation between the bands. The design concept is then extended to achieve a quad-band antenna design with 30~dB of measured isolation between all of its bands. Compared to other methods the proposed one preserves the low-profile of microstrip antennas and provides a simple way to design and fabricate multi-band and multi-mode antennas. Therefore, this method is very well suited for designing antennas for next generation communication systems.
