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When microstrip antennas (MSA) are loaded with monolithic stubs, shorting pins or slots, the electrical resonant length of the patch gets modified and hence tunable or multiple frequency antennas can be realized. As a stub protrudes from the periphery of the patch, this configuration occupies more space. Hence, where compact dual frequency antennas are required, cutting slots in the patch is more suitable. Using both CIM and IE3D, we theoretically analyze a rectangular microstrip antenna (RMSA) embedded with a rectangular slot along the non-radiating edge of the patch, yielding dual frequency operation with a broadside radiation pattern. The slot modifies the resonant length of the fundamental mode of the simple RMSA and introduces a new resonant mode similar to TM11. The effects of slot dimensions and slot location on both of the resonance frequencies have been investigated. The proposed configuration is tunable for appropriate choice of the slot length and location of the slot. The simulation results have been verified experimentally and the theoretical resonance frequencies are within 1.5% of the measured values.