A highly-shielded and easy-integration Antenna-in-Package (AiP) configuration based on the rectangular hollow dielectric resonator antenna (DRA) is presented. The surfaces of the hollow are made conductive to form a metal cavity, isolating the DRA from the RF circuits. Both passive and active antennas for 2.4-GHz WLAN applications are designed, measured, and compared. The two antennas are excited in the TExẟ11 mode and perform similarly. To know the effect of the perfect-electric-conductor (PEC) boundary, the DRA without any metal cavities are also studied for comparison. Two sets of experiments are designed to investigate the electromagnetic interference (EMI) between the AiP and a low noise amplifier (LNA) inside the hollow. The influences of the DRA on the packaged LNA are measured and analyzed for both the no-PEC-boundary and PEC-boundary cases. It is found that the output of the LNA will be significantly affected by the DRA input power when the latter is sufficiently high.