A planar antenna with an isotropic radiation pattern is provided. The planar antenna includes a substrate, a dipole antenna, a microstrip line set, and a channel selection module. The dipole antenna is disposed on a first surface of the substrate, and the microstrip line set and the channel selection module are disposed on a second surface of the substrate. A first microstrip line and a second microstrip line of the microstrip line set are spirally extended along two opposite rotation trails on a vertical projection plane to form a high-frequency path with the dipole antenna. The planar antenna controls the on/off state of the channel selection module so that a low-frequency path is formed when the dipole antenna is connected to a first line and a second line. A plurality of channels having different operating frequencies is respectively generated within the high-frequency path and the low-frequency path.