Planar antenna with isotropic radiation pattern

A planar antenna with an isotropic radiation pattern, comprising:

• a substrate, having a first surface and a second surface;
• a dipole antenna, disposed on the first surface, and having a first radiating body and a second radiating body;
• a microstrip line set, disposed on the second surface and electrically connected to the dipole antenna, wherein 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 respectively with ends of the first radiating body and the second radiating body as starting points, so as to form a high-frequency path with the dipole antenna;
• and
• a channel selection element, disposed on the second surface and electrically connected to the microstrip line set, wherein a low-frequency path is formed when the first radiating body is connected to a first line and the second radiating body is connected to a second line, wherein a plurality of channels having different operating frequencies is respectively generated within the high-frequency path and the low-frequency path.