Self-diplexing antenna using the square patch with a hole

The paper proposes a double-layered circularly polarised self-diplexing antenna (SDA) using a square patch with a shorted hole, which is a new type antenna with improved diplexing characteristics. The equivalent relative permittivity can be adjusted by changing the size of hole. The E-plane radiation energy at ±90° is largely reduced when the relative permittivity is unity. To utilise this characteristic, a double-layered SDA is proposed which consists of the lower layer of a square patch with a hole and the upper layer of a 0.5 λ₀ airgap square patch. Using this antenna, the isolation level was improved by a minimum of -54 dB