Bandwidth enhancement of a low-profile monopole antenna embedded in a circular cavity on a manhole cover

This article presents a low-profile disk-loaded monopole antenna specifically designed to be embedded into a manhole cover and a method to improve the bandwidth of the antenna. The antenna is placed deep inside a circular cavity formed on a manhole cover and is covered with a thick radome to protect the antenna from strong external impacts. The effect of the cavity size on the bandwidth of the antenna was studied, and as a result, it was found that the bandwidth of a low-profile monopole antenna is greatly enhanced when the cavity diameter is about one wavelength. The bandwidth enhancement mechanism due to the cavity size is analyzed in terms of the quality factor (Q) of the antenna. A prototype was fabricated and measured; the disk-loaded monopole antenna with a diameter of 0.2λ₀ and a height of 0.04λ₀ (λ₀ is the free-space wavelength at the center frequency) provides a wide impedance bandwidth of 14.3% (10-dB return loss) when placed inside a cavity with a diameter of 0.82λ₀ and a depth of 0.1λ₀. The antenna radiates vertically polarized omnidirectional fields in the horizontal direction and has a stable measured peak gain that varies from 4.5 to 5.5 dBi across the measured impedance bandwidth.