Bandwidth of Gain in Metasurface Antennas

Metasurface Antennas: Design and Performance

“…Here the antenna bandwidth is the frequency range in which the gain is larger than −3 dB with respect to the gain at the design frequency. The rate at which the antenna loses gain when changing frequency is related to the dispersion of the MTS [16]. In fact, this dispersion introduces a variation β sw → β sw + ∆ β sw and α → α + ∆α implying a phase mismatch between the SW-wavenumber and K s(ρ), as well as a distortion of the amplitude of the aperture field.…”

“…where (ka) 2 is the directivity of a uniformly illuminated circular aperture of radius a and ε is the antenna efficiency. From (16), the reduction of gain with frequency is translated into a decrease of the antenna efficiency. Naming ε the efficiency at the design working frequency, and ε the efficiency when a shift ∆ β sw arises on the wavenumber due to the frequency change, the unilateral bandwidth is obtained when the condition ε = 0.5ε is met.…”

“…A representation useful in practice relates the antenna gain in dBi for ε feed ε tap = 1 to the percentage bandwidth B % . To this purpose, we evaluate the product between the antenna gain in (16) and the half-power relative bandwidth in (18) when ε feed ε Ω = 1 and ε conv ε tap = a λ / (a λ + 2), obtaining:…”

Wideband Active Region Metasurface Antennas

“…The main drawback of conventional MTS antennas is their limitation in terms of product bandwidth gain. In particular, circular apertures with period d of the MTS modulation independent of the radial coordinate provide broadside high gain at the frequency where d matches the wavelength of the exciting SW. One can estimate the bandwidth as in [34], where an optimal choice of the amplitude modulation index m is assumed for antennas with radius a>3λ0 (with λ0 being the free-space wavelength at the center frequency f0). Then, the fractional bandwidth is given by Δf/f0=1.2(a/λ0)(vg/c), where c is the speed of light in free space, and vg is the group velocity of the SW at f0 when it propagates along a uniform average impedance.…”

“…Then, the fractional bandwidth is given by Δf/f0=1.2(a/λ0)(vg/c), where c is the speed of light in free space, and vg is the group velocity of the SW at f0 when it propagates along a uniform average impedance. In addition, the aperture gain in absence of losses (directivity) is G=(2πa/λ0) 2 (a/λ0)/(a/λ0+2), for an optimal m [34]. In the case of electrically large antennas, the latter expressions provide a fractional bandwidth gain product approximately equal to 47(a/λ0)(vg/c).…”