  Microwave phased array antennas (PAAs) are very attractive to defense
applications and high-speed wireless communications for their abilities of fast
beam scanning and complex beam pattern control. However, traditional PAAs based
on phase shifters suffer from the beam-squint problem and have limited
bandwidths. True-time-delay (TTD) beamforming based on low-loss photonic delay
lines can solve this problem. But it is still quite challenging to build
large-scale photonic TTD beamformers due to their high hardware complexity. In
this paper, we demonstrate a photonic TTD beamforming network based on a
miniature microresonator frequency comb (microcomb) source and dispersive time
delay. A method incorporating optical phase modulation and programmable
spectral shaping is proposed for positive and negative apodization weighting to
achieve arbitrary microwave beam pattern control. The experimentally
demonstrated TTD beamforming network can support a PAA with 21 elements. The
microwave frequency range is $\mathbf{8\sim20\ {GHz}}$, and the beam scanning
range is $\mathbf{\pm 60.2^\circ}$. Detailed measurements of the microwave
amplitudes and phases are performed. The beamforming performances of Gaussian,
rectangular beams and beam notch steering are evaluated through simulations by
assuming a uniform radiating antenna array. The scheme can potentially support
larger PAAs with hundreds of elements by increasing the number of comb lines
with broadband microcomb generation.
