For rectangular waveguide to transfer energy, the cross-section should be at least:

A rectangular waveguide functions mathematically like a high-pass filter, meaning it will only allow frequencies above a specific 'cutoff frequency' to propagate. Any frequency lower than this cutoff will be rapidly attenuated and fail to travel down the tube. For electromagnetic waves to successfully transfer energy down the waveguide, the wider internal dimension of the rectangular cross-section must be at least one-half of the signal's wavelength in free space. This fundamental physical requirement establishes the lowest frequency (longest wavelength) the waveguide can possibly support.