def calc_SEFD(A, Tsys, eff=1.0):
    """ Calculate SEFD
    Tsys = system temperature
    A = collecting area
    Ae = effective collecting area
    eff = aperture efficency (0.0 to 1.0)
    """
    kb = 1.3806488e3  # 1.38064852e-23    Boltzmann constant
    Ae = A*eff
    return 2 * Tsys * kb / Ae