def lookup_frequency(path_length, frequency_lookup):
    """
    Lookup the maximum allowable operating frequency.

    """
    if path_length < 10000:
        return frequency_lookup['under_10km']
    elif 10000 <= path_length < 20000:
        return frequency_lookup['under_20km']
    elif 20000 <= path_length < 45000:
        return frequency_lookup['under_45km']
    else:
        print('Path_length outside dist range: {}'.format(path_length))