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If we integrate the combined radiation flux from the base of the jet to
beyond the point where the jet becomes optically thin to the lowest radiation frequency, we obtain a flat spectrum over the complete frequency range. This integration may be represented as the summation of self-absorbed synchrotron spectra from electron populations located at increasingly greater radii along the jet axis, as shown above.

In the BK79 model the flux is integrated from the base of the jet to the
point where the magnetic field strength is too weak for electrons to emit at a given frequency. This frequency dependent upper integral limit ensures a flat spectrum mathematically, though it assumes that the emission region is sufficiently large to encompass this radius for all frequencies.