  An experiment was performed at the scientific neutron source FRM II in
Garching to determine the cumulative antineutrino spectrum of the fission
products of $^{238}$U. This was achieved by irradiating target foils of natural
uranium with a thermal and a fast neutron beam and recording the emitted
$\beta$-spectra with a gamma-suppressing electron-telescope. The obtained
$\beta$-spectrum of the fission products of $^{235}$U was normalized to the
data of the magnetic spectrometer BILL of $^{235}$U. This method strongly
reduces systematic errors in the $^{238}$U measurement. The $\beta$-spectrum of
$^{238}$U was converted into the corresponding antineutrino spectrum. The final
$\bar\nu_e$-spectrum is given in 250 keV bins in the range from 2.875 MeV to
7.625 MeV with an energy-dependent error of 3.5 % at 3 MeV, 7.6 % at 6 MeV and
$\gtrsim$ 14 % at energies $\gtrsim$ 7 MeV (68 % confidence level).
Furthermore, an energy-independent uncertainty of $\sim$ 3.3 % due to the
absolute normalization is added. Compared to the generally used summation
calculations, the obtained spectrum reveals a slight spectral distortion of
$\sim$ 10 % but returns the same value for the mean cross section per fission
for the inverse beta decay.
