The polarization and mass density modulation associated with antiferroelectric ordering in two ferroelectric nematic liquid crystals, a prototypical single compound (DIO) and a commercial mixture (FNLC919), is studied experimentally as a function of temperature. The wavenumber of the polarization modulation, $q_A$, is determined by small angle X-ray diffraction from the weak, accompanying density wave (wavenumber $q_S=2q_A$) using a conventional laboratory source. Combined results for $q_S$ and the saturated value of the polarization are analyzed in terms of Landau theory previously developed to describe the para-/antiferro-/ferroelectric sequence of phase transitions in solid ferroelectrics. The analysis of $q_S$ indicates that the polarization modulation can be reasonably well approximated by a simple sinusoid in DIO, whereas in FNLC919 a sharp decrease in $q_S$ close to the antiferro- to ferroelectric transition indicates development of a soliton-like profile.