The study of the dynamic spectra of hectometric Jovian emission (f<1.3 MHz) for the period January to June 1978 from the planetary radio astronomy (PRA) experiment on the Voyager 1 and 2 spacecraft shows that its shape is stable with the rotation of the planet. However, there are noticeable differences between the spectra observed at one-month intervals by one spacecraft, or simultaneously by the two spacecraft, which can be clearly related neither to an effect of Io nor the the Jovicentric decination of the observer.