Jet structure in the quasars 3C 345 and 1803+784

The fine structure of the AGN objects 3C 345 and 1803+784 is studied. Their jets are shown to have conical helical structures. The helix diameter and pitch are proportional to the distance from the nucleus (injector). The hel ix axis is curved. This structure is attributable to a short- and long-peri od precession of the accretion-disk rotation axis. The jet is cocooned in a thin layer of thermal plasma with l approximate to 0.01 pc, whose transpar ency increases as one recedes from the nucleus. The thermal-electron densit y in the cocoon wall near the nuclear region is N-e greater than or equal t o 10(6) cm(-3). The nuclear activity causes the cocoon transparency to chan ge, which results in a low-frequency variability of radio emission from the quasars and in a change of polarization parameters. The cocoon transparenc y determines the visible part of the jet and "displaces" the nucleus from t he injector. The nucleus is a compact, bright region of the jet, whose brig htness temperature does not exceed the Compton limit. The jet spectrum exhi bits a low-frequency cutoff. The cutoff frequency in the spectrum of the nu clear region lies at short centimeter wavelengths and shifts to lower frequ encies as one recedes from the injector. The low-frequency spectral index i s steep and reaches alpha = 4, which is attributable to additional absorpti on of synchrotron radiation in the cocoon wall. The high-frequency spectral index is typical of optically thin sources and is alpha approximate to -1. 5 at large distances from the nucleus.