| Active galactic nucleus is the general term for an object containing nonstellar activity in its nucleus and usually optical/UV emission lines. Generally speaking, AGN phenomena refers to the energetic phenomena taken place in the galactic nuclei. The radiation from typical AGNs covers a broad frequency range. The standard model considers that the high energy of AGN derives from the accretion process by a super-massive black hole locating at the galactic center. The "uniform schemes" attributes the diverse observational characters of AGNs to the orientation effect. The relativistic emission from the radio jet is Doppler boosted due to moving towards the observer. The brightness of radio core, aligned with the line of sight, would be magnified due to the relativistic beaming. The apparent motions of jet components, in some cases, could be amplified to exceed the light speed, i.e., superluminal motion. There are increasing evidence supporting that the emission in 7-ray loud blazars is highly relativistic, i.e., the jet apparent motions are much higher than the light speed; the brightness temperature of the radio core is approaching or higher than the equipartition limit; the parsec- and kiloparsec-scale radio structures are preferentially aligned. I took part in the observational research project of an EGRET-detected blazar sub-sample, and leaded in the studies of 3 individual sources: two EGRET-detected blazars 1611+343 and 3C 273, and one gamma-ray loud blazar candidate 1502+106, as well as a CSS source 3C 286.There are increasing observational evidences supporting that some neighboring spiral galaxies contain compact radio sources in their nuclei. These radio sources are similar with luminous AGNs in many observational characters, but in extremely lower luminosity. They are temporarily named Low Luminosity AGN (LLAGN). It is believed that these LLAGNs also hold super-massive black holes, just like typical powerful AGNs. I joined in the multi-frequency observational study of a representative LLAGN ?Sgr A*, the compact radio source at the Galacic Center.The main body of the thesis consists of two major parts : the first part includes the first 5 chapters, in which the radio properties of a number of highly luminous active galactic nuclei (AGNs) are studied; and the second part, i.e. the 6th and 7th chapters, analyzes the multi-frequency radio observations of a typical low-luminosity AGN (LLAGN), Sgr A*, the compact radio source at the Galactic Center. The further working is prospected in the last part, Chapter 8.The present thesis begins with a brief introduce of the radiation properties of AGNs inChapter 1. In the last section of Chapter 1 we outline the subsequent parts of the thesis.In Chapter 2, the observational properties of HPQ 1502+106 at radio frequencies are discussed. The research is based on multi-epoch multi-frequency analysis at 2.3, 5.0, 8.3, 24.4 and 43.1 GHz, with the observing epoch ranging from June 1994 to December 2002. Different mas-scale radio images are exhibited, and the source is characterized by a multi-component one-sided jet at all epochs. The higher-frequency images indicate a curved jet trajectory in the source. We detect superluminal motions in four discrete jet components, and the estimated velocities range from 10.5 c and 37.3 c. The observed results of 1502+106 show evidence for highly relativistic jet close to the line of sight. The radio properties of 1502+106 suggest that it could be an EGRET blazar, though the 7-ray nature of the source is not confirmed yet.In Chapter 3, the EVN observation of OVV 1611+343 at 5 GHz is analyzed. The total intensity image shows that an almost 90?jet bending from the south to the east at ~ 3 mas south to the radio core. The polarization intensity image indicates that the polarization intensity peaks where the jet bends structure. The reflection by an oblique shock could result in the observed bending. The brightness temperature of 1611+343 exceeds the equipartition limit, and the apparent velocities of jet components are larger than the light speed...
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