| Sensitivity and resolution are the most important parameters of astronomical facilities. This thesis will discuss methods of improving the sensitivity and angular resolution of VLBI system, and use these methods to study some radio sources. Section I discusses the phase-reference technique and studies two phase-referenced datasets. Phase-reference technique uses the visibility data of a nearby strong compact source to calibrate those of a target source. Phase-reference technique improves visibility data's coherence time as well as the system's sensitivity. Phase-referenced data contains the target source's positional information with respect to the reference source. The use of this technique in astronometry can yield sub-milliarcsecond accuracy, and it's one of the popular techniques in differential VLBI astronometry. Chapter 2 presents the first quasi-simultaneous multi-frequency (2.3, 5.0, 8.4 and 15.4GHz) VLBI observations of the Compact Steep Spectrum (CSS) source 3C 138, and the fast-switching phase-reference observation was used only at 15.4GHz. Three methods (hybrid mapping, phase-reference mapping, and another hybrid mapping using phase-reference + self-calibration) are applied to study these phase-reference data at 15.4GHz. The second hybrid map (phase-referencing + self-calibration) has the highest resolution and dynamic range, and the phase-reference map gives the relative position of the target source 3C 138 with respect to reference source PKS 0528+134 with a high accuracy. This is the first time that the fourth compact component B2 is detected after Cotton et al (2003). For the first time, the spectral distribution of the components within its central 10 milli-arcsecond region is obtained and the component A is most likely to be the flat-spectrum core. Chapter 3 presents a quasi-simultaneous multi-frequency (2.3, 8.4 and 22.3 GHz) VLBA phase-reference study of the radio galaxy 3C 66B. The angular seperation of target source 3C 66B from the reference source 3C 66A is only 6′, so the phase-reference maps of 3C 66B are made with high dynamic ranges at all three frequences. These phase-reference maps provide accurate relative position of the target source 3C 66B with respect to the reference source 3C 66A. The reference source 3C 66A has a curved jet close to the core, but it is will collimated at large-scale. The light curve of 3C 66A at 15.4GHz appears to be periodical with a period about 4.5 years. Section II discusses Space VLBI and studies high resolution VSOP observations of the southern blazar J1924-29 at 1.6GHz. Extending baseline is the most effective way to improve system's angular resolution. In 1997, Feb. 12, the radio telescope HALCA was successfully launched and firstly extended the effective baseline out of the earth. It improved the VLBI's resolution by three times and especially improved the ability to detect the sources on the southern sky. The space-VLBI observations ofJ0924-29 reveal a typical jet struture with milli-arcsecond accuracy, and the two jet components extend to the northeast direction with a position angle difference of 10°, which suggests a curved jet. The Gaussian model-fitting results indicate that the brightness temperature of J0924-29's core in the rest frame of the source exceeds 1012K.
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