The Design Construction and Measurment of A Long—Periodic Dipole Antenna

The major work of this thesis is to design and make a good LPDA(log-periodic dipole antenna) as the feed of the ten-meter parabolic reflectors at the Yunnan Observatory. What we have done are as follows: 1) designing a pair of crossed dipoles; 2) analyzing the characteristics of the LPDA; 3) estim ating the characteristics of the parabolic; 4) discussing the change from linear polarization to circular polarization. We are generally satisfied with the LPDA measurement. The radiation patterns of the antenna, average Input Impedance, 3dB down beam-width and 10 dB beam- width ware concordant and stable with small variations in the bandwidth which is from 0.5GHz to 1.5GHz. Side-lobe levels of practical well-designed directional antennas typically range from about 13dB to about 40dB, and the worst side-lobe level of our antenna occurs at 1300M Hz, which is only 8dB down. But at other fixed frequencies of the measurement, the highest side-lobe levels are lower than -13dB. And then the antenna gain is about 2.5dB lower than what we want, but higher gain may be obtained by using the antenna as feed for parabolic reflectors. We can say that the antenna we made is frequency independent. Many of the advances that have been made in antenna theory such as fast Fourier transform, geometric theory of diffraction, plane-ware-spectrum (pws) scattering and method of moments etc., are related to digital computing in the sense that they would be impracticable as design tools for the engineer if the com putations had to be made by manual methods. therefore in this thesis, we also make full use of the com puter to design the antenna, to analyze the characteristics of the antenna, and to compute the parameters from all the data-files measured. The FORTRAN programs we made make all these work easier. At last, we use Hybrid coupler to change voltages from linear-polarization into circular-polarization.