2.2 Principles of Radio Telescopes
47
2.2.2 Two-Element Radio Interferometer
It is well known that the wavelength of radio-frequency band is 10
4
−10
7 times that
of visible frequency band. It is known from formula (2.2) that if the resolution of
a optical telescope with a aperture of 10 cm is achieved by single radio telescope,
the antenna aperture of the radio telescope will be required to get to 1−1000 km.
Apparently, it is impossible to make the radio telescope with so huge aperture. At
present, the world’s largest aperture for the rotation parabolic antennas is only 100 m,
and that for the Earth-fixed spherical antenna also reaches 500 m. The resolutions
of the single radio telescopes are always far less than those of the optical telescopes.
Therefore, the radio interferometry is introduced to improve the resolutions of the
radio telescopes. It is a powerful technique for aperture synthesis in astronomy.
According to the interferometric principle of electromagnetic waves, the combination of two radio antennas separated from certain distance can be used to improve
the resolution of the radio telescope. The combination is usually called two-element
radio interferometer. Two same radio antennas A 1 and A 2 are installed, respectively, at two ends of baseline D, and connected together to the same receiver by
the transmission lines with equal length. A schematic diagram of the two-element
radio interferometer is illustrated in Fig. 2.4. Generally, the radiation signals from
the same radio source do not arrive simultaneously at the two antennas. Supposed
that the radio incident angle is α, the time when the radiation signals arrive at antenna
Fig. 2.4 Schematic diagram of two-element radio interferometer
47
2.2.2 Two-Element Radio Interferometer
It is well known that the wavelength of radio-frequency band is 10
4
−10
7 times that
of visible frequency band. It is known from formula (2.2) that if the resolution of
a optical telescope with a aperture of 10 cm is achieved by single radio telescope,
the antenna aperture of the radio telescope will be required to get to 1−1000 km.
Apparently, it is impossible to make the radio telescope with so huge aperture. At
present, the world’s largest aperture for the rotation parabolic antennas is only 100 m,
and that for the Earth-fixed spherical antenna also reaches 500 m. The resolutions
of the single radio telescopes are always far less than those of the optical telescopes.
Therefore, the radio interferometry is introduced to improve the resolutions of the
radio telescopes. It is a powerful technique for aperture synthesis in astronomy.
According to the interferometric principle of electromagnetic waves, the combination of two radio antennas separated from certain distance can be used to improve
the resolution of the radio telescope. The combination is usually called two-element
radio interferometer. Two same radio antennas A 1 and A 2 are installed, respectively, at two ends of baseline D, and connected together to the same receiver by
the transmission lines with equal length. A schematic diagram of the two-element
radio interferometer is illustrated in Fig. 2.4. Generally, the radiation signals from
the same radio source do not arrive simultaneously at the two antennas. Supposed
that the radio incident angle is α, the time when the radiation signals arrive at antenna
Fig. 2.4 Schematic diagram of two-element radio interferometer