The antenna is the most obvious part of a radio telescope. It is analogous to the lens of an optical telescope.
The antenna gathers the minute amount of radio frequency energy from the sky and transforms it to a tiny electrical current which, after much processing, we can then measure. Most radio telescope antennas are quite large due to the resolving power desired.
Larger antennas may better focus the energy from a smaller region on the celestial sphere. This region of the sky to which the antenna is most sensitive may be thought of as the beam pattern of the antenna.
The resolving power
RP=1.22L/D x 206265
Where L= wavelength in millimeters
D= collector diameter in millimeters
206265 is deducted from the parallax (1pc /1 AU)
The resolution power is expressed in arc seconds.
The resolution of a radio telescope is linked to the frequency by the herewith formula. We immediately see that the dish diameter becomes rapidly huge if we want a resolution similar to optical telescopes. We can work at a few GHz but here silicon components are useless. The only solution is using an inter-ferometry base and to integrate results (VLA, VLBI, etc).
While the type of antenna most often thought of in relation to radio astronomy is the parabolic dish antenna, many other types of antennas are also used. Large arrays of dipole antennas have been used to discover pulsars and probe the noise storms of Jupiter. Long trough-like antennas, the cylindrical parabolic, are still used in observatories around the world. Arrays of Yagi antennas, horn antennas, Mills crosses, and many others have contributed to radio astronomy. Virtually any antenna which has a reasonably small beam pattern has been used.
Very often, amateur radio telescopes will keep the direction of the antenna fixed along the north-south line, or meridian. The antenna is adjusted in elevation to a given angle and the cosmic radio source allowed to pass through the antenna beam as the Earth rotates. This is called a meridian drift scan observation. As the radio source passes through the antenna pattern, an increase in energy is recorded as a rise and then a decline in the data recording device. Meridian drift scans offer the advantage that calculation of the source coordinates becomes a simple matter. The right ascension of the source is equal to local sidereal time at which the source passes.
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