Types of Antennas

A diverse variety of antennas have been used for radio astronomy (see eg. Chapter 3) the principal reason for this diversity being the wide range of observing wavelengths: from $\sim 100$ m to $\sim 1$ mm, a range of $10^5$. However the most common antenna used for radio astronomy is the paraboloid reflector with either prime-focus feeds or cassegrain type feed arrangement.

Prime-focus parabolic antennas although mechanically simple have certain disadvantages, viz. (i) the image-forming quality is poor due to lower ${f/D}$ ratios in prime-focus antennas, and (ii) the feed antenna pattern extends beyond the edge of the parabolic reflector and the feed hence picks up some thermal radiation from ground. The cassegrain system which uses a secondary hyperboloid reflector and has the feed located at the second focus of the secondary solves these problems. For cassegrain systems the $f/D$ ratio is higher and further the feed ``looks'' upwards and hence pick up from the ground is minimized. This is a great advantage at higher frequencies, where the ground brightness temperature ($\sim 300$ K) is much higher than the brightness temperature of the sky. However this is achieved at the price of increased aperture blockage caused by the secondary reflector.

A primary advantage of paraboloid antennas (prime focus or cassegrain) is the ease with which receivers can be coupled to it. The input terminals are at the feed horn or dipole. A few other advantages are: (i) high gain, a gain of ${\simeq 25 }$ dB for aperture diameters as small as $10{\lambda}$ is easily achievable, (ii) full steerability, generally either by polar or azimuth-elevation mounting. Further the antenna characteristics are to first order independent of pointing, (iii) operation over a wide range of wavelengths simply by changing the feed at the focus.

Compared to optical reflectors paraboloid reflectors used for radio astronomy generally have a short ${f/D}$ ratio. Highly curved reflectors required for higher ${f/D}$ ratios result in increased costs and reduced collecting areas. Although the reflecting antennas are to first order frequency independent, there is nonetheless a finite range of frequencies over which a given reflector can operate. The shortest operating wavelength is determined by the surface smoothness of the parabolic reflector. If ${{\lambda}_{mn}}$ is the shortest wavelength,

$${{\lambda}_{mn}} \: {\approx} \: {\sigma}/20$$ (19.2.1)

where, ${\sigma}$ is the rms deviation of the reflector surface from a perfect paraboloid. Below $\lambda_{mn}$ the antenna performance degrades rapidly with decreasing wavelength. The longest operating wavelength ${{\lambda}_{mx}}$, is governed by diffraction effects. As a rule of thumb the largest operating wavelength $\lambda_{mx}$ is given by

$${{\lambda}_{mx}} \: < \: 2 {\bar{L}} \\$$ (19.2.2)

where, ${\bar{L}}$ is the mean spacing between feed-support legs. At $\lambda = \bar{L}$ the feed support structure would completely shadow the reflector.