PARABOLIC REFLECTOR ANTENNA

Ankit JainB.Tech 5th Sem (ECE)

IIIT Allahabad

Features :

High Gain ( 30-40 db) Low cross polarization Reasonable bandwidth, Fractional Bandwidth

being at least 5% on commercial models. Can be very wide in huge dishes. (like the

Stanford "big dish" above, which can operate from 150 MHz to 1.5 GHz).

Smaller dish antennas (2-28GHz) Large dishes - VHF region (30-300 MHz)

Basic Structure :

Consists of a feed antenna pointed towards a parabolic reflector. The feed antenna is often a horn antenna with a circular aperture.

Unlike resonant antennas like the dipole antenna which are typically approximately a half-wavelength long at the frequency of operation, the reflecting dish must be much larger than a wavelength in size. The dish is at least several wavelengths in diameter, but the diameter can be on the order of 100 wavelengths for very high gain dishes (>50 dB gain). The distance between the feed antenna and the reflector is typically several wavelenghts as well.

Major Features :1)All rays emanating from the focal point (the source or

feed antenna) will be reflected towards the same direction.

2)The distance each ray travels from the focal point to the reflector and then to the focal plane is constant.

As a result of these observations, it follows the distribution of the field on the focal plane will be in phase and travelling in the same direction. This gives rise to the parabolic dish antennas highly directional radiation pattern.

Parabolic antennas are meant to work for microwave signals i.e waves that have very small wavelengths. This is mainly because of two reasons-

For any wave lower in frequency, the wavelength becomes so big that the dish becomes too large for practical use!

Obviously, the costing…

Limitati ons with Designing

Problems of horn feed antenna

Operates in a single mode only

Excess of cross polarization

Desired focal distribution is not achieved

Different forms of Parabolic reflector