Power gain and directive gain are related by radiation efficiency ,

Radiation efficiency- ratio of total power radiated by antenna to power accepted by antenna.

Antenna radiation pattern

Plot of radiation intensity as a function of angles(degrees)

2 angle coordinates employed by a ground based radar are generally azimuth and elevation angles.

Main lobe-At zero degreesRemaining lobes are side lobe region.

Vestigial lobe- irregularity in the radiation pattern at 22dB below the peak ,with increase in degrees.

Near-in-Side lobe - generally decrease in magnitude as the angle increases.

Spill over radiation- part of feed radiation pattern that is not intercepted by the reflector.

Ranges b/w 100 to 115 degrees.

Contd...

Back lobe- due to diffraction around the edges of reflector in backward direction.

Effective apertures

Effective aperture of receiving antenna is Measure of effective area presented to the incident wave by antenna.

Transmitting gain G,receiving affective area Aeof a lossless antenna are related by

Side lobe radiation

Radiation from an antenna that is not radiated by a main beam

Theoretical ,it is possible to have antenna radiation pattern with main lobe and no side lobes. But impractical to achieve.

Highest side lobe is the one which is adjacent to the main lobe

Peak side lobe is generally 13.2 dB or -13.2 dB compared to the main lobe.

Low antenna side lobes are usually desirable for minimizingthe effect of strong jamming and interference.

Aperture efficiency and polarization

Aperture efficiency Usually occurs at the center of the main beam.

Radiation efficiency is less than unity -energy is lost.

Aperture efficiency less than unity- energy is redistributed in angle.

Polarization of an EM wave is defined by orientation of electric field.

Radar antennas are linearly polarized (either horizontal or vertical)

Air surveillance radar employ horizontal polarization.

Tracking radars-vertical polarisation.

Circular polarization- when electric field rotates at rate equal to RF frequency.

enhances the detectability of aircraft targets in the midst of rain.

Elliptical polarisation-when electric field rotates at rate equal to RF frequency,amplitude of elliptically polarized electric field varies during rotation period.

Reflector antennas