Similar questions

1.Explain different types of antenna used in satellite communication?

2.Explain briefly the “offset feed” used with paraboloidal reflector antenna stating its main advantages and disadvantages. Explain with neat diagram the antennas used in satellite communication.

The different types of antennas used in satellite communication are as follows:

1. Horn Antenna
2. Parabolic Reflector Antenna
3. Parabolic Reflector Antenna with offset feed
4. Double Reflector Antenna
5. Shaped Reflector Antenna

1.Horn Antenna

• The horn antenna is an example of an aperture antenna that provides a smooth transition from a waveguide to a larger aperture that couples more effectively into space.
• Horn antennas are used directly as radiators aboard satellites to illuminate comparatively large areas of the earth, and they are also widely used as primary feeds for reflector type antennas both in transmitting and receiving modes. There are two types of horn antenna:

Conical horn antennas

a. The smooth-walled conical antenna is the simplest horn structure. The horn may be fed from a rectangular waveguide, but this requires a rectangular-to circular transition at the junction. Feeding from a circular guide is direct and is the preferred method, with the guide operating in the TE11 mode.

b. The conical horn antenna may be used with linear or circular polarization, but in order to illustrate some of the important features, linear polarization will be assumed. The smooth-walled horn does not produce a symmetrical main beam, even though the horn itself is symmetrical. The radiation patterns are complicated functions of the horn dimensions. This lack of symmetry is a disadvantage where global coverage is required.

c. By operating a conical horn in what is termed a hybrid mode, which is a nonlinear combination of transverse electric (TE) and transverse magnetic (TM) modes, the pattern symmetry is improved, the cross-polarization is reduced, and a more efficient main beam is produced with low sidelobes.

Pyramidal horn antennas

a. The pyramidal horn antenna is primarily designed for linear polarization. In general, it has a rectangular cross section and operates in the TE10 waveguide mode In general, the beamwidths for the pyramidal horn differ in the E and H planes, but it is possible to choose the aperture dimensions to make these equal.

b. The pyramidal horn can be operated in horizontally and vertically polarized modes simultaneously, giving rise to dual-linear polarization.

2. Parabolic Reflector Antenna

• Parabolic reflectors are widely used in satellite communications systems to enhance the gain of antennas. This is the type seen in many home installations for the reception of TV signals. The circular aperture configuration is referred to as a paraboloidal reflector.
• The main property of the paraboloidal reflector is its focusing property, normally associated with light, where parallel rays striking the reflector converge on a single point known as the focus and, conversely, rays originating at the focus are reflected as a parallel beam of light.
• The phase centre of the feed is kept at the focus of reflector and the feed is connected to the high power amplifier (HPA) via orthogonal mode transducer (OMT) which is a three port device.
• When the antenna is in the transmit mode the energy from HPA is given to the feed phase centre where the horn antenna is present. The horn antenna then illuminates the reflector by radiating the energy towards it. The reflector then radiates to form parallel beam of energy.
• When the antenna is in the receive mode the reflector captures energy from all directions and converges it to the focal point, from there it is fed to the LNA via OMT.

3. Parabolic Reflector Antenna with offset feed

• In this the radiation pattern of the horn is offset so that it illuminates only the upper portion of the reflector. The feed horn and its support can be placed well clear of the main beam so that no blockage occurs.
• With the centre fed arrangement described in the previous section, the blockage results typically in a 10% reduction in efficiency and increased radiation in the sidelobes. The offset arrangement avoids this.
• The main disadvantages of the offset feed are that a stronger mechanical support is required to maintain the reflector shape and because of the asymmetry, the cross-polarization with a linear polarized feed is worse compared with the centre fed antenna.
• Polarization compensation can be introduced into the primary feed to correct for the cross-polarization, or a polarization-purifying grid can be incorporated into the antenna structure
• The advantages of the offset feed are sufficiently attractive for it to be standard on many satellites.
• It is also used with double-reflector earth station antennas, and is being used increasingly with small receive-only earth station antennas.

4. Double Reflector Antenna

• With reflector-type antennas, the feeder connecting the feed horn to the transmit/receive equipment must be kept as short as possible to minimize losses. This is particularly important with large earth stations where the transmit power is large and where very low receiver noise is required.
• The single-reflector system does not lend itself very well to achieving this so more satisfactory arrangements are possible with a double-reflector system.
• The feed horn is mounted at the rear of the main reflector through an opening at the vertex.
• The sub-reflector, which is mounted at the front of the main reflector, is generally smaller than the feed horn and causes less blockage.
• Two main types are in use are the Cassegrain antenna and the Gregorian antenna, named after the astronomers who first developed them.

5. Shaped Reflector Antenna

• The process referred to as reflector shaping, employs computer-aided design methods. With the shaped reflector dimples and/or ripples are created on the surface.
• The depth of these is no more than a wavelength, which makes them rather difficult to see, especially at the Ka band. Reflections from the uneven surface reinforce radiation in some directions and reduce it in others.