Electro Magnetic wave theory (Antenna-Radiation pattern etc.) | Types of Antennas
A microwave antenna system consists of the antenna itself, some form of transmission lines connecting the antenna to the transmitter and receiver, plus some sort of coupling device, either a circulator or an isolator.
This handout describes different microwave antenna, their characteristics, construction and mounting arrangements. Also different kinds of transmission lines ie. feeders are described.
2.0 Characteristics of Microwave Antennas
Highly directional antennas are used with point- to-point microwave systems. By focusing the radio energy into a narrow beam that can be directed towards the receiving antenna, the transmitting antenna can increase the effective radiated power by several orders of magnitude over that of an omni directional antenna. The receiving antenna also, in a manner analogous to that of a telescope, can increase the effective received power by a similar amount.
Although gain is a primary characteristic, there are other antenna characteristics which are of importance in communications systems. Antenna beam width, side-lobe magnitudes, off-axis radiation, directivity patterns and polarization discrimination are of a, great significance for frequency co-ordination purposes. Impedance match [usually expressed’ as VSWR although return loss is a much more useful parameter] across the band to’ be used is of great importance in situations where echo distortion is significant. Consequently it is no longer sufficient merely to select an antenna for optimum gain efficiency. Lastly, antennas must be moderate in cost, easy to install and strong enough to give service in rugged environments for over twenty years.
2.1 PARABOLIC ANTENNA
The parabolic antenna is used almost universally in point-to-point systems. The parabolic antenna utilizes a reflector consisting of a paraboloid of revolution and primary radiator at the focal point [Fig.1]. The reflector converts the spherical wave radiating from the focus to the planar wave across the face of the paraboloid to concentrate the energy in a beam much like a searchlight beam as discussed below.
The parabola is a plane curve, defined as the locus of a point which moves so that its distance from another point (called the focus) plus its distance from a straight line (directrix) is constant. These geometric properties yield an excellent microwave or light reflector, as will be seen,
2.1.1 Geometry of the parabola
Figure 1 shows a parabola CAD whose focus is at F and whose axis is AB. It follows from the definition of the parabola that
FP+ PP1 = FQ.+ QQ’ – FR + RR’ = K
Where k= a constant, which may be changed if-a different shape of parabola is required
AF= focal length of the parabola.
Consider a source of radiation placed at the focus. All waves coming from the source and reflected by the parabola will have traveled the same distance by the time they reach the directrix, no matter from what point on the parabola they are reflected. All such waves will thus be in phase. As a result, radiation is very strong and concentrated along the AB axis, but cancellation will take place in any other direction, because of path-length differences. Thus the parabola lead to the production of-concentrated beams .
A practical reflector employing the properties of the parabola will be a three
dimensional surface, obtained by revolving the parabola about the axis AB.
The resulting geometric surface is the paraboloid, often called a parabolic
reflector or microwave dish. When it is used for reception, exactly the same behavior is manifested, so that this is also a high- gain receiving directional antenna reflector. Such behavior is, of course, predicted by the principle of reciprocity, which states that the properties of an antenna are independent of wheather it is used for transmission or reception.
The reflector is directional for reception i.e., rays normal to the directrix, are
brought together at the focus. On the other hand, rays from any other
direction are cancelled at that point, again owing to path-length differences.
The reflector provides a high gain because, like the mirror of a reflecting
telescope, it collects radiation from a large area and concentrates it all at the local point.
2.2.2 Feed Mechanism
As already discussed, the primary antenna is placed at the focus of the paraboloid for best results in transmission or reception. However, the direct radiation from the feed, which is not reflected by the paraboloid, tends to spread out in all directions and hence partially spoils the directivity. Several methods are used to prevent this, one of them being the provision of a small spherical reflector to redirect all such radiation back to the paraboloid.
Yet another way of dealing with the problem, a horn antenna pointing to the main reflector. It has a midly directional pattern in the direction in which its mouth points; thus direct radiation from the feed antenna is once again avoided. lt should be mentioned at this point that although the feed antenna and its reflector obstruct a certain amount of reflection from the paraboloid when they are placed at its focus, the obstruction is slight indeed. For example, if a 3Ocm diameter reflector is placed at the centre of a 3-m dish, simple arithmetic shows that the area obstructed is only 1 percent of the total. Similar reasoning is applied to the horn primary, which obstructs an equally small proportion of the total area.
2.2.3 There are different types of feed designs for various frequency bands and different system applications. Feeds for the 890 to 2,300 MHz bands are generally coaxial dipoles, slot excited circular wave guide reverse horns or printed circuit arrays.
The Cassegrain feed is used when it is desired to place the primary antenna at a convenient position and to shorten the length of the transmission line or wave guide connecting the receiver (or transmitter) to the primary. This requirement in the line or waveguid may not be tolerated, specially over lengths which may exceed 30 m in large antennas. Another solution to the problem is to place the active part of the transmitter or receiver at the focus. With transmitters this can almost never be done because of their size, and it may also be difficult to place the RF amplifier of the receiver there. This is either because of its size or because of the need for cooling apparatus for very low-noise applications in which case the RF amplifier may be small enough, but the ancillary equipment is not. In any case, such placement of the RF amplifier causes servicing and replacement difficulties, and the Cassegrain feed is often the best solution.