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.
An obvious difficulty results from the use of a secondary reflector namely, the obstruction of some of the radiations from the main reflectors, because the dimensions of the hyperboloid are determined by its distances from the horn primary feed and the mouth diameter of the horn itself, which in turn is governed by frequency used: One of the ways to overcome this obstruction is by means of a large primary reflector together with feed placed as close to the sub-reflector as possible.
3.0 ELECTRICAL CHARACTERISTICS OF PARABOLIC ANTENNA
- Antenna Gain
Microwave antenna gain is stated in dBi, indicating decibels relative to the
gain of an isotropic antenna. This is a hypothetical ‘ideal antenna’ or ‘point
source’ which radiates equally in all directions. In some literature, ‘dBi1 is
shortened to just ‘dB’.
The gain of a parabolic antenna depends upon its size, frequency and
illumination. Maximum gain would occur if the .illumination was uniform in
phase and equal in amplitude across the aperture of the parabola. In this
case, the gain would merely be the ratio of the aperture of the parabola to the area of the hypothetical isotropic antenna.
The gain, G, is given by the equation :
Conventional feeds provide an illumination of approximately -10 dB at the
edge of the parabola from that at the centre, which results in an antenna
efficiency of 58 to 63 percent for production antennas. Taking other factors
into account, most manufacturers guarantee antenna efficiencies of 55 percent. Typical gains of microwave parabolic antennas are given in Table – 1.
TYPICAL ANTENNA GAINS AND BEAMWIDTH FOR VARIOUS SIZES AND FREQUENCIES
|2 GHz||6 GHz||11 GHz|
|Beamwidth degrees||Gain dBi||Beamwidth degrees||Gain dBi||Beamwidth degrees|
3.2 The half power beamwidth is the beamwidth, in degrees, at the -3 dB power point. The beamwidth, in degrees for a conventional parabolic antenna is given by the equation :
= 70 lemda/D
Where Theta = beamwidth in degrees
D = the parabolic antenna diameter
lemda = wave length in the same units.
The value of Theta is approximately 1.1 degree at 6 GHz and 3.4 degree at 2 GHz for a 3.0 m diameter antenna. The main lobe drops off to a null at 1.1 degree beamwidth off axis. This may mean that signal could drop as much as 40 dB if a 3.0 m antenna at 6 GHz is moved 1.1 degree off axis. One can appreciate the need for sturdy mounts and careful tower design.
3.3 VOLTAGE STANDING WAVE RATIO (VSWR)
The antenna VSWR is the ratio of the amplitude of the voltage standing wave at the maximum to the amplitude at the minimum. VSWR is always equal to or greater than 1.0. A 1.000 VSWR indicates that an antenna is perfectly matched to a transmission line.
Since the feedhorn, located at the focus, has some physical size, it will catch some reflected energy from the parabola causing a mismatch [Fig. 2]. This is termed dish effect, and the VSWR contribution may be 1.02 or more, depending upon size and frequency. Placing a raised circular plate, of the proper thickness, called a vertex plate, at the centre of the reflector can cancel out these dish reflections. Vertex plates are usually installed along with the feed when the antenna is assembled.
The VSWR over the operating frequency bands for standard microwave
antennas will be approximately 1.10. Through extreme care in manufacture and additional tuning and matching, low VSWR antennas can achieve a VSWR of 1.04 to 1.06.
For some system calculations VSWR in terms of the return loss in decibels is useful. In this case,
Return Loss [in db] = 20 log VSWR +1 / VSWR –1
|VSWR||R. L.||VSWR||R. L.||VSWR||R. L.|
Low VSWR antennas are necessary to ensure minimum echo distortion in long haul microwave systems. A high antenna VSWR will cause some of the transmitted signal to be reflected back down the transmission line. This reflected signal can be again reflected at the RF equipment and sent towards the antenna. This delayed signal causes unwanted noise which can be compounded in a long microwave system. A low VSWR antenna minimizes the amount-of reflected signal.
Standard Parabolic Antennas- make a reliable economic choice for the majority of thin route systems. These antennas have a VSWR of about 1.10:1 which is satisfactory for low to medium channel densities and moderate length systems.
Low VSWR Parabolic Antennas- use the same reflectors but offer feed design with a VSWR of 1.04 to 1.06 essential for medium to high channel densities and long multi-hop systems.
3.4 RADIATION PATTERNS
The radiation patterns of antennas have become more important with the increase in microwave congestion and the need for careful coordination to prevent interference between systems. In planning a route, a system engineer will evaluate the potential interference from microwave systems operating on the same frequencies up to 160 to 320 km. away. If the carrier to interference signal ratio, C/l, is 70 dB or more, interfering noise is negligible. However if the C/l ratio is low, then preventive measures must be taken such as using a high performance or ultra high performance antenna.
Interference coordination analysis normally uses envelopes of the antenna radiation called radiation pattern envelopes [RPE’s] Radiation pattern envelopes are prepared using the 360 degree azimuth radiation patterns of the antenna at representative frequencies in the band, usually low, middle and high [Fig. 4]. Radiation pattern envelopes are smoothed by drawing a line over all the peaks of all the loves to provide a ‘worst case’ envelope. For symmetrical antennas the envelope is folded to 1800over so that the 180 degree to 360 degree half is super – imposed on the 0 degree half [Fig.5.].
Some antennas, like ultra high performance, are designed to have appreciably different patterns left and right of boresight. In these cases, a full 360-degree radiation pattern envelope may be shown [Fig.6]. The feed is these antennas may by rotated 180 degree to select the preferred side it cases where the asymmetry can be used to advantage by the system engineer.
3.5 The front-to-back ratio of an antenna is defined as the ratio of the power received from [or transmitted to] the main beam of the antenna to the power received from [or transmitted to] the back side. Front-to-back ratio for a standard parabolic antenna is defined at 180 +5 degrees. For high performance or ultra high performance antennas, this is defined as 180 + 80 degrees, in order to operate satisfactorily on a two frequency plan, using the same transmitting frequencies in two directions at a repeater, it is necessary to have high front- to back ratios.
While the antenna radiation pattern envelopes whole the expected radiation performance of the particular antenna, the actual pattern achieved in the field is dependent upon site conditions and foreground reflections. Careful site planning is therefore essential.
4,0 Horn Reflector Antennas
The horn reflector [cornucopia] antenna has a section of a very large parabola mounted at such an angle that the energy from the feed horn is simultaneously focussed and reflected at right angles. A horn antenna having the equivalent gain of a 3 meter parabolic antenna is over 6 meters in height and causes a much greater load from wind on the tower, However, it has a much higher front- to- back ratio than the standard parabolic antenna, but has about the same front -to-back ratio as higher performance antenna of the same gain'(Fig.8).
This type of antenna has good VSWR characteristics and with suitable coupling network’s [which are quite complex and very expensive], can be used for multi-band operation on both polarizations. However, there are moding problems, particularly at the higher frequencies which, if uncorrected, can cause severe distortions, Correcting of these moding Problems is a very difficult task.
Disadvantages are that this antenna is very big, heavy and complex to mount. The cost of one antenna with suitable coupling networks to provide dual polarization at 4 GHz and 6 GHz band far exceeds the cost of two separate parabolic high performance antennas, providing equivalent or better electrical performance.
6.0 WAVEGUIDES AND TRANSMISSION LINES
Wave guide and transmission line is important, not only for its loss characteristics, which enter into the path loss calculation, but also for the degree of impedance matching attainable, because of the effect on echo distortion noise. The later becomes important with high-density systems having long waveguide runs.
6.1 Coaxial Transmission Lines
In bands up to 2 GHz, coaxial cable is usually used, and except for very short runs, it is usually of the air dielectric type. Typical sizes are: 2.2 cm. diameter. Andrew type HJ 5-50, with attenuation of about 6dB per 100 meters at 2 GHz, and 4.1 cm. diameter, Andrew type HJ 7-50 with an attenuation of about 3 dB per 100 meters. These cables are normally ordered in the exact length required with factory installed and sealed terminal connectors. Both these types of cables are flexible enough to provide direct connection at the rear of the antenna provided that the mount allows direct access in horizontable plane. If the vertical run of the coaxial cable is down the side of the tower away from the antenna, this can be easily accomplished. In any coaxial cable system where VSWR is important, the number of connections should be kept to a minimum.
However, at the equipment end, it may be necessary to reduce the larger size cable if used, to the smaller size cable with a suitable runs, with suitable transitions for flexibility in connection to the radio equipment.
In some rare circumstances, where high power are necessary, it may be
necessary to use elliptical flexible waveguides with vertical runs, with suitable transitions to coaxial cable at the top of the run and at the bottom of the run.
6.2 Wave guides
Bands higher than 2 GHz require the use of waveguides almost exclusively and one of three basic types may be used rigid rectangular, rigid circular, and flexible elliptical. The latter is of continuous construction, having the advantages of minimizing the number of flange connection usually of two. one at the antenna end, one at the equipment end. If rigid rectangular or circular waveguides are used, it is necessary to use short section” of flexible waveguide for connection to the antennas and to the equipments. In some cases, it may be necessary to use rigid rectangular waveguide inside the equipment building because of restrictions of space. However, in all cases it is desirable to keep the number of flanges and length of flexible sections as small as possible since each flange and each flexible section, besides having higher, losses, have poor VSWR than the main waveguide types.
6 .2.1 Rectangular Guide
Rigid rectangular waveguide is the most commonly used, with oxygen-free,
high conductivity copper (OFHC), the recommended material. The types and approximate characteristics are as follows:
4 GHz band : WR 229 is standard for most installations. It has a loss of approximately 2.79 dB per 100 meters.
6 GHz band : WR 137 is normally used. It has a loss of approximately 6.6 dB per 100 meters. In cases where, due to high towers, a reduced transmission loss is required, transitions can be supplied for use with WR 159, which has a loss of about 4.6 dB per 100 meters.
7-8 GHz : WR 112 is normally used. Attenuation is approximately 8.8 dB per 100 meters.
11 GHz : WR 90 is normally used. Attenuation is approximately 11.5 dB per 100 meters.
12-13 Ghz :WR 75 is normally used. Attenuation is approximately 14.7 dB per 100 meters.
For the most critical applications, where extremely low VSWR is required to meet stringent noise performance specifications, special precision waveguide, manufactured to very tight tolerance, is recommended.
6.2.2 Circular Guide
Circular waveguide has the lowest loss of all, and in addition, it can support two orthogonal polarizations within the single guide. It is also capable of carrying more than one frequency band in the same guide. For example, WC 281 circular, guide is normally used with horn reflector antennas to provide two polarizations at 6 GHz. But circular guide has certain disadvantages. It is practical only for straight runs, requires rather complicated and extremely critical networks to make the transitions from rectangular to circular and can have significant moding problems, when the guide is large enough to support more than one mode for the frequency range in use. Consequently, though circular waveguide is available in several different sizes, and its low losses to make it attractive, it is recommended that it be used with considerable caution.
6.2.3 Elliptical Guide
Semi-flexible elliptical waveguide is available in sizes comparable to most of the standard rectangular guides, with attenuations differing very little from the rectangular equivalents. The distinctive features of elliptical guide is that it can be provided and installed as a single continuous run, with no
intermediate flanges. When carefully transported and installed it can provide good VSWR performance but relatively small deformations can introduce enough impedance mismatch to produce severe echo distortion noise. However, usually the effect of small deformations can be ‘tuned’ out.
The most commonly used types and their approximate characteristics are as follows:
|4||GHz band :||EW – 37||Approximately||2.8||dB per 100 meters|
|6||GHz band :||EW – 56||Approximately||5.7||dB per 100 meters|
|7-8||GHz band :||EW – 71||Approximately||8.2||dB per 100 meters|
|11||GHz band :||EW – 107||Approximately||12.1||dB per 100 meters|
|12-13||GHz band :||EW – 122||Approximately||14.7||dB per 100 meters|
All attenuation figures given at mid band.
In all types of waveguide systems it is desirable to keep the number of bends, twists, and flexible sections to a minimum. It is also vitally important to use great care in installation, since even very slight misalignments, dents, or introduction of foreign material into the guides can create severe discontinuities. As manufacturing techniques improve, and installers become more familiar with elliptical guide, the return losses have been significantly reduced to the point that waveguide systems utilizing premium elliptical waveguides provide minimum noise contributions to the overall transmission system.