Satellite Receiving Antennas By Mark Long First published in Middle East Satellite Today magazine. Copyright 1998 Mark Long.	Search this site powered by FreeFind
	THE DIGITAL SATELLITE TV HANDBOOK is an entire course in digital satellite TV technology, complete with self-grading exams and supporting IBM-PC compatible software on CD-ROM.

All satellite dishes incorporate a parabolic curve into the design of their bowl-shaped reflectors. The parabolic curve has the property of reflecting all incident rays arriving along the reflector's axis of symmetry to a common focus located to the front and center. The parabolic antenna's ability to amplify signals is primarily governed by the accuracy of this parabolic curve. Poor antenna performance can result from inaccuracies in the dies used to manufacture the reflector surfaces. More often, however, low antenna efficiencies are caused by the installer's failure to grasp the importance of using good antenna assembly techniques.

Antenna Materials & Construction
The reflector must be constructed out of metal in order to reflect the incoming microwave signals. Some antenna reflectors appear to be manufactured out of plastic or Fiberglas; however, these dishes contain an embedded metal mesh material that reflects the incoming satellite signals.
Solid one-piece antennas are most often the best performers because there are no assembly errors and they will maintain their exact shape over the lifetime of the system. Solid petalized antennas constructed out of four or more segments are generally the next best performers. Potential assembly errors are limited to variations along the seams between petals. The installer can easily visually inspect these seams during assembly to ensure that there are no variations in the surface curve from one petal to the next. One-piece and petalized antennas are also available in a perforated form. The diameter of the perforation holes is a function of signal wavelength: too small to pass or resonate with the wavelength of the incoming microwave signals but large enough to pass light in order to minimize the antenna's environmental impact.
Mesh antennas are the most susceptible to construction errors. The two-part construction process consists of the building of a support frame and a series of flexible mesh panels. The installer attaches the mesh to the frame using a series of metal clips or sheet metal screws. Mesh antennas also are highly susceptible to environmental effects. Heavy wind storms, for example, can loosen the clips holding the mesh to the frame and distort the curve from its original shape or even blow out one or more of the mesh panels.
The installer should examine the antenna at intervals during the installation process. Close attention should be paid to how the various petals fit together. The reflector surface should appear to be continuous, with minimal variation from petal to petal and few noticeable bumps or waves along the surface of mesh antennas.
Antenna symmetry is also very important. Improper construction of a petalized antenna can warp the reflector curvature. The installer should sight along a side view of the reflector from the near to far edge of the antenna rim. If the near and far rims of the dish do not line up in parallel with each other then the installer will need to loosen the bolts holding the petals together and re tighten them in such as way that the reflector conforms to the manufacturer's intended shape. Another way to detect warp is to run strings across the antenna's rim. All strings should lightly touch over the center of the dish. Any gaps between strings indicate a flaw in the reflector surface.

Prime Focus Antennas
The basic design principle of the parabolic curve can be incorporated into antenna designs in a variety of ways. Dishes with a focal point directly at the front and center of the reflector are called prime focus antennas.
Prime focus antennas are easy to construct and point toward the desired satellite. There are two main design disadvantages, however: the feedhorn and feed support structure block part of the reflector surface and the feedhorn must look back at the dish at such an angle that it can also intercept noise from the "hot" earth located directly behind the reflector. The feedhorn's illumination of the antenna must be attenuated or tapered to minimize noise contribution from the perimeter of the dish. This design necessity acts to reduce the antenna's efficiency.
Prime focus antennas use two different types of feedhorn support bracket. A three or four-legged support provides a rigid support structure for the feedhorn and LNB over the center of the dish and at the distance specified by the manufacturer. The main disadvantage of this structural approach is that it may be difficult to make minor variations in the focal length, that is the distance from reflector center to the lip of the feed opening. The buttonhook structural design uses a single support member to position the LNB and feedhorn. This tubular leg can usually be slid in and out of a clamp or bracket at the center of the dish, allowing the installer to fine-tune the focal length. However, the buttonhook support may not always position the feed at the precise center of the dish, especially when the feedhorn is weighted down by multiple LNBs.
Motorized dishes may experience feedhorn movement when the antenna is moved from one satellite to the next; heavy winds can also temporarily move the feedhorn away from the antenna's focus. Guy wire kits are available which the installer can use to provide additional structural rigidity to the buttonhook support if required for a given installation.

Offset-fed Antennas
The dish design of choice for most digital DTH systems is called an offset-fed antenna. Here the manufacturer uses a smaller subsection of the same parabolic curve used to produce prime focus antennas, but with a major axis in the north/south direction, and a smaller minor axis in the east/west direction.
With the offset-fed design, the feedhorn is no longer positioned at the front and center of the reflector but rather offset to the bottom of the dish. However, the feed would be centrally located if we extended the parabolic curve of the offset fed dish to the full length of a prime focus parabola.
The offset fed antenna design offers several distinct advantages over its prime focus counterparts. There is no feedhorn blockage, an important consideration when the antenna aperture is less than one meter in diameter. Moreover, the offset angle at which the feedhorn tilts up toward the reflector is such that if the feed looks over the antenna's rim it will see the cold sky rather than the hot earth. Due to these advantages, the offset-fed antenna can achieve higher efficiency levels than prime focus antennas can generally attain.
The low inclination angles required by offset antennas also may be beneficial in certain climate zones. In tropical or semi-tropical environments, rain will not collect inside the reflector. In cold weather climates, snow will slide off of the antenna surface rather than accumulating inside the reflector.

Cassegrain Antennas
The cassegrain antenna is most often used for dishes that exceed five meters in diameter. Its use is primarily restricted to uplink earth stations and cable TV head ends. The cassegrain design incorporates a small sub reflector located at the front and center of the dish. The sub reflector deflects the microwaves back toward the center of the reflector, where the feedhorn is actually mounted.
Like the prime focus dish, the cassegrain antenna's view of the satellite is partially obscured, in this case by the sub reflector. However, when the diameter of the dish exceeds 5m, the percentage of blockage is actually quite small.
This type of antenna obtains higher efficiencies because the feedhorn looks up at the cold sky and the required illumination taper is reduced. The precise manufacturing tolerances required to implement this dual reflector approach, however, increases the manufacturing cost and adds complexity to the installation process.

Spherical Antennas
The spherical antenna design creates multiple focal points located to the front and center of the reflector, one for each available satellite. The curvature of the reflector is such that if extended it outward far enough along both axes it would become a sphere.
Spherical antennas are primarily used for commercial SMATV and cable installations where the customer wishes to simultaneously receive multiple satellites with a single dish. These satellites must be within +/- 20 degrees of the reflector's axis of symmetry.

Planar Arrays
Some digital DTH systems in Japan and elsewhere have elected to use an alternate antenna design called the planar array. These flat antennas do not rely on the reflective principles used by all parabolic dishes. Therefore no feedhorn is required.
Instead a grid of tiny elements is embedded into the antenna's surface. These elements have a size and shape which causes them to resonate with the incoming microwave signals. A spider's web of feed lines is used to interconnect all the resonant elements in such a way that their signal contributions are all combined in phase at a single terminal located at the center of the array which connects directly to the LNB.
Planar arrays are relatively unobtrusive: there is no feedhorn and the LNB is located to the rear of the antenna out of sight. Since these antennas are most always dedicated to the reception of a single satellite or constellation of collocated satellites, they can be mounted in a fixed position on an outer wall or rooftop.
One main disadvantage of the planar array is its limited frequency bandwidth which is about 500 MHz. Parabolic antennas, however, have a broad bandwidth; a single dish, for example, can be used to receive S, C, and Ku-band satellite signals. Another disadvantage of the planar array is the high construction cost: more than four times the cost of manufacturing a feedhorn and parabolic reflector with equivalent signal amplification characteristics.

Antenna Gain and G/T
The gain of a satellite antenna is the measurement of its ability to amplify the incoming microwave signals. Gain, which is expressed in decibels, or dB, is primarily a function of antenna capture area or aperture: the larger the antenna aperture, the higher the antenna gain. Gain also is directly related to antenna beam width: the narrow corridor or "boresight" along which the antenna looks up at the sky.
The antenna's efficiency rating is the percentage of signal captured by the parabolic reflector that actually is received by the feedhorn. As we have previously seen, the feed-horn's illumination of the outer portion of the dish is attenuated or tapered, which leads us to conclude that antenna gain is not as important a factor as it might first appear to be.
The ultimate figure of merit for all receiving antennas is the G/T (pronounced "G over T"); that is, the gain of the antenna (in dB) minus the noise temperature of the receiving system (in dB). A typical C-band DTH system will have a G/T of around 20 dB/K, while most Ku-band digital DTH systems have a G/T of 12.7 dB/K. The more powerful the satellite signal, the lower the G/T value that will be needed at the receiving system down on the ground.
The noise value (T) primarily comes from two sources. The antenna noise is a function of the amount of noise that the feedhorn sees as it looks over the antenna rim towards the hot earth (which has a noise temperature of 290 K). Antenna noise generally ranges between 30 and 50 K.
The noise contribution of the LNB's internal circuitry is the other major source of concern. C-band LNB performance now ranges as low as 20 K. If we add an antenna/feed noise of 40 K to LNB noise of 35 K = 75 K. Ten times the Logarithm of 75 K equals a (T) of 18.8 dB. A typical 1.8m diameter C-band antenna will produce a gain of 38 dB. Therefore the G/T of the system described above would be (G) 38 dB minus (T) 18.8 equals 19.2 dB/K.

Deep Versus Shallow Dishes
The parameters of the parabolic curve that the antenna designer selects can be adjusted to create a variety of focal length to antenna diameter (f/D) values. Antennas which have an f/D greater than .38 are said to be shallow, whereas dishes will an f/D less than .33 are said to be deep.
Although the long focal length afforded by the shallow dish design increases the feedhorn's ability to illuminate the entire reflector surface, we have already seen that there are distinct disadvantages to doing this. Moreover, antenna noise increases as antenna elevation increases. Shallow dishes are more susceptible to intercepting earth noise when pointing at low elevation angles. Finally, the shallow dish is more susceptible to picking up terrestrial interference from microwave relay stations.
The deep dish trades off gain in order to lower antenna noise performance. The deep-dish design is an attractive alternative for locations that potentially may experience terrestrial interference problems or at installations which require low antenna elevation angles. The deep-dish design positions the feedhorn relatively close to the rim of the reflector. Therefore the deep dish has a greater ability to shield the feedhorn from potential TI sources. However, the feedhorn is so close to the reflector that it cannot illuminate the entire surface.

Antenna Side Lobe Rejection
The explosive worldwide growth in satellite telecommunications is leading to closer spacing between satellites in geostationary orbit. What's more, the very latest satellites are transmitting higher-powered signals than ever before. Both of these developments act to increase the potential for interference from nearby or adjacent satellites.
The perfect parabolic antenna would only receive signals from the satellite at which it was pointed while rejecting all signals coming from other directions. In the real world, however, each antenna design will produce a main beam along the axis of symmetry as well as other beams of lower intensity that look out at adjacent angles. These beams of lower intensity are called "sidelobes".
The goal of all satellite TV antenna manufacturers is to reduce the gain of these sidelobes to levels that are at a minimum of 15 dB below the gain of the main lobe. This level of sidelobe attenuation is usually sufficient for preventing adjacent satellites from causing interference to reception of the desired satellite. The location off axis of each sidelobe is a function of antenna diameter and signal frequency. The installer can therefore select an antenna that is large enough to put the adjacent satellites in the first "null" of the antenna receiving pattern or use an antenna which has a sidelobe that is at least -15 dB down from the main beam.

Antenna Mounts
The steel mount and the bearing that supports the antenna reflector must be able to maintain a precise position once boresighted onto the desired satellite. A misalignment of the mount of as little as two inches can make the difference between perfect TV reception and no reception at all. The installer should check the rigidity of the mount by grasping the rim and gently shaking it to see if there is any "play" as wind or rainstorms may push the dish off of boresight, causing erratic reception.
All mounts incorporate adjustments that permit the installer to point the dish at the desired satellite. Digital DTH antennas commonly have what is known as a fixed mount that is adjusted once at the time of installation and then left alone thereafter. The fixed mount has separate settings for the required azimuth (compass bearing corrected to true north) and elevation (the angle at which the reflector tilts up at the sky).
Motorized DTH antennas must rotate in an arc that mimics the curvature of the geostationary arc where all the satellites are located. A modified form of the polar mount used by astronomers on their telescopes is used to achieve this effect. The axis of the modified polar mount must be aligned with the earth's axis of rotation at an angle that corresponds to the latitude of the receiving site. Precise tracking of the geostationary arc also requires a declination adjustment that tilts the antenna downward slightly in the direction of the geostationary arc. This modification to the polar mount is required because of the relative closeness of the satellites in comparison to the stars that astronomers view with polar mounted telescopes.