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9.3.1
Antennas for Earth Stations
A considerable amount of technical information on the properties and performance
of antennas was given in Chapters 4 and 7. In this section, we focus on the physical
characteristics of antennas specifically used in Earth stations: the prime focus fed
parabola and the cassegrain, shown in Figure 9.9(a) and Figure 9.9(b), respectively.
An alternate feed system called the splash plate is shown in Figure 9.9(c). As
discussed in Section 9.3.1.3, a splash plate is a flat reflecting plate introduced into
the antenna, usually to allow a different mounting geometry.
9.3.1.1
Prime Focus Fed Parabola
Antennas with a diameter less than 4.5m and particularly those used in RO service
may employ the prime focus fed configuration shown in Figure 9.9(a). Within this
Figure 9.9
Earth station antenna configurations commonly used for receive and two-way communi-
cations links. (a) Prime-focus fed reflector antenna, (b) Cassegrain feed system, and
(c) splash plate feed system.
9.3
Radio Frequency Equipment
301
class is the offset fed parabaloid, used extensively for antennas less than about 2m
in diameter. The cost usually is the lowest because of its simplicity and ease of
installation. To minimize RF loss and noise, a compact LNB package is located
directly on the waveguide port of the feed to minimize loss between the front-end
amplifier and the feed. The RF cable to connect between LNB and indoor receiving
equipment is attached to one of the struts. Unless the electronics can be fitted
at the feed, the prime focus configuration is not attractive for transmit/receive
applications.
In a variant of the prime focus approach, the feedhorn is at the end of a hook-
shaped piece of waveguide that extends from the vertex of the parabolic reflector.
The LNB is attached to the end of the waveguide behind the reflector, and the
assembly is supported at the feedhorn with either struts or guy wires. That has the
obvious advantage of allowing the electronics to be accessed more easily and
provides placement out of the way of the main beam. The mechanical rigidity and
thermal stability of the bent waveguide system are limited, but small-diameter
antennas used for TVRO purposes can get by with that approach.
9.3.1.2
Cassegrain Optics
Many of the shortcomings of the prime focus fed antenna are overcome with the
cassegrain system, shown in Figure 9.9(B). Using the folded optics principle of the
reflector telescope, the focus of the parabola is directed back by a hyperbolic
subreflector to a point at the vertex of the center of the main reflector. Proper
illumination is still provided, but the feed and the electronics can be joined at an
accessible point in the antenna structure. In large antennas, a hub enclosure is
provided at the back for placement of essentially the entire RF terminal in close
proximity to the feed. The cassegrain system eliminates the cable or waveguide
runs between RF electronics and feed.
9.3.1.3
Splash Plate
The splash plate feed system, shown in Figure 9.9(c), relies on a subreflecting
surface to direct energy back toward the reflector. In this case, the surface is a flat
disk that is supported by a dielectric material like styrene plastic positioned in
front of a feed tube. There is no need for a feed horn or feed support because the
assembly is supported by the feed tube itself and the RF electronics can be located
behind the reflector. This arrangement benefits from its mechanical simplicity and
strength, as well as its avoidance of blockage from support struts.
9.3.1.4
Offset Fed Geometry
The offset fed geometry, which was reviewed in Chapter 7 for spacecraft antennas,
also has been applied to Earth stations. Shown in Figure 9.10 is the common home
Ku-band TVRO used in high power DBS. A main benefit of the offset approach
is the elimination of main beam blockage by the feed and struts, which tends to
improve antenna efficiency and reduce sidelobe levels. It also permits rain and
snow to fall off the surface, because the reflector is at a more acute angle relative
to the ground. In a particular design, there may be mechanical or cost advantages
302
Earth Stations and Network Technology
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