Parabolic antennas

The radiotelescopes at IRAM (Institut de Radioastronomie Millimdtrique), a French-Spanish-German consortium, can lay claim to a good few discoveries of new cosmic molecules over the past ten years. The IRAM interferometer on the Plateau de Bures in France combines signals gathered by five parabolic antennas. It has an angular resolution of 0.5 arcsec at 1.3 mm. 

Fig. 2.16 A parabolic antenna has a tapered aperture distribution and will therefore scatter more in the forward scattering direction than it absorbs. Thus, it is not an MSA even in the relaxed sense.

We shall return in much greater detail to feeding parabolic antennas in Chapter 8 and advise alternate ways to alleviate scattering problems. In Chapter 7 we present invisible antennas with omnidirectional radiation patterns. 

Furthermore, in Chapter 8 we will show that in order to design a parabolic antenna with a low RCS, it is crucial to use a feed with low RCS. Such a feed is readily obtained by a relatively simple modification of the omnidirectional antenna presented in this chapter. Note that no power is lost in the omnidirectional antenna or in the low RCS feed when transmitting. See also Section 7.6. 

We should point out that the RCS level of a parabolic antenna never can attain the low levels possible with an array. They are, however, popular in some camps and will therefore be discussed in Chapter 8. 

The backscatter of a parabolic antenna consists essentially of two components ... 

When discussing scattering from a parabolic antenna the question is often asked, Did you incorporate the edge currents Our answer is, Certainly ... 

Passivation of the surface of n-GaAs is possible with thin films of plasma polymerized thiophene [255]. A composition containing an electrically nonconductive polymer matrix and 3-octylPT is claimed as a material for parabolic antennas, reflectors for radar, heating systems, photoelectric devices, and electric circuits and apparatus [256]. PTs are used for manufacturing a nonlinear two-terminal device this device is not asymmetrical, gives stable electrical characteristics, and is useful as a display device [257, 258]. PT and 3-octylPT as electrically conductive polymers are claimed to be useful for liquid crystalline display devices [259-261]. PTs are also used for the production of color filters for liquid crystal displays [262]. 

The first successful measurements of thermal emission from the planets were made in 1956 at the Naval Research Laboratory in Washington, D. C. C. H. Mayer, T. R McCullough, and R. M. Sloanaker scanned Venus, Mars, and Jupiter with a 15-m parabolic antenna equipped with a new 3-cm-wavelength radio receiver. They detected weak thermal emission from these three planets when each was observed at its closest distance to the Earth. 

Microwave signals are reflected by flat surfaces. Plane reflectors may be used to bounce a microwave signal around a hill or a building that would otherwise block its path. Flat reflectors are often placed at the top of tall microwave-relay towers. Parabolic dish antennas at ground level face skyward, directed toward the reflectors that bounce their signals to the horizon. 

Parabolic reflectors are used in other devices as well. Radar antennas, the dishes used to pick up satellite television signals, and the reflectors used to concentrate sound from distant sources are all parabolic. 

C. Antenna. The antenna is used to make a transition from a guided wave (from the transmission line) to a radiated electromagnetic wave. The design of the antenna is influenced by many factors such as size, frequency, and electrical impedance. Antennas are normally of two types - omnidirectional and directional. The omnidirectional antennas are element type antennas such as monopoles or dipoles. The directional are horn-type antennas, parabolic dish type antennas such as a satellite communications antenna (SATCOM), or a phased-array antenna which can emit many beams at once. The characteristics of the antenna are a very important aspect of hazard evaluation. 

A nonwoven mat of smooth flexible aluminum coated oriented glass fibers, Metafil, has been developed by Tracor Aerospace and BGF Industries, USA, for electrically or thermally conductive applications, such as parabolic dish antennae. It is compatible with most resin systems and processing is said to be similar to uncoated glass mat, but cost is lower than other conductive materials. 

A large flat aperture is most often associated with a narrow pencil beam. However, we shall in Chapter 7 consider antennas with omnidirectional pattern and low visibility in the backward direction over a broad band. Further discussed in Chapter 8 is how to design a feed for a parabolic cylinder that will produce an RCS about 6dB lower than with no feed at all. The design of such a feed is closely related to the omnidirectional design. We emphasize, however, that parabolic systems never can attain the inherently low RCS level encountered for the flat aperture over a broad frequency band. 

The reaction to this suggestion is typically something like Well, we have measured dipoles, horns, parabolic dishes, flat spirals as well as helical antennas with groundplane, and what not, and we have never come across an antenna with no residual scattering. In fact we are not even sure whether an antenna without residual scattering violates certain fundamental rules ... 

For an in-depth discussion of the scattering from horns when used as a feed antenna in a parabolic cylinder, see Chapter 8. There you will also find ways demonstrating how to alleviate some of the problems. 

Parabolic reflector antennas cannot compete with arrays in respect to low RCS. However, by using a newly designed feed, it can be reduced substantially. Since this type of antenna still is popular in some camps, it is discussed in Chapter 8. 

Parabolic reflector antennas are usually illuminated by a flared-horn antenna with a flare angle of less than 18°. A rectangular horn with a flare angle less than 18° has approximately the same aperture field as the dominant-mode rectangular waveguide feeding the hom. 

High-power direct broadcast satellites (DBS) or direct-to-home (DTH) satellites operate atKu-band. In the U.S., satellites operating in the broadcast sateUite service (BSS) with downlink frequencies of 12.2-12.7 GHz, deliver TV directly to home receivers having parabolic dish antennas as small as 46 cm (18 in) in diameter. DBS with digital modulation and compressed video is providing more than 150 National Television Systems Committee (NTSC) TV channels from a single orbital location having an allocation of 32 transponder channels, each with 24-MHz bandwidth. 

Reflector antennas typically employ a parabolic structure that is highly reflective at the carrier frequency of the radar so as to focus energy at either a feed or subreflector. The commonly used Cassegrain reflector antenna employs a hyperboloidal subreflector to focus energy on a feed located at the base of the parabohc reflector so as to minimize antenna aperture blockage. The feed is generally some manner of horn antenna designed to produce a specified illumination function across the reflector. 

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