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ABSTRACT

In this paper, there is an introduction in chapter one, there are motivation, modes of operation, analysis of helix and modified helices in chapter two, there are helical antenna wave dispersion and radiation resistance in chapter three about helical antenna. We can see an example for helical antenna in chapter four. In chapter five, there are five topics. First one is feed impedance, second is polarization, third is summary, fourth is conclusions and the last one is references.

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CHAPTER ONEINTRODUCTIONA helical antenna is an antenna consisting of a conducting wire wound in the form of a helix. In most cases, helical antennas are mounted over a ground plane. Helical antennas can operate in one of two principal modes: normal (broadside) mode or axial (or end-fire) mode.

A helical antenna is a specialized antenna that emits and responds to electromagnetic fields with rotating (circular) polarization. These antennas are commonly used at earth-based stations in satellite communications systems. This type of antenna is designed for use with an unbalanced feed line such as coaxial cable. The center conductor of the cable is connected to the helical element, and the shield of the cable is connected to the reflector. To the casual observer, a helical antenna appears as one or more "springs" or helixes mounted against a flat reflecting screen. The length of the helical element is one wavelength or greater. The reflector is a circular or square metal mesh or sheet whose cross dimension (diameter or edge) measures at least 3/4 wavelength. The helical element has a radius of 1/8 to 1/4 wavelengths, and a pitch of 1/4 to 1/2 wavelength. The minimum dimensions depend on the lowest frequency at which the antenna is to be used. If the helix or reflector is too small (the frequency is too low), the efficiency is severely degraded. Maximum radiation and response occur along the axis of the helix. Helical antennas are commonly connected together in socalled bays of two, four, or occasionally more elements with a common reflector. The entire assembly can be rotated in the horizontal (azimuth) and vertical (elevation) planes, so the system can be aimed toward a particular satellite. If the satellite is not in a geostationary orbit, the azimuth and elevation rotators can be operated by a computerized robot that is programmed to follow the course of the satellite across the sky.

The helical antenna has a long history and has been the object of much study and development over the last half century since its invention in 1946 [Kraus, 1976]. It is an interesting antenna with unique characteristics, being capable of high gain, wide bandwidth, and circular polarization. As a result, it has been used in a wide range 2

of applications including satellite communications, radio astronomy, and wireless networking. This dissertation presents a fundamental advancement of the basic axial mode helix design. This new form of helix antenna, called the Stub Loaded Helix (SLH) antenna, offers the advantage of a significant reduction in helix antenna size with only a relatively small corresponding reduction in performance. The performance reduction in many applications is not relevant and the application requirements are still satisfied. The result is a new antenna design that offers the performance characteristics and advantages of the conventional axial mode helix but in a much more compact physical size envelope. The original aspects of the work described in this dissertation are covered by U.S. patent #5,986,621. All intellectual properties related to this patent are controlled and administered by Virginia Tech Intellectual Properties, Inc.

The helical antenna is a hybrid of two simple radiating elements, the dipole and loop antennas. A helix becomes a linear antenna when its diameter approaches zero or pitch angle goes to 90 o . On the other hand, a helix of fixed diameter can be seen as a loop antenna when the spacing between the turns vanishes (a ! 0o ) . Helical antennas have been widely used as simple and practical radiators over the last five decades due to their remarkable and unique properties. The rigorous analysis of a helix is extremely complicated. Therefore, radiation properties of the helix, such as gain, far-field pattern, axial ratio, and input impedance have been investigated using experimental methods, approximate analytical techniques, and numerical analyses. Basic radiation properties of helical antennas are reviewed in this chapter.

The geometry of a conventional helix is shown in Figure 2.1a. The parameters that describe a helix are summarized below. D ! diameter of helix S !spacing between turns N ! number of turns C ! circumference of helix ! D A ! total axial length !NS a !pitch angle 3

If one turn of the helix is unrolled, as shown in F igure 2.1(b), the relationships between S, C, a and the length of wire per turn, L , are obtained as:

S !L sin a ! C tan a

=

Figure 1.1 A sketch of a typical helical antenna

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CHAPTER TWOSURVEY OF HELICAL ANTENNAS

2.1 MotivationThe original motivation for this work came from the desire to develop a reduced size helix antenna for use in satellite communications links for both earth terminals and on space platforms. Although technological trends are toward the use of higher frequencies in the microwave and millimeter regions where greater bandwidths are available, there are still a number of satellite systems that utilize the VHF and UHF frequency bands. The most prominent of these is the U.S. Navy FLTSATCOM system which utilizes a network of geosynchronous satellites to provide worldwide coverage to bases, ships, and mobile ground forces.

At VHF and UHF frequencies, the physical dimensions of a conventional axial mode helix can be large enough to present difficulties, particularly for mobile forces and on the ever 2 increasingly crowded top sides of today's naval ships. A typical helix for FLTSATCOM applications can have a helix diameter on the order of one foot (30.5 cm) and an axial length of 12 feet (4 meters) or greater with a ground plane on the order of 4 feet (1.3 meters). Mounting and pointing of such a large structure presents mechanical problems. Any reduction in antenna size without significantly impacting performance would be very desirable.

In every aspect of wireless communications today, there is a desire to minimize antenna size. The technological progress that has produced significant advances in the miniaturization of components and circuitry has not been mirrored by corresponding advancements in antenna miniaturization, for a very fundamental reason. Solid state components are only now approaching structure sizes that are comparable to the wavelength of an electron. Most antennas operate in the size regime where their physical dimensions are on the order of the wavelength of operation. Much theoretical 5

work over the years, as well empirical results, indicate that antenna size reduction results in compromises of some key performance characteristics, most notably efficiency and bandwidth. It is the goal and art of engineering to minimize and optimize these compromises to provide a solution that meets the requirements of each specific application. It is hoped that the Stub Loaded Helix antenna has achieved an appropriate balance between performance and compromises in order to be considered a useful contribution.

2.2 Modes of Operation2.2.1 Transmission Modes An infinitely long helix may be modeled as a transmission line or waveguide supporting a finite number of modes. If the length of one turn of the helix is small compared to the wavelength, L , the lowest transmission mode, called the T0

mode, occurs. Figure 2.2a shows the charge distribution for this mode. When the helix circumference, C , is of the order of about one wavelength (C }1 ) , the second-order transmission mode, referred to as the T1 mode, occurs. The charge distribution associated with the T1 mode can be seen in Figure 2.2b. Higherorder modes can be obtained by increasing of the ratio of circumference to wavelength and varying the pitch angle.

2.2.2 Radiation Modes

When the helix is limited in length, it radiates and can be used as an antenna. There are two radiation modes of important practical applications, the normal mode and the axial mode. Important properties of normal-mode and axial-mode helixes are summarized below.

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Figure 2.1 (a) Geometry of helical antenna; (b) Unrolled turn of helical antenna

Figure 2.2 Instantaneous charge distribution for transmission modes: (a) The lowest-order mode (T0); (b) The second-order mode (T1)

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2.2.2.1 Normal Mode

For a helical antenna with dimensions much smaller than wavelength (NL

),

the current may be assumed to be of uniform magnitude and with a constant phase along the helix . The maximum radiation occurs in the plane perpendicular to the helix axis, as shown in Figure 2.3a. This mode of operation is referred to as the normal mode. In general, the radiation field of this mode is elliptically polarized in all directions. But, under particular conditions, the radiation field can be circularly polarized. Because of it small size compared to the wavelength, the normal-mode helix has low efficiency and narrow bandwidth.

2.2.2.2 Axial Mode When the circumference of a helix is of the order of one wavelength, it radiates with the maximum power density in the direction of its axis, as seen in Figure 2.3b. This radiation mode is referred to as axial mode. The radiation field of this mode is nearly circularly polarized about the axis. The sense of polarization is related to the sense of the helix winding.

In addition to circular polarization, this mode is found to operate over a wide range of frequencie

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