Communications: radio wave antennas – Antennas – High frequency type loops
Reexamination Certificate
2001-07-12
2003-02-04
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
High frequency type loops
C343S742000
Reexamination Certificate
active
06515632
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to antennas and, more particularly, to a loop antenna of moderate electrical size having an omnidirectional far-field pattern similar to that of an electrically-small loop.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Electric and magnetic dipole antennas having ideal omnidirectional patterns are very useful for design, operation and testing of various electromagnetic systems. For example, electric dipoles are often used to make so-called site attenuation measurements and for characterizing test sites used in testing antenna systems. Site attenuation measurements are essentially insertion loss measurements made with two precision dipoles carefully positioned a fixed distance apart. The deviation in insertion loss between the two dipoles as compared with the insertion loss between the dipoles in “free space” (actually, a reference site) gives an indication of the quality of a test site. However, the electric dipoles can mask some problems with a site in that they do not radiate in all directions; they exhibit radiation nulls located on their dipole axes. A magnetic dipole also has radiation nulls on its dipole axis (a perpendicular running through the center of the loop). However, by using two electric (horizontal and vertical) and two magnetic dipoles (horizontal and vertical), masking effects of the nulls may be overcome. An “omnidirectional” or “isotropic” pattern as used herein refers to a pattern having constant field amplitude with direction within a two-dimensional plane perpendicular to the axis of an electric dipole, or, in the case of a magnetic dipole loop, the plane containing the loop. In other words, the dipole cannot be literally omnidirectional because of its radiation nulls, but it is desirable that the dipole be omnidirectional in the plane perpendicular to the direction containing the nulls. Dipoles having such idealized patterns are needed to obtain accurate characterization of test sites.
Isotropic patterns are also desirable in mobile communications systems, in which the direction from which an incoming signal comes may be constantly changing. The large amount of scattering and reflection encountered in typical mobile communications systems makes it desirable to employ antennas with different polarizations, so that the chance of detecting a signal having an arbitrary polarization is increased. An electrically-small magnetic loop dipole radiates a dipolar pattern which is orthogonal to that of an electric dipole. Thus, such an antenna is useful when pattern diversity is required.
Electric (linear wire) and magnetic (loop) dipoles exhibit omnidirectional far-field patterns when used at frequencies for which they are electrically-small, or for which the physical size of the antenna is small compared to the wavelength of radiation. For the purposes of this disclosure, “electrically-small” refers to an antenna having its largest dimension smaller than about {fraction (1/10)} of a wavelength. Electric dipoles having omnidirectional patterns may be realized fairly easily. A wire dipole exhibits a fundamental series resonance (the linear or wire dipole exhibits a minimum susceptance input admittance; it is an open circuit at DC) when it is slightly less than one-half wavelength long. At this point the input impedance to the dipole is about 73-80 Ohms and thus is very nearly intrinsically matched to a 50 Ohm source. Furthermore, the pattern of the so-called half-wave dipole differs only slightly from that of an electrically-small dipole. Patterns of ideal (electrically-small) and half-wave electric dipoles are discussed further in pages 200-222 of
Antennas
by John D. Kraus (McGraw-Hill, 1988, hereinafter “Kraus”), which pages are hereby incorporated by reference as if fully set forth herein.
Practical realization of a magnetic dipole having an omnidirectional pattern, on the other hand, is more difficult. A single-turn loop antenna, or magnetic dipole, exhibits its fundamental parallel resonance (a loop exhibits a minimum reactance input impedance; it is a short circuit at DC) at a frequency when it is very nearly one wavelength in circumference. However, the pattern of a self-resonant loop is completely different from that of an electrically-small loop, and is not omnidirectional. In fact, the maximum field amplitude is not even in the plane of the loop, as it is for the electrically-small loop. Patterns of magnetic dipoles of various electrical sizes are discussed further in pages 238-255 of Kraus, which pages are hereby incorporated by reference as if fully set forth herein.
A classical magnetic dipole therefore needs to be electrically-small to produce an omnidirectional pattern. There are several reasons, however, for using an antenna which is not electrically-small. An electrically-small loop has a very small radiation resistance and very high radiation Q. The high radiation Q corresponds to narrowband radiation characteristics. Furthermore, it is much easier to match an antenna of moderate electrical size to a 50-ohm source (50-ohm sources are most common, and other typical impedances, such as 75 ohms, are also relatively large). In a metrology antenna, the matching network can contribute significantly to measurement uncertainty. This is because of necessarily non-zero tolerances in matching components and because of temperature sensitivity of the matching components. In addition, at higher UHF frequencies and above it becomes difficult to implement an electrically-small antenna with precision. This is because the short wavelength requires a very physically-small antenna with the attendant tight dimensional tolerances. That is, the dimensional tolerances are related to the wavelength and the overall size of the antenna.
Finally, while in principle it is possible to scale any linear electromagnetic device, some details cannot easily be scaled in practice. For example, connectors and coaxial transmission lines are commercially available only in specific sizes and geometries. It is not at all worthwhile to design and manufacture custom connectors for a specific antenna. Furthermore, if custom connectors were developed, adapters to allow interconnection with industry-standard connectors would also be required. Thus, it is best if designs can employ standard coaxial connectors such as SMA connectors. If, for example, it were necessary to implement an electrically-small antenna at 2450 MHz, the antenna would be roughly the same size as the SMA connector. Obviously, in this case, the external geometry of the connector would influence the radiation pattern of the antenna. In most cases, it is useful if the external geometry of the connector and feed transmission line have minimal influence on the operation of the antenna.
Further discussion of the use of omnidirectional antennas and problems with electrically-small loops is included in U.S. Pat. No. 5,751,252 to Phillips (hereinafter “Phillips”), which is hereby incorporated by reference as if fully set forth herein. An approach described in Phillips to making an omnidirectional loop antenna involves “breaking” the loop at a point opposite the feed point of the loop, and bridging the break with a capacitive element. By effectively open-circuiting the loop at what would be the maximum current point of the (unbroken) loop, this approach lowers the overall current variation around the loop, resulting in a more omnidirectional pattern. The diameter of the loop described in Phillips is {fraction (1/7)} of a wavelength, which although larger than a classical electrically-small loop, may still be undesirably small, particularly for operation at higher frequencies (e.g., greater than one GHz). There further appears to be no indication in Phillips of how the small capacitor values needed (0.7 pF at 800 MHz) are to be realized with the precision necessary for a metrology grade antenna.
Another approach is to simulate a large loop using f
Conley & Rose & Tayon P.C.
Daffer Kevin L.
TDK RF Solutions
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