Communications: radio wave antennas – Antennas – Active sleeve surrounds feed line
Reexamination Certificate
2001-12-21
2003-12-02
Ho, Tan (Department: 2821)
Communications: radio wave antennas
Antennas
Active sleeve surrounds feed line
C343S791000, C343S821000
Reexamination Certificate
active
06657601
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of antenna systems. More particularly, the present invention relates to metrology antenna systems. The present invention is intended to serve as a reference radiator or receiver of electromagnetic radiation and provides a near perfect or canonical dipolar radiation pattern.
BACKGROUND OF THE INVENTION
An antenna system consists of radiating/receiving elements as well as a feed network, which couples the radiating elements to an external device, such as an amplifier, or system, such as a receiver. A well-designed antenna system provides an input impedance that closely matches that of the external device or system to which the antenna system is connected at resonance. In this way reflections and standing waves are minimized. Thus, one task of the feed network can be to match the input impedance of the radiating element(s) to the impedance level of the system. Additionally, the feed network may convert a single-ended or unbalanced source into a balanced configuration. This is necessary if the antenna is a symmetric or balanced antenna such as a dipole and the source utilizes a coaxial port. Metrology antenna systems are one type of antenna system that, by design, should produce accurate and repeatable electromagnetic field measurements. Some electromagnetic field measurements include, but are not limited to site attenuation, anechoic chamber characterization, antenna characterization, in-situ telecommunication device characterization, and Specific Absorption Rate (SAR). In a metrology antenna system very little mismatch or imbalance can be tolerated. Therefore, the requirements for the feed network of a metrology antenna system are quite exacting. In order to provide the greatest possible confidence in measurements, it is desirable that a metrology antenna system be capable of being comprehensively modeled numerically or possibly analytically in a straightforward manner. In particular, it is desirable that the antenna system be designed such that well-established and extensively-verified numerical models such as the Numerical Electromagnetics Code (NEC-2, NEC-4) can be used to accurately model it. This limits the geometry of and the materials used in the antenna to those that can be accurately represented in the numerical model. In particular, the NEC code is extremely well adapted to representing linear antennas.
A linear antenna is essentially a one-dimensional antenna, that is, one that looks substantially like a linear wire. Linear antennas, include, but are not limited to, half-wave linear dipoles, quarter-wave linear monopoles, electrically-short linear dipoles, electrically-short linear monopoles, folded dipoles, folded monopoles, sleeve dipoles, and sleeve monopoles.
FIG. 9
depicts an idealized center driven linear dipole antenna. As can be see in
FIG. 9
, the self-contained voltage source V
o
cos(&ohgr;t) feeds the antenna system, which comprises two linear wire elements. Linear antennas are also referred to as wire antennas because sometimes they are fabricated from wire stock or other conducting materials. While it is possible to fabricate linear antennas from wire, such antennas are more often fabricated from rigid metal tubing or circular metal bar stock. One comprehensive reference on linear antennas is R. W. P. KING, THE THEORY OF LINEAR ANTENNAS WITH CHARTS AND TABLES FOR PRACTICAL APPLICATIONS, herein incorporated by reference in its entirety. One such linear antenna is shown in FIG.
9
.
FIG. 9
depicts a dipole antenna which utilizes a self-contained source. Most practical implementations of dipole antennas do not use a self-contained source. Instead, the antennas are driven via feed transmission lines. In most practical situations, this transmission line is a coaxial cable.
Even though a dipole is a symmetric antenna, it must be driven with a symmetric or balanced source, also known as a differential source in order to obtain a symmetric radiation pattern. As can be seen in
FIGS. 10 and 11
, the linear dipole, identified by two linear wire elements has a balanced source derived from two single-ended sources, namely the two ½ V
0
(cos(&ohgr;t)) voltage sources. Schematic depictions of two equivalent, balanced sources are shown in
FIG. 5. A
balanced source produces two voltages equal in magnitude and opposite in phase with respect to a common reference. If the sources of a linear dipole are not symmetrically balanced, common mode current will flow on the feed transmission line. Common mode currents produce distorted radiation patterns and cross-polarized radiation thereby eliminating the principle benefit of the linear dipole, namely radiation patterns which are easily modeled. Some references describing these effects are in W. L. WEEKS, ANTENNA ENGINEERING, §4.5 (McGraw Hill 1968) and C. A. BALANIS, ANTENNA THEORY ANALYSIS AND DESIGN,” §9.8.6 (John Wiley & Sons 1997) herein incorporated by reference in its entirety.
Accordingly, to prevent common mode currents which produce distorted radiation patterns, linear dipoles must be fed with balanced sources. Sources originally unbalanced may be converted to balanced sources using a BALanced-to-UNbalanced (BALUN) transformer or network. One simple example of a balun is a transformer with a center-tapped secondary, such as is shown in FIG.
6
. With this configuration, as shown in
FIG. 6
, a single-ended source, V
0
(cos(&ohgr;t)), is connected to the primary and the center tap of the secondary winding is connected to ground. It should be noted that V
0
represents the magnitude of the AC voltage and &ohgr; represents its radian frequency. This produces two voltages equal in magnitude but opposite in phase, namely ½ V
0
(cos(&ohgr;t)) and −½ V
0
(cos(&ohgr;t)). Linear dipole antennas are usually coupled to coaxial transmission lines through baluns. Some prior art baluns include, but are not limited to, the Marchand or Roberts balun, the choke balun, and the split sleeve balun. The Roberts dipole, a linear dipole driven by a Marchand balun, is a metrology standard and is specified in ANSI standard C63.5-1998, herein incorporated by reference in its entirety. These prior art baluns have a number of inadequacies. As for the Roberts balun these include: (1) calibration procedures using an automatic vector network analyzer are difficult to implement, (2) acceptable manufacturing tolerances for physically small devices such as are required for high frequency operations are difficult to achieve; and (3) spurious radiation from the balun which can significantly perturb the linear dipole's radiation pattern. Choke baluns suffer from drawbacks similar to the Roberts balun. These shortcomings include: (1) the difficulty of implementing a calibration procedure using an automatic vector network analyzer; (2) physical limitations in magnetic materials, such as ferrite, which limit the operating frequency range of such devices to several GHz at the highest; and (3) spurious radiation from the balun.
As noted earlier, in order to increase confidence in electromagnetic field measurements, it is desirable to employ an antenna system that can be simply and accurately modeled analytically or numerically. This is particularly important for metrology or reference antenna systems. Because linear dipoles are among the simplest antenna structures and have been extensively analyzed, they are widely used in conjunction with metrology applications. Despite their simplicity, there are some difficulties encountered in the realization of practical dipole antennas. One difficulty involves the techniques used to feed the dipoles. Radiation originates from these feed mechanisms, and simple numerical and analytical models cannot accurately account for this radiation. Feed region radiation can cause the behavior of a practical dipole to depart markedly from that of an ideal or canonical dipole. Besides the distortion of the ideal canonical dipole radiation pattern caused by the feed source, radiation from the feed source also complicates t
Darby & Darby
Ho Tan
TDK RF Solutions
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