Communications: radio wave antennas – Antennas – With variable reactance for tuning antenna
Reexamination Certificate
1999-04-16
2001-03-27
Ho, Tan (Department: 2821)
Communications: radio wave antennas
Antennas
With variable reactance for tuning antenna
C343S795000
Reexamination Certificate
active
06208306
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to antennas, and more particularly to compact, broadband antennas utilizing a combination of folding and top-loading techniques.
2. Description of the Related Art
Frequency-independent antenna designs, in particular log-periodic dipole arrays (LPDAs), are widely used for broadband electric field generation applications. However, at the lower end of their operating range (the frequency range over which they exhibit frequency-independent behavior), such antennas must be approximately one-half wavelength in width. Thus, an LPDA with a lower operating frequency of 30 MHz (10 meter wavelength) must be approximately 5 meters wide. Because such dimensions are unacceptably large and because operating frequency ranges extending from below 20 MHz to above 2 GHz are required by the EMC testing industry, design techniques for a reduced-size hybrid antenna have been sought.
Reducing the size of an antenna such that its dimensions are smaller than one-half of a wave-length at its operating frequency may be described as making the antenna “electrically small”. Electrically small antennas are typically defined as those which fit within a sphere having a radius of ½&pgr; wave-lengths. Electrically small antennas are inherently more narrowband and inefficient than larger antennas, making design of compact antennas at relatively low frequencies challenging.
One common technique for extending the frequency range of an LPDA while limiting its size is the use of a broadband dipole to replace the lowest frequency element in the LPDA. For example, Brown-Woodward or bowtie dipoles can be used in conjunction with a 150 MHz LPDA (one having a low-frequency operating limit of 150 MHz) to extend the response of the antenna system down to 30 MHz. Examples of such a design are the model 3142 and 3143 antennas available from EMC Test Systems, L.P. Another possibility which is currently commercially available is to use a biconical dipole element to replace the lowest frequency elements.
While currently-available hybrid antennas are superior to LPDA antennas alone, they are still quite ineffective at the low-frequency end of their operating range. This weak performance at the low-frequency end imposes the amplifier requirements for an electric field generation system. Because the amplifier is generally the most costly component of the system, relaxing amplifier requirements would have a very significant effect on system cost.
Using dimensions and requirements typical for the EMC testing industry, we consider an antenna which occupies a 0.5 meter radius spherical volume, for which an electric field intensity of 20 V/m at 26 MHz is desired at a test distance of 3 meters from the antenna. It can be shown using radiation and power considerations that the theoretical lowest input power to achieve this performance is about 109 Watts. (It is assumed that the antenna exhibits a dipole radiation pattern.) Currently available EMC testing antennas, however, require amplifiers of at least 500 Watts to achieve this performance.
It would therefore be desirable to develop more efficient broadband antennas for operating frequencies at which the antennas are electrically small.
SUMMARY OF THE INVENTION
The problems described above are addressed by a compact, electrically-small broadband antenna' which combines a folded, top-loaded antenna geometry using broadband radiating elements with a series capacitance applied at the antenna feed. The broadband radiating elements are used both as the antenna feed and as shunting (sometimes called “swamping”) elements (giving rise to the “folded” geometry), and have a tapered form. Examples of these elements 'are bowtie or biconical elements. The folded geometry preferably includes an inductive load at the shunting element.
The top loading of the antenna may be obtained in any of several configurations. In general, a plate, or a wire-frame approximation to a plate, provides capacitance and also decreases the radiation Q of the dipole by increasing the current at its outer ends. This type of loading has been utilized for many years in unfolded antenna designs. In the antenna recited herein, the plate is attached to the above-described broadband radiating elements, such as bowties. A variety of orientations of the plate with respect to the broadband elements may be used. For example, the loading is believed to be most effective when an edge of the plate is attached to the shunting element such that an “F” configuration is obtained. Other plate orientations, such as one forming a symmetric “T” geometry with respect to the broadband elements, or one in which the plate is mounted asymmetrically, may also be used. Although these geometries are believed to provide slightly less effective loading than the “F” configuration, they are more effective than many currently-used designs, and the use of such a configuration may provide needed flexibility in meeting mechanical design constraints.
The combination of a folded geometry and top-loading as used in the antenna recited herein is believed to provide the antenna with a series resonance at a frequency for which the antenna is about one-tenth of a wave-lenth in length, and a parallel resonance at a frequency for which the antenna is close to one-half of a wave-length in length. Length as used herein refers to the distance between the ends farthest from the feed of the broadband radiating elements (if a dipole configuration is used), or between the ground plane and the end farthest from the feed of the broadband radiating element (if a single element in a monopole configuration is used). This length typically also corresponds to the distance between the top-loading elements (in a dipole configuration) or the distance between the top-loading element and the ground plane (in a monopole configuration). The operating band of the antenna may extend from approximately the series resonance frequency to about twice the parallel resonance frequency. For example, a typical operating band may extend from a frequency for which the antenna is about one-tenth of a wave-length long to a frequency for which the antenna is about two-thirds of a wave-length long. For an antenna which is 1.3 meters long, this would correspond to an operating band from about 25 MHz to about 150 MHz.
The parallel resonance exhibited by the folded antenna is positioned within the operating band of the antenna using tuning elements. Generally, the parallel resonance would be positioned fairly close to the lower limit of the operating range. In the vicinity of its parallel resonance, the radiation Q of the folded antenna is significantly lower than that of an unfolded (conventional) antenna at the same frequency. Therefore, this placement of the parallel resonance, combined with the use of a series capacitance at the input, results in significantly improved low-frequency performance of the antenna. The antenna is fed through series capacitors which essentially cancel the inductive reactance on the low side of the parallel resonance. Because the overall impedance level is high, an impedance transformer/balun will usually be required. In particular, a 50:200 ohm balun works well, with the source on the 50-ohm side and the antenna on the 200-ohm side.
The antenna recited herein may also be combined with an LPDA or other frequency-independent antenna to form an extremely broadband hybrid antenna. While the new broadband antenna given here provides a significant improvement in the performance of compact, broadband antennas, further improvement of the performance of the combination of this element with an LPDA may be provided by adding additional broadband elements, such as bowties, between the LPDA and the folded, top-loaded antenna. These additional elements help to cover the transition region between the LPDA and the new folded, top-loaded antenna.
REFERENCES:
patent: 5926150 (1999-07-01), McLean et al.
Crook Gentry E.
McLean James S.
Clinger James
Darby & Darby
EMC Automation, Inc.
Ho Tan
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