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A high gain dual-polarized conical antenna.

Many of today's highly sophisticated electronic warfare environment simulators require high performance antennas in order to accurately reproduce the spatial and spectral signal environment encountered in a real engagement. One such application is for radio frequency simulators (RFS) that are used for guided weapons evaluation. In these systems, one wall of an anechoic enclosure is lined with precision radiating sources, each of which is fed with appropriate signals to simulate targets and their movements in real time, as well as clutter and jamming. Figure 1 shows antennas developed recently to be used for RFS applications in the Air Force Development Test Center's (AFDTC) new guided weapons evaluation facility at Eglin Air Force Base. Over 500 of these antennas have been produced to date. Derivatives of the basic antenna design are well suited for additional applications in electronic support measures, self-protect jamming and radar warning receivers.

The RFS antenna is a precision, dual-polarized conical horn operating over the 6 to 18 GHz frequency band. Its high gain (22 to 30 dBi), efficient design utilizes a surface-matching lens to minimize the length of the device. High polarization orthogonality (greater than 30 dB), good phase and amplitude tracking between ports, and good monotonic pattern performance are obtained via an optimally designed coax-to-quadridged waveguide feed section.

A smaller version of the RFS antenna, shown in Figure 2, also was produced. The smaller version's aperture is eight inches in diameter, and its peak gain measures 15 to 22 dBi across the same 6 to 18 GHz frequency band. Both of these antennas can be extended easily to include right- and left-hand circular polarization (axial ratios less than 1 dB) with the incorporation of a stripline feed network. This modification provides switchable or simultaneous access to four antenna polarization states.

PERFORMANCE DESCRIPTION

The high gain antenna is designed to meet the stringent parameter specifications driven by the RFS application. Key performance specifications are listed in Table 1. Two field-replaceable SMA connectors provide the orthogonal inputs to the critical feed section of the antenna. The feed consists of the two inputs, offset along the horn axis, feeding a quadridged waveguide section and a conical short-circuit termination. Linear compensation to one feed is required to achieve phase tracking balance across the band to within [+ or -]10 [degrees] typical. Optimization of this feed region proved critical in achieving a low SWR and a high isolation between inputs. Since the coaxial-to-waveguide transition is a major contributor to SWR performance, much effort was devoted to optimizing the impedance match. The final antenna configuration resulted in SWR performance of 1.4 typical and 2 maximum over the entire 3:1 frequency band. Port-to-port isolation was greater than 30 dB.
TABLE I


PERFORMANCE SPECIFICATIONS


Frequency range (GHz)                            6 to 18
Polarization                                   Dual Linear


SWR
maximum                                            2.0
typical                                            1.4


Gain (dBi) (min)                                 22 to 30
Isolation (d)                               [greater than]25
Cross polarization (dB)                     [greater than]25
Gain tracking (dB)                             [+ or -]0.75
Phase tracking ([degrees])                     [+ or -]10
CW power capacity (W)                               20


During the development, tradeoffs were necessary to find an acceptable balance between antenna radiation pattern performance and SWR. Computer modeling, aperture field probes and network analyzer measurements utilizing time domain techniques were employed as optimization design tools. As such, the quadridged circular waveguide section was chosen to achieve the desired bandwidth and to provide dual orthogonal linear polarizations. Various ridge shapes (stepped, cosine and exponential) and axial lengths were evaluated. In addition, numerically controlled machining of the feed section was employed since precise control of mechanical dimensions was necessary, to obtain the required electrical performance. This computer-aided manufacturing approach was utilized to maintain the required precise control of the ridges orthogonality (to within 2 [degrees]) in the feed region, in order to meet the greater than 25 dB cross-polarization specification. Levels in excess of this electrical specification are realized consistently in production acceptance testing. In order to control and keep production unit costs to a minimum, less expensive manufacturing techniques were used in the less critical regions of the horn.

The dimensions of the tapered-ridged transition were chosen to maximize the peak gain and realize the lowest possible peak SWR. The horn measures 22 inches in length with a 12-inch diameter aperture. Peak gains in excess of 22 dBi at 6 GHz and 30 dBi at 18 GHz were realized. To overcome the spherical wave phase error inherent with compact horns and to achieve the desired gain and antenna sidelobe levels, a low dielectric constant lens is used. The lens includes a multistage transformer for surface matching. Figures 3 and 4 show typical H- and E-plane antenna patterns at 8 and 12 GHz, respectively. In addition, unit-to-unit gain tracking is typically less than 1 dB, satisfying a key simulator specification. The antenna array application requires precise control of the amplitude of various antenna groupings to simulate moving targets.

CONCLUSION

High performance dual-polarized antennas are ideal for anechoic chamber array applications, such as RFS. Currently, the antennas are being employed in a number of anechoic facilities.

ACKNOWLEDGMENT

This antenna was developed under contract to the USAF, Eglin Air Force Base, AFDTC. The company would like to thank Richard Hamilton and Vance H. Maples for their technical and programmatic support throughout the effort.
COPYRIGHT 1996 Horizon House Publications, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1996 Gale, Cengage Learning. All rights reserved.

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Publication:Microwave Journal
Date:Jun 1, 1996
Words:871
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