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Designer Polarization for EW Antennas.

Historically, radar warning receivers (RWR) employed cavity-backed, spiral antennas to detect and classify threats to an aircraft and to determine their direction of arrival. These spiral antennas are circularly polarized and can sense signals of all polarizations with varying sensitivity, except the opposite sensed circular polarization. The spiral antenna performed efficiently but was unable to meet the challenge posed by hostile signals that were arbitrarily polarized. A common aperture element was needed that was polarization diverse.

In 1983, Randtron Antenna Systems (Menlo Park, CA), a division of L-3 Communications (New York, NY), developed the dual-polarized sinuous antenna (DPSA), a common-aperture, frequency-independent antenna with performance equal to or better than a spiral antenna of the same size and capable of simultaneously responding to all polarizations, including orthogonal circular, elliptical and linear.

The performance of the new DPSA emulated a spiral antenna of the same size and was capable of simultaneously receiving or transmitting radio-frequency (RF) signals of any two polarizations on two isolated output ports. When used in existing RWRs, two receive channels or a switch is needed.

The sinuous antenna consists of two interleaved, orthogonal pairs of sinuous conductors that are etched onto a dielectric substrate, which is mounted above a RF-absorbing cavity in a manner similar to a cavity-backed spiral. The devices are available in a variety of sizes for operation at a wide range of microwave frequencies.

In 1985, Randtron Antenna Systems made the sinuous antenna available as a drop-in replacement for the single-polarization spiral antenna. Since that time, DPSAs have become widely used on airborne, land-based, shipboard and undersea platforms in applications such as RWRs; electronic-warfare (EW) systems; electronic-support measures (ESM), such as interferometers; RF repeaters for electronic countermeasures (ECM) and SIGINT systems, as well as functioning as sensors for polarimeter systems and feeds for reflectors in antenna test ranges.

Alternative Solution

Recently, an alternative solution has been developed by Randtron Antenna Systems that makes use of the unique polarization-diversity properties of the sinuous antenna and eliminates the modifications to the remainder of the RWR entirely by providing virtually all the sinuous improvements with only one port.

Research determined that by combining the signals from the two DPSA outputs with varying amplitude and phase ratios, any polarization could be created. Thus, a specific designer polarization can be selectively created. The idea is to provide a tailored polarization that receives all major polarizations with good, predictable antenna patterns and only shows poor polarization response to some obscure polarization. This tailored polarization would be based on the relative strengths and likelihood of the signals expected in common mission RF environments.

Sinuous-Antenna Operation

Polarization theory revealed how the dual-polarization capability of the sinuous antenna could be exploited to create a targeted, designer polarization. However, before we delve into the designer-polarization technique, it is important to briefly review, the operation of the sinuous antenna.

Figure 1 depicts the block diagram of a sinuous antenna, which includes the horizontal- and vertical-polarization elements, balanced-to-unbalanced transformers (baluns), a 90[degrees] hybrid coupler and receiver channels. This implementation is capable of sensing all polarizations.

Each conductor pair is excited at the center by a balun that combines opposing conductors in a 0[degrees]/180[degrees] phase relationship. The baluns are mounted to the bottom of the cavity and extend into the cavity to connect to the sinuous conductors.

Signals received at each of the dual balun outputs are combined in a [plus or minus]90[degrees] phase relationship by the four-port hybrid. The two output ports thus provide right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP) simultaneously.

The two DPSA output ports provide maximum response to left- and right-hand circularly polarized incoming signals and also respond well to any linear polarization. As a result, one port or the other will always receive any polarized signal with a sensitivity that is no less than 3 dB below the matched polarization sensitivity.

Looking at Designer Polarization

As previously stated, when the signals from the two DSPA outputs are combined with varying amplitude and phase ratios, any polarization can be created.

All polarizations fall into the general category of elliptical polarization -- the general case, of which linear and circular polarizations are limiting cases. Figure 2 shows the relationship between the amplitude and phase of the power received at the two sinuous outputs and the polarization of the approaching wave.

For the cases of [plus or minus][infinity]dB, we have the limiting case of linear vertical and horizontal polarizations. For the case of 0 dB and [plus or minus]90[degrees] we have left- and right-hand circular polarizations. Slant polarizations result for the in- and out-of-phase conditions of 0[degrees] and 180[degrees]

To implement designer polarization, the DPSA incorporates a tandem quadrature hybrid to generate circular polarization with linear antenna elements. This produces a predominantly leftslant, left-hand, elliptical polarization with a desired left-slant to right-slant linear polarization ratio. Most important, this antenna will have reasonable gain and well-behaved patterns to all four main polarizations (right and left circular, vertical and horizontal linear) from a single port. Furthermore, no system-hardware changes are required to implement this increased polarization capability.

By contrast, a spiral antenna responds to its opposite sense circular polarization with unpredictable gain and pattern characteristics rendering the information unusable in a typical direction-finding or EW system.

The Future of Designer Polarization

A technique for generation of arbitrary antenna polarization has been analyzed and successfully tested. Designer polarization can be used to simplify airborne RWR, ESM and EW systems when upgrading to greater polarization diversity. The key to success of designer polarization is the use of the sinuous antenna, which has the unique feature of simultaneously generating or receiving all polarizations from a common spiral-like aperture. Furthermore, the sinuous antenna, like no other, can maintain this capability over a very wide frequency band and very wide field of view.

The importance of this common aperture with coincident phase centers to the success of designer polarization cannot be over-stressed. A pair of antennas that do not collect signals at the same physical location as the sinuous antenna will produce a variation in phase that degrades the polarization parity over space.

Robert O. Magatagan is a microwave staff engineer at Randtron Antenna Systems.
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Title Annotation:electronic warfare
Comment:Designer Polarization for EW Antennas.(electronic warfare)
Author:Magatagan, Robert O.
Publication:Journal of Electronic Defense
Geographic Code:1USA
Date:Dec 1, 2000
Previous Article:Protecting the Big Boys: Directed IR Countermeasures for Large Aircraft.
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