Mark G. Parent

Fiber-optic prism true time-delay antenna feed

Experimental results for an optical-control technique for implementing a true time-delay function for array antennas are reported. A microwave signal is transmitted on a wavelength-tunable optical carrier through a fiber-optic prism-a set of nominally equal-delay fibers with differing net dispersion-to photodetectors that feed each antenna element. The relative interelement time-delay (beam angle) adjustment is accomplished by tuning the optical carrier wavelength. Measured antenna patterns of a two-element array clearly demonstrate beam steering and true time-delay operation over a two-octave bandwidth of 2-8 GHz.<<ETX>>

Array transmitter/receiver controlled by a true time-delay fiber-optic beamformer

We demonstrate a phased-array antenna controlled by a fiber-optic beamformer in true time-delayed transmit mode and in phase-delayed receive mode. The wavelength-tunable optical carrier propagating in a high-dispersion fiber realizes a true time-delay, with the steering direction set by a single voltage controlling the wavelength. The linear antenna is a sparsely-populated array with eight broadband spiral elements. The transmitter operates in 2-18 GHz frequency range over /spl plusmn/50/spl deg/ azimuth steering, and the receiver operates in 6-16 GHz frequency range over /spl plusmn/35/spl deg/ azimuth steering.<<ETX>>

Preliminary Investigations of a Low-Cost Ultrawideband Array Concept

A design concept is presented that achieves ultra wideband (UWB) array performance with significantly fewer elements than the traditional approach of using a single wideband antenna element type to fully populate the array. Starting from a conventional 8:1 bandwidth array design of a given aperture size, an array of equivalent aperture and bandwidth is created using scaled elements of three different sizes. This wavelength-scaled equivalent array has fewer than 18% of the original element count, i.e., roughly 6-times fewer elements, a similar reduction in weight, and most importantly, a significant reduction in electronics required to feed the array. If proven viable, array architectures of this type could make UWB arrays significantly more cost effective. In this preliminary numerical study, rigorous full-wave simulation tools are used to test the performance of small but informative wavelength-scaled array configurations of flared-notch radiators for the single-polarization case.

Design and Performance of Frequency Selective Surface With Integrated Photodiodes for Photonic Calibration of Phased Array Antennas

The design, fabrication, and integration of a frequency selective surface (FSS) with integrated photodiodes to allow for photonic calibration of phased array antennas is presented. The design includes embedding electrically short dipole antennas in each unit cell of the FSS, with a zero-biased photodiode placed across the gap of the diode. Fibers from an optical distribution network are passed through the honeycomb core of the frequency selective surface and pigtailed to the photodiodes. The RF performance of the frequency selective surface with integrated optics is investigated via simulations and measurements, and the results show that the structure maintains RF-transparency.

RF Characterization of Zero-Biased Photodiodes

The characteristics of zero-biased InGaAs p-i-n photodiodes were studied for use as optical transducers in an in situ phased array radar calibrator. Radio frequency (RF) operation is especially important for the intended application in which the zero-biased photodiodes drive patch antennas embedded in a radome. The photodiodes act as sources for real-time calibration of the phase and amplitude of each array element and will operate with only optical input from a modulated optical fiber link. Unfortunately, little is known of the RF performance of photodiodes operated without voltage bias. In this paper, both the dc and RF performance of a particular photodiode were studied to determine the optimum operating conditions and to understand the type and severity of any limitations.

Two optical-control techniques for phased array: interferometric and dispersive-fiber true time delay

We describe and present experimental results for two optical control techniques for phased array antennas. The first technique is based on interferometric heterodyning of two narrow- linewidth YAG lasers for the generation of required microwave signal and for simultaneous steering of the radiated beam. The constructed system is simple and well-suited for narrowband applications, and it may be built without any active mechanical components. The measured radiated antenna patterns are in close agreement with the predicted ones. The second technique is a novel and elegant method for implementing a true time-delay function for optical control. It relies on using a wavelength-tunable laser to provide the optical carrier for the microwave signal and a fiber-optic prism--a set of equal delay fibers with differing net dispersion. The relative interelement time-delay (beam angle) adjustment is accomplished by tuning the optical carrier wavelength. The experimental results obtained on a compact antenna range clearly demonstrate beam-steering and true time-delay operation over a two-octave bandwidth.

Microwave time delay beamforming using optics

A practicable method of driving 2-D arrays using time delay waveforms for both transmit and receive is presented. An optical subsystem generates a set of delayed signals which are transferred to the elements by fiber optics. The time delays are generated by creating frequency-dependent microwave phase shifts in the optical system. An optical fiber array spatially samples optical wavefronts and transfers the optical signals to the array where they are heterodyne detected to recreate the microwave signals at the input to transmit-receive (TR) modules. This process essentially extends the backplane of the antenna back to the control system where element level operations may be performed. All beamforming and nullforming functions may be implemented, e.g. monopulse. The requisite TR modules have no phase shifters and a minimum of logic.<<ETX>>

True time-delay fiber-optic control of a phased-array transmitter with three-octave bandwidth

We demonstrate a true time-delay fiber-optically controlled phased-array transmitter with eight broadband spiral elements in a sparsely-populated array. The transmitter bandwidth is microwave-component limited to 2-18 GHz frequency range. The transmitter shows /spl plusmn/50/spl deg/ azimuth steering with no observed squint over a complete frequency range.We demonstrate a true time-delay fiber-optically controlled phased-array transmitter with eight broadband spiral elements in a sparsely-populated array. The transmitter bandwidth is microwave-component limited to the 2-18 GHz frequency range. The transmitter shows /spl plusmn/50/spl deg/ azimuth steering with no observed squint over a complete frequency range.<<ETX>>

High frequency vector sensor design and testing

A new antenna design that exploits the simultaneous measurement of the complete electric and magnetic vector fields (vector sensor) has been designed, built, and tested experimentally. The sensor is designed for use in the high-frequency band (3-30MHz). Special consideration for potential operation near the ground/earth is incorporated into the antenna design, yielding a symmetric response amongst the individual field sensing components. The vector sensor consists of three orthogonally oriented electrically short dipoles as well as three electrically small orthogonal loop elements. Appropriate active impedance matching circuits transform the individual field sensors to a nominal 50 ohms for connection to a multichannel receiver system. Experimental measurements indicate that to first order the sensing components are sufficiently decoupled in spite of their relatively close electrical spacing.

Low-cost phased array antenna for satellite communications on mobile earth stations

Future US Navy ships are expected to use multifunction, low radar cross section (RCS) phased array antennas for satellite communications. In this paper, we present a unique phased array concept in which a single planar array antenna (on a mobile earth station) can be used to communicate simultaneously with several geostationary (GEO) satellites by generating multiple independent beams. This array will have full electronic beam scanning capability in the azimuth direction with fixed beam positions in the orthogonal (elevation) plane using one or at most two hard-wired squints without the need for redesigning the antenna for different earth station locations. The proposed technique will reduce the cost and complexity of phased array antennas designed for mobile earth stations.