Impulse optoelectronic systems are finding growing use for most Ultra Wide Band applications, particularly for foliage, ground and wall penetrating radar. Optical systems with ultrafast laser sources generate without jitter ultrashort duration electrical waveforms. This characteristic allows to create UWB multielement arrays. The aims of these systems are to increase the peak radiation power of a single antenna and to point the radiation beam in an accurate direction. The laboratory XLIM has designed UWB antennas for ten years in the frequency range [100MHz - 3GHz], for transient applications such as Radar Cross Section measurements (RCS) and impulse Synthetic Aperture Radars (SAR). These antennas present some specific qualities suitable for radiation and measurement of ultra-short pulses. They are able of radiating high power pulses with a minimum of distortion.

The optical multielement system which is presented here is composed of four antennas (Valentine Antenna : XLIM design) and four photoconductive switches illuminated with an optical pulse and synchronised with a low jitter (<2ps). This hybrid optoelectronic system generate a short-duration electrical waveform (10kV, tm=130ps,FWHM=300ps) per antenna.

The possibility to drive each photoswitch with timedelay optical lines (accuracy<5ps) permits to point the radiation beam between -20° and +20°.

This paper describes all the capabilities of this ultrawideband optoelectronic system in particular : - the radiation power increase with the antenna number,
- the beam steering.

Creation of this type of optical sources antenna array allows to increase radar range, to scan, to point and to track with electronic driving antennas.

The paper presents an overview of research results from FOI on antenna apertures and integrated beamforming components of dual polarized phased array antennas for wideband and wide angle beamforming. The research has been motivated by their potential use in future applications in electronic warfare, multiband satellite communication, radar and multifunction systems. The paper presents the electromagnetic design and experimental evaluation of two dual polarised antenna arrays in the 2-6 and 6-18 GHz bands. Each array consists of two displaced co-located linear polarized arrays, with 7 x 8 elements each. The wide angle beam scanning performance has been modelled using infinite array approximations with commercial HFSS and in-house developed FDTD codes. The array designs have been experimentally characterised regarding circuit properties, such as active reflection coefficient and array efficiency, and radation properties including active element patterns and embedded array gain. The results show that the arrays have good reflection and radiation properties in a frequency range of up to 1:3 and for scan angles of up to 60°. The polarisation properties vary with scan angle, from linear polarization in the principal E- and H-planes to highly elliptical polarisation in the diagonal planes, as theoretically expected.

The frequency dependence of the polarisation is small in the principal planes and more pronounced in the diagonal planes.

A concept for wideband arrays with reconfigurable beamforming with frequency tapered aperture areas is also presented. One objective is to enable adaptation of beamwidths and beamshapes to different requirements, for example for different RF functions. A second objective is to enable a frequency independent beamwidth over a wide frequency range. Key components for wideband beamforming with MMIC true-time delays and reconfigurable active power distribution designed to operate in the full 2-18 GHz range are also presented. Simulation and measurements indicate performance adequate for use in reconfigurable wideband array systems.

Figure 1. Dual polarized 6-18 GHz array antenna.

Figure 2. Simulated radiation pattern showing a constant beamwidth over a frequency band of 6-18 GHz using frequency dependent tapering.

This paper deals with the design, realization and measurements of large band or ultra wide band high gain antennas in the W-band. It describes various developments conducted over the past years. Main applications are turned to radar systems or antennas for metrology. Since high gain is required, focusing antennas are developed. Among them, we distinguish between lens or classical reflector antennas, and printed reflectors. In the first case, the phase correction is frequency independent, and the frequency bandwidth is generally given by the primary source. Therefore, the expected bandwidth can be high when the primary source is optimized. In the second case, the phase correction is achieved using patches that are frequency dependent. But in this case, the expected bandwidth is lower.

We have developed several W-band printed reflectors based on Fresnel zone phase compensation in classical [1] or folded configurations, achieving about 10% bandwidth centered at 95 GHz as shown in figure 1. Efforts were made on primary sources in order to obtain the best trade-off between bandwidth and radiation characteristics that have to remain stable over the investigated bandwidth. Losses in gain do not exceed 3 dB between 90 and 100 GHz. This antennas were initially dedicated to radar systems.

In parallel, we designed and measured some lens antennas covering the entire W-band [2]. The lens itself has an hyperbolic profile and can be either rexolite made [2] or more cheaper PVC made. They are been used as reference antennas for measurements. In this case, return loss, but also diagram stability have to be achieved over the whole W-band. In particular, side lobe levels should be minimized. As it is for laboratory use, it is also important to maintain relatively low cost and easy to make antennas. As mentioned above, efforts are carried out on the primary source. Corrugated horns are well known as very good candidates for satisfying both S11 and diagram requirements. Nevertheless, they are quite complicated, when not impossible to fabricate with standard tools, when frequency increases. Therefore, the horn radiation pattern is optimized in order to radiate a prolate spheroidal function [3] that insures very low side lobes over the investigated bandwidth with quite a few steps in the horn profile. Figure 2 shows the simulated radiation pattern of the horn in the W-band. Fig. 3 shows the simulation for a compact lens of f/D =0.5 whereas fig.4 shows the measurements with f/D =1.

References

[1] B. D. Nguyen, C. Migliaccio, Ch. Pichot : "94 GHz Zonal rings reflector for helicopter collision avoidance", Electronic Letters, 2004, Vol.40 n°20 , pp. 1241-1242, 30 Septembre 2004.
[2] C. Migliaccio, J-Y. Dauvignac, L. Brochier, J-L Le Sonn and Ch. Pichot : "W-band high gain lens antenna for metrology and radar applications", Electronic Letters, 2004, Vol.40 n°22 , pp. 1394-1396, 28th October 2004.
[3] D.Slepian and H.O Pollak: "Prolate Spheroidal wave functions, Fourier analysis and uncertainty", Bell System Technical Journal, January 1961.

We start this paper with a four- band classification of high-power electromagnetic (HPEM) wave-forms based on bandwidth that has been recently proposed. An antenna system that radiates impulse-like waveforms making use of paraboloidal reflectors has been called the Impulse Radiating Anten-nas (IRA) and is an excellent example of a hyperband system. More recently, prolate-spheroidal sur-faces are being considered to fabricate such antennas for specialized applications. The paper also describes some examples of such antennas and potential applications.

We first present the classification of high-power electromagnetic signals (HPEM) into 4 bands based on bandwidth using the "percent bandwidth pbw" and the band ratio of the EM spectrum as .The impulse radiating antennas (IRAs) as shown in figure 1 is an example of Hyper-band system. Hyperband systems can be built in many forms such as reflector IRAs described below in Figure 1, or TEM horns, and lens IRAs. The reflector IRAs can also built with a Paraboloidal sur-face or a Prolate-Spheroidal surface.

Figure 1. Photograph of the 3.67m Prototype IRA

They have useful applications such as:

Ultrawideband (UWB) technology is exploited in wireless communications and radar. An antenna array capable of electronic beam steering can adapt its RF beam to locate targets and may minimize undesirable interferences to existing narrowband system. Current regulations from FCC expects UWB to occupy bandwidth of at least 25% or a minimum of 500 MHz. The low frequency band for UWB operates below 1 GHz band and higher bands from 2 to 10 GHz. Multiple time delay elements are required to steer the beam of an UWB antenna array. Work have been reported on UWB arrays but none of these address the performance of beam-steering and system design in the context of an approved emission mask. This paper will discuss these topics. In particular, we will report on design of beam steering sub-system using Direct Digital Synethesizer (DDS) and associated circuits such as Pulsed forming network(PFN). Measured results of a linear UWB phased array, including radiation patterns and pulse-width distortions, will be discussed in the context of FCC’s emission mask. The scanned angles of UWB array depends on the radio pulse-width and spacing between adjacent antenna elements, d. The angle between the direction of radiated signal and perpendicular to the axis of the linear array is given by β as shown in Fig. 1. Typically, the UWB source transmits mono-pulsed signals. To achieve maximum signal in a direction, the outputs of array must be in-phase. For a linear array with uniform spacing, d, between the antennas, the relative delay is given in Eqn. (1) for a scanned angle of β. Partial or non-overlapping of the summed signal causes time dispersion when time delays are out of phase. The relative delay between the first element to the i^th element is δ_i such that: δ_i=idsin(β)/c Eqn(1) where c is the speed of light.

The time delay for a beam scanning UWB array requires the following: the relative time delay between adjacent antenna elements should be a fraction of the pulsewidth to steer fine scanned angle; the time delay should be variable to change the scanned angle adaptively; timing jitter of the true time delay element should be minimised to avoid errors. The full width at half maximum (FWHM) power in time domain characterizes the pulse-width of the UWB signal and measures the time interval at -3 dB level from peak power. Wider time dispersion is measured at -6 dB. The pulse-width at -3 dB (FWHM) and -6dB are shown in Fig. 2 to 3 at 0 ps and 600 ps delay. At 0 ps delay, the main lobe is pointing at 0° and 180° as expected. The FWHM is about 67 ps here. This increases to about 740 ps at ±90°. The pulse-width at -6 dB widen more rapidly up to 910 ps at these 2 angles as compared to the FWHM(-3 dB). The pulse-width across the angles looks quite symmetrical with 0 ps. At 600 ps delay, the main lobe is directed at 60 °and 120 °. Here, the FWHM remains stable at 67ps. This increases to a maximum value of 1.29 ns at -90 °as shown in Fig. 3. This is much larger than the FHWM at 90 °with 214 ps. The results show that time dispersion increased dramatically away from the main-lobe. Hence, the emission mask for the UWB array has to be considered carefully since there is a change of spectrum across angles which may deviate from the approved emission mask.

Ground Penetrating Radar (GPR) is a useful tool for detection of all kind of electrical inhomogeneities in the ground, which is widely used in geological, geotechnical, archeological and forensic investigations. Exact localization of detected inhomogeneities and their characterization requires mechanical 2D scanning of the area under investigation with GPR, which makes GPR data acquisition very time consuming. If mechanical scanning over the area can be replaced by an electrical scanning of the subsurface with a focused antenna footprint similarly to the antenna-beam scanning in far-field phase arrays, the GPR investigation time might be drastically reduced. Principal difficulty however lies in the fact that the antenna footprint focusing should be achieved in the near field rather than in the far field of the antenna system.

To investigate possibilities of the near-field focusing we designed and built a GPR antenna system with digital footprint steering. The down-looking antenna system consists of a single transmit antenna and a linear receive antenna array with 7 elements. The receive array is placed in H-plane of the transmit antenna 40cm beneath it. For the transmit antenna a dielectric wedge antenna has been used, while a shielded loop antenna has been used as a receive one. To combine a sufficient penetration depth with the radar down-range resolution of the order of 3cm, the operational frequency band of the array (based on the antenna system transfer function) has been selected between 500 MHz and 3.5 GHz. Focusing of the footprint and its electronic steering has been realized via digital time delays in a multi-channel receiver.

The receive antenna array has been optimized to provide high-resolution images of the shallow subsurface. The goals of optimization were minimal mutual coupling between the elements (while keeping receive antenna separation smaller than half-wavelength at the highest operational frequency), maximal sensitivity of the antenna element and minimal end effects. The optimizing parameters were loop radius, separation between the elements and their mutual position.

Performance of the antenna system has been investigated experimentally. Focusing of the antenna footprint in the ground at the depth of 17cm by the antenna system elevation of 20cm has been observed. Capability of the electrical steering of the footprint along the array has been demonstrated (Fig.1). The received signal bandwidth remains remarkably stable by any position of the footprint, which proves correctness of the array design. Figure 1: Peak-to-peak footprints of the receive array with in-phase feeding (a) and progressive time delay shift of 100ps per element (b) and 200ps per element (c)

Conclusion: we have developed a UWB antenna system for GPR with an electronically steered linear receive array. The operational bandwidth of the system is of about 3GHz with a central frequency of 2GHz. The near-field footprint of the receive array is smaller than the footprint of each of its elements. Furthermore, the footprint position along the array is electronically steered. This antenna development resulted in realization a first ever GPR with real-time 1D electrical scanning.

Ultra Wideband Antennas can require balance to unbalance transformers (Baluns) that are able to operate over a bandwidth of in excess of one decade. Baluns claiming to fall into this class in the literature are the Marchand, Tapered line/Split coax, Double-Y along with a number of others. A previous study of these Baluns has lead to their classification into three groups with respect to their operation and this paper compliments these initial results by presenting quantitative data on the performance of these Baluns, with specific reference to those suitable for wide band operation. The data is based on computational simulations of the antennas and their feeding structures and although some measurement is presented these measured results are of confirmation value only as the measurement of common mode impedance is highly dependent on the measurement detail.

Introduction

Antenna Baluns are commonly used to connect balanced antennas to unbalanced feed lines, but the literature on how these work and on how to design them is not always clear and in some important cases it is in error. To better understand the operation of Baluns so as to introduce a simple physical understanding, they have previously been grouped by the fundamental method by which balance is forced on the feed and radiating structure. These three groups which require a physical symmetry of the balanced part of the antenna are retained and refined in this paper. The three groups are not definitive in sense that some Baluns may be considered as belonging to more than one group. The groups may be summarized as follows:

Many commercial and military applications require small low profile UWB antennas that operate from 50 MHz to 2000 MHz. Using conventional designs to cover such a vast frequency range with a single antenna would require an aperture size and profile which are too large for practical applications. In this paper, advanced antenna miniaturization techniques and theoretical limitations on antenna size for small UWB antennas with realistic impedance matching in mine are discussed.

Antenna miniaturization techniques such as dielectric or reactive loading are commonly used to increase the antenna’s electrical size without increasing its physical size. However, each of these miniaturization techniques by itself faces important performance trade-offs for large miniaturization factors. In this paper, a hybrid approach that involves both dielectric and reactive loading is used to maximize the miniaturization factor while minimizing any adverse effects. Our approach to miniaturizing an UWB antenna involves the use dielectric material on both sides of the antenna (substrate and superstrate) to maximize the miniaturization factor for a given dielectric constant. In addition, the thickness of the dielectric material is tapered to suppress dielectric resonance oscillation modes and surface waves as well as to maintain high-frequency performance. To maximize the miniaturization factor while minimizing the negative effects of dielectric loading, reactive loading or the artificial transmission line (ATL) concept is also used. This allows us to minimize the dielectric constant which results in less impedance reduction, a minimal antenna weight and reduces possible surface wave effects.

As it is well known, metallic cavity backings reduce antenna gain particularly for broadband configurations at heights less than wavelength/15. This problem can be addressed with the proper use of both lossy ferrite and PEC. An example is given for a low profile spiral antenna where lossy ferrite is used in the outer region to reduce antenna cavity coupling and preserve the free-space antenna gain at low frequencies. Towards the center, the backing tapers to a conductive coating making it suitable for high frequency operation. It is shown that this hybrid surface (ferrite to metallic) improves the antenna performance significantly at lower frequencies without compromising the high frequency gain.

Short pulses of gigawatt power level ultrawideband (UWB) radiation are necessary to detect and recognize remote objects. It is desirable to provide a wave beam steering mode and to radiate UWB pulses with dual polarization. The optimum length of an UWB pulse from the point of view of object sounding at a distance of 100 km and longer is 1 ns. In this case distortions of the pulse waveform are small at its propagation in the atmosphere that is significant at solving the problem of object recognition since non-coordinate information about the object is contained in the waveform of a reflected UWB pulse. The task of creation of high-power sources of UWB radiation for radars disintegrates into two parts, and namely: development of multielement arrays and pulsed power drivers for their excitation. Investigations in this direction have been carried out at the Institute of High Current Electronics for more than 10 years [1].

This paper gives special attention to analysis of investigation results of array antennas for high-power UWB radars. Compact combined radiators with cardioid pattern and efficiency by energy of 90 per cent and by peak power of 100 per cent at their excitation by bipolar pulses have been created on the basis of the developed approach to synthesis of UWB antennas [2]. A combination of electric and magnetic type radiators is used in the antennas. Linear and plane arrays of different configuration on the basis of combined antennas were investigated. It is shown that radiation characteristics remain satisfactorily at the wave beam steering in the limits of ± 45 degrees. To radiate pulses with dual polarization, the plane array was divided into two sub-arrays excited by two pulses with time delay. The geometry of the sub-arrays providing low level of background radiation as well as coincidence of the main directions of the patterns of two wave beams has been found out.

The developed array antennas were used in high-power sources of UWB radiation at their excitation from 0.5-2-ns high-voltage bipolar pulse generators. The bipolar pulse choice is conditioned by the higher efficiency of transformation into radiation by energy and peak power in comparison with the monopolar one. Besides, bipolar voltage pulse application allows decreasing the amplitude in two times in comparison with the monopolar one at close values of the radiated field peak strength that is important for reaching the utmost peak power of radiation determined by the electrical breakdown of the radiator. Created UWB sources based on 16-element arrays radiate linearly and dual polarized pulses with the efficient potential (product of the electric field peak strength by the distance) of 0.3-1.7 MV at the pulse repetition rate of 100 Hz. Pattern width by peak power is 20 degrees. The approach under development allows increasing the efficient potential of radiation at the expense of increasing the number of elements in the array and power of exciting pulses. Pattern width will decrease resulting in improvement of radar spatial resolution.

1. Koshelev V.I. Advances in Russian Ultra Wide Band Microwave Sources. Proc. Inter. Conf. on Directed Energy Weapons. 2005. London. UK..
2. Koshelev V.I., Buyanov Yu.I., Kovalchuk B.M., et al. High-power ultrawideband electromagnetic pulse radiation // Proc. SPIE. 1997. V. 3158. P. 209-219.