Although antennas are followed by RF front ends made of electronic components in most radio communication and radar systems, they are typically separated in design and fabrication. This paper provides some effort to unify these two entities to create new avenues for efficient system design. The paper provides examples in two categories; active integrated antennas, and retrodirective systems. In the first category, both antennas and electronic devices are benefiting from each other by placing the device within or in close proximity of the antenna. The latter typically provides circuit functions in addition to radiation capability. The second category is an interesting system capable of providing special radiation properties from the system in which antennas and devices work together.

Planar Antenna Arrays Using a Feed Network with Nonradiative Dielectric (NRD) Waveguide Ulf Schmid1, Wolfgang Menzel1 1University of Ulm, Microwave Techniques, Albert-Einstein-Allee 41, 89081 Ulm, Germany Tel.: +49-7 31-502 63 50, E-Mail: ulf.schmid@uni-ulm.de, wolfgang.menzel@ieee.org - Please use attached PDF file -

Introduction
The off-axis properties, i.e. the ability to steer the beam, of an inhomogeneous lens antenna, namely the half Maxwell fish-eye (HMFE) lens antenna [1], are investigated in this paper. Since the application aimed is the automotive radar, this study is carried out at 77 GHz.
The HMFE lens is composed of three homogeneous shells and is fed by a waveguide as shown in figure 1 and 2.

Simulation and Characterization
The normalized computed magnitude for four different off-axis configurations is shown in Figure 3 whereas the scan angle as a function of the off-axis displacement is reported in Fig. 4.

Conclusion
The ability to form multiple beams with a HMFE lens has been investigated in this paper. Measurement results of a three-shell HMFE lens fed by a waveguide, displaced linearly at the bottom of the lens, show the validity of this study.

This work was supported by Rennes Metropole.

[1] Lafond, O., Himdi, M., Rondineau, S., Fuchs, B. : 'Lentilles inhomogènes à gradient d'indice de type Oeil de Poisson de Maxwell, système d'antenne et applications correspondants', french Patent Request 0507188, Jul. 5th, 2005.

[2] CST Microwave Studio, ver. 4.2.

Recently, there has been an increasing interest in the development of automotive radar sensors for adaptive cruise control and pre-crash safety systems, using a millimeter-wave band from 76 to 77 GHz. For these systems, a field of view (FOV) with a length of 150 m, covering about 20deg. is sufficient, which can be provided by most sensors on the market today. In contrast, new developments like stop & go adaptive cruise control and collision avoidance assist systems require the observation of a broader FOV up to 60deg. with a maximum range of 60m to deal with cut-in situations. An electronically scanned composite right/left-handed (CRLH) leaky wave (LW) antenna using the varactor diodes has been presented (1). This antenna has the advantage of wide beam scanning performance at the fixed frequency, but the diodes are too lossy to use in the millimeter-wave band. We propose a novel structure of a frequency-independent steerable CRLH LW antenna for the millimeter-wave band applications. The antenna has the advantages of wide beam scanning, high gain and a simple structure. The proposed antenna has features wherein a movable dielectric slab is placed above the CRLH LW antenna, and the radiation angle can be steered by changing the distance between the slab and the antenna using compact actuators.

Figure 1 shows the structure of the proposed CRLH LW antenna with a movable dielectric slab. The CRLH LW antenna consists of a series connection of unit cells which simple gap capacitors and symmetrical straight shunt stubs are used in. The dielectric slab is placed close to the CRLH LW antenna, and the effective dielectric constant varies by changing the distance h between the microstrip line patterns and the dielectric slab, so the radiation angle can be steered at the fixed frequency. The prototype CRLH LW antenna shown in Figs. 2 was fabricated and tested in the millimeter-wave band. A Teflon substrate was used as the movable slab. Its relative dielectric constant is 2.2, with a thickness of 0.127 mm. Moreover, slots have been added to the CRLH LW antenna to control the aperture amplitude distribution of the array antenna in order to enhance antenna gain. Backward-to-forward beam scanning characteristics at 76 GHz have been demonstrated successfully by the measurements. A wide scanning angle from 73 to 114 deg. has been achieved experimentally. The peak gain of the antenna with the slots is 12.3 dBi, which is 1.0 dB higher than that of the antenna without the slots. The sidelobe level is also improved by 7.8 dB. The aperture efficiency of the antenna is 25.3%. In the near future, it may be possible to realize automotive radar antenna systems with a high gain over 20 dBi by using the proposed antenna.

Reference (1) S. Lim, C. Caloz, and T. Itoh,"Metamaterial-based electronically controlled transmission-line structure as a novel leaky-wave antenna with tunable radiation angle and beamwidth$B!&(B IEEE Trans. Microwave Theory Tech., vol. 52, no. 12, pp. 2678-2690, Dec. 2004.

This paper presents a class of simple antennas that produce diversity action and offer performance improvement over conventional single-port (nondiversity) antennas. It is characterized by two features adding value to published structures. Firstly, two orthogonal polarizations can be obtained for each of the three switchable patterns. Secondly, the optimization of the terminating reactances is based on the minimization of the calculated correlation factor between the patterns (typ. <0.7).

The antenna operating at 5.8 GHz is depicted in Fig. 1. The biasing circuit is not represented in the Figure. It is a three-element parasitic antenna array where aperture-coupled patch antennas are used as radiating elements. The central patch antenna can be excited by one of the two orthogonal crossed slots, the slot selection resulting either in an E-plane or H-plane coupling with the adjacent parasitic patches. The selected slot also enforces one of the two linear orthogonal polarizations. The slot selection is performed with a SPDT connected to the feeding microstrip line below the ground plane. To keep the same diversity patterns for both polarisations, the gap between adjacent patches is adjusted so that equal E-plane and H-plane couplings are obtained. Each of the slot pairs in the parasitic patches is loaded by a switchable stub (pure reactance) through a SPDT. The stub lengths are adjusted by pin diodes. The SPDT role is to select a slot parallel to the slot of the central patch to keep the same polarization in the three patches.

To build the optimization charts mentioned above, the S-parameters of the structure and the active element patterns of each patch antenna are extracted from an electromagnetic simulation (HFSS). These data are then exported in a dedicated software (Matlab) to calculate the return loss and radiation pattern of the antenna array for any loading reactance. An optimization procedure is finally developed to select the reactive loads (or stub lengths) for the best trade-off between beam direction and return loss.

Measurements of the six switched patterns are presented in Fig. 2. Calculated correlation coefficients between all patterns range from 0.05 to 0.3. Based on an indoor measurement platform and a statistical treatment of data, the diversity gain combining all 6 channels (3 patterns times 2 pol) has been measured. Depending on the antenna position (along a wall, on the ceiling...) and the emitter features, measured diversity gains ranging from 9 to 13 dB have been observed for a 0.01 outage.

During the last two decades, a very wide variety of lens antennas has been proposed (e.g. G. Godi, IEEE TAP, vol. 53, April 2005). Among those, the so-called substrate lens antennas are of particular interest because they allow the design of high-gain and low-cost radiating structures with reduced trapped surface modes. Nevertheless, this kind of lens suffers from one main drawback: they only generate pencil beams whose directivity, gaussicity and direction of maximum radiation are mainly adjusted, at first order, by the diameter of the lens, the height of the cylindrical extension and the offset of the primary feed(s), respectively. On the contrary, the main motivation of shaped lenses is to produce shaped beams, for instance for ultra-low side lobes or constant flux illumination.

The purpose of this paper is twofold: (1) Development and description of a powerful CAD tool for the optimization of multi-shell arbitrarily-shaped substrate lens antennas for shaped beam applications, (2) Design and comparison of the radiation performance of compact optimized single- and double-shell lens antennas.

The aim of the design procedure consists in finding the best shape for each of the dielectric shells so that the radiation characteristics of the lens comply with user-defined target specifications (e.g. far-field radiation pattern template, such as Gaussian, secant-squared or flat-top beams). Compared to the prior state of the art, this CAD tool allows the global optimization of arbitrarily-shaped 3D lenses, whereas the most recent papers about lens optimization only deal either with the local optimization of 3D lenses (gradient-type approach), or with the global optimization of axis-symmetrical lenses. Here, our design methodology is based on the use of a binary Genetic Algorithm (GA) combined with the hybrid Geometrical Optics / Physical Optics (GO / PO) approach implemented for arbitrarily-shaped 3D multi-shell configurations. First we describe in detail the capabilities of our tool with emphasis on (i) the analysis of dielectric lenses with arbitrary shapes (representation of 3D shaped lens boundaries and differential coding) and on (ii) the GA solver (fitness, convergence). Then three compact lens antennas fed by printed patch antennas and radiating an elliptical Gaussian beam, are designed in Ka-band. The first two antennas are single-shell lenses made of Rexolite and Ceramic HIK-500 K=6 respectively, and the double-shell configuration combines both materials. Mechanical constraints (convexity, maximum diameter) are taken into account during the optimization procedure of double-shell structures. Whereas low-permittivity large extended hemispherical substrate lenses do not suffer from the effects of multiple internal reflections, we show here that the latter have a significant influence on the radiation performance of reduced-size shaped lenses. To maintain a compact configuration, we propose the use of a high-k material (HIK) for the inner shell and low-k material (Rexolite) for the outer shell. This alternative allows a significant improvement of the radiation characteristics and power transfer efficiency (up to 86%), compared to single-shell high-k configurations.

The 63-64 GHz band has been set aside by CEPT for inter-vehicle and roadside to vehicle communications (IVC and RVC). Roadside beacons would be spaced along the road at regular intervals. This paper describes the development of antennas at Birmingham University for the Miltrans project, led by BAE Systems in collaboration with Qinetiq, Panorama Antennas. The small sizes of antennas in the mm-wave band make dimensional tolerancing and loss reduction more challenging than meeting the 1.57% bandwidth requirement. Designing for a wider bandwidth can ease the tolerance problem.

In the ideal radiation pattern for the elevation plane, the gain falls off sharply below 8° elevation to reduce the cancellation effect due to the reflected signal from the road. A link budget with 10dB margin over 500m range (for 1km beacon spacing), assuming identical antennas at either end and a source power of 60mW, requires antenna directivity of ~21dBi. For optimum IVC and RVC coverage, either a broad beam pattern in the azimuthal plane, or a switchable pencil beam is required. In the first practical demonstration of a 63 GHz vehicular communications system, a fixed beam antenna design was used.

The antenna options considered included tapered printed slots, microstrip and air spaced patch arrays and slot arrays.

The final design selected was a slot array, using a low-cost multilayer metallic feed network produced by precision chemical etching, with approximate dimensions 60×35×4mm. In the measured antenna pattern, a narrow beam in elevation is the result of the array factor, whilst the broader beam in the horizontal plane is consistent with the required footprint for a range of curved and straight roads scenarios. The gain of 15 dBi is lower than the ideal discussed above, but is a practical upper limit taking account of manufacturing issues. The nominal 1 km beacon spacing may thus need to be reduced. In the system demonstration, RVC and IVC ranges of 165 m were demonstrated at 6 Mb/s, and 110 m at 18 Mb/s. Adjacent lane IVC coverage was demonstrated with static vehicles up to 70^o degree from boresight, and with passing speeds up to 70 mph at approximately 45^o. Further details of system tests and their relationship to the antenna properties will be reviewed in the paper.

The paper will review the range of layered metallic patch and slot arrays studied, and consider manufacturing issues and possible wider applications, such as dual band versions for combined communications and radar systems.

Fig. 1. Slot Array Antennas Mounted on Vehicle for System Demonstration.

Fig. 2 Slot Antenna Array Patterns.

Using of active array antennas has been shown as a suitable technique to improve the figure of merit (G/T) of a radio-link, increase the effective gain (the parameter that must be introduced in Friis equation to analyse the link) or make a better distribution of the power what can reduce the number of amplifiers used in the feeding network [1]-[2].

Besides of these characteristics other improvements can be achieved by studying the performance of the corresponding service where the antenna is used. Then, for conventional line-of-sight point-to-point radio-relay system antennas [3], it can be seen that the recommendation is devoted to the gain of the radio-relay system. As one of the characteristics of active antennas is that it can improve the effective gain [4], a trade-off between the increase of effective gain and the reduction of antenna size can be done. The effect of including active elements to increase the effective gain while maintaining the radio-link gain implies a reduction in the antenna size. The paper presents the reduction that can be achieved in the antenna size by using of an active array. A study of the level of active element integration (patch level or subarray level) has been done to obtain the optimum situation between the size reduction factor and the power associated to any of the amplifiers in the active array.

The proposed line of sight reflector antenna is circular polarised antenna with a gain of 23 dBi working at a frequency of 3.5GHz in a receiving way. The active array has been done with printed radiators with a slot in the printed patch to achieve circular polarisation. This radiator has been chosen since it provides a more compact design. The radio electrical characteristics of this radiator are axial ratio 1 dB and gain larger than 4 dB. The overall array is composed of four subarrays. Each of the subarrays is composed of four circularly polarised patch antennas. The integration level (patch or subarray level) has been studied in order to achieve the optimum trade-off between the following parameters: G/T, reduction factor and power distribution. This study will be shown in the whole paper.

Finally, the proposed array composed of four subarrays provides a G/T of -11.27 dB/K in front of the reflector G/T of -19.65 dB/K. This implies an improvement of more than 8 dB. The size has also been reduced in a ratio larger than 6. For a gain in the reflector antenna of 23 dBi its size for an efficiency of 65% is 24.5 ¦Ë2 while for the proposed active array is 4¦Ë2. This reduction has implied an increase in the antenna beamwidth; this increase is not critical since in the recommendation [3] the key parameter is the system gain to be introduced in the Friis equation.

[1]D. Segovia, V. Gonz¨¢lez: ¡°Progress in Active Antenas: Broadbanding, Mixing and Diplexing¡±, Invited ICECOM 2005. [2]Y. Qian, T. Itoh, ¡°Progress in Active Integrated Antennas and Their Applications¡±, MTT. Nov. 1998, pp. 1891-1900. [3]Recommendation ITU-R F-1245* [4]D. Segovia, V. Gonz¨¢lez, J.L. V¨¢zquez, L. Incl¨¢n and C. Mart¨ªn: ¡°An active broadband transmitting patch antenna for GSM-1800 and UMTS¡±, M. and Optical Technology Letter, june 2004

Applications such as automotive adaptive cruise control as well as imagery systems for unmanned air and ground vehicles have prompted the need for millimeter-wave electronically scanned polarimetric radars that are lightweight, low-cost, and low-power. Also, with the increasing need for more advanced radar systems capable of distinguishing the shape, size, and material properties of a target, there has been a growing focus toward the design of broadband dual-polarized antennas for millimeter-wave polarimetric radar systems. As a result, the selection and design of the antenna element is crucial in order to realize compact element spacing, independent control of both polarizations, and broad bandwidth. In this presentation, a new millimeter-wave dual-polarized horn antenna array is presented, which is perfectly suited for low cost phased array radar with advanced target identification capabilities.

To maintain compact size while allowing for the independent control of both the vertical and horizontal polarizations, an L-shaped horn antenna was investigated and optimized for 34 to 40 GHz. In contrast to conventional feeding mechanisms such as orthomode transducers, two orthogonally positioned waveguides feed the horn antenna, which do not exceed the half-wavelength spacing in both the E- and H-planes. Following the orthogonally positioned waveguides, a low-loss broadband ninety degree waveguide step-twist junction is implemented for one of the two waveguides. This allows for the in-line waveguide to microstrip transitions, which are placed onto either side of a common ground plane. Circuitry for the control of the phase and amplitude may be easily integrated with the microstrip feed lines.

The L-shaped horn antenna is scalable with frequency, and in this presentation, a center frequency of 35 GHz was considered, which corresponds to an aperture size of 4.35 mm by 4.35 mm. Due to the compact size of the millimeter-wave antenna and waveguide feed, a stereolithographic process is used to fabricate a 1x10 linear array, which is metallically plated. Dielectric material with relative permativity 3.0 is used to fill the antenna and waveguide feed array, which also allows for the low loss waveguide to microstrip transition on TMM3 substrate with relative permativity 3.27.

Through measurements of the L-shaped horn antenna and waveguide feed, the 10 dB return loss for both waveguide ports showed better than 16% bandwidth for 34 to 40 GHz operation. In addition, better than 15 dB isolation was achieved throughout this frequency range. The measured gain of the antenna at 35 GHz is 5.3 dB with 19 dB cross-polarization discrimination. The L-shaped horn antenna demonstrates approximately 96 degree beamwidth in the E-plane and 70 degree beamwidth in the H-plane, which therefore allows for the wide-angle beam scanning for phased arrays. Finally, the antenna array was implemented as part of a broadband millimeter-wave phased array radar and better than 5 cm range resolution was demonstrated.

Rescue helicopters have to be versatile for any weather conditions. By flying at low altitude they are facing targets, such as power lines [1], that may not be detected in the visual flight rules. In order to overcome this problem, a global system achieving an image fusion between a CCD, an infrared camera and a MMWave FMCW radar has been developed at 94 GHz [2]. Cost, gain but specially weight and overall dimensions of the radar are crucial since we have to consider an on-board system. An original antenna, of Cassegrain type provides high gain (44 dBi) but his cost and overall dimensions are still critical as shown in fig.1. Printed reflectors and reflectarrays offer an alternative solution to these conventional antennas [3].
Several printed Fresnel reflector were realized and tested on board for this application. The first one, is an enhanced Fresnel reflector using circular elements [4] combining (l/8) et (l/4) phase compensation zones as shown in fig. 2. Radiation pattern is given in figure 3. The second one (fig. 1) whose radiation pattern is given in fig. 4, is a folded one [5]. Particular attention was put on the primary sources of the reflector by using a small FSS in order to achieve return loss levels bellow -25 dB. Fig. 5 shows gain measurements and comparisons with the classical (l/2) FZP [6]. The enhanced Fresnel reflector achieves a maximal gain of 38 dBi at 94 GHz which is 3 dB and 5 dB higher than the folded and half-wavelength Fresnel reflectors respectively.
Finally, antennas were tested on board. Contrary to anechoic chamber measurements, best results were obtained with the folded reflector. Power lines were detected up to 600m instead of 800 m with the Cassegrain antenna.The reason might be its better stability to the high vibrations existing during helicopters flights. Nevertheless, printed Fresnel reflectors have demonstrated their use for on-board systems. Some upcoming efforts will to be made in order increase the gain up to 40 dBi.
References

[1] K.Sarabandi, M. Park,''A Radar Cross-Section Model for Power Lines at Millimeter-Wave Frequencies" IEEE Transaction on Antennas and Propagation, vol 51, NO 9, pp.2353-2360, September 2003.
[2] K.Yamamoto, K.Yamada, N.Yonemoto, H.Yasui, B. D. Nguyen, C. Migliaccio, Ch.Pichot, ''The performance of 94 GHz Radar for Obstacle Detection '', Proceedings of the International radar symposium, Berlin 6-8 September 2005, Germany, pp.147-151.
[3] D. Pozar, "Microstrip Reflectarrays: Myths and Realities", JINA, International Symposium on Antennas, Nice 8-10 November 2004, pp.175-179.
[4] Y.J. Guo, S.K. Barton, "Phase correcting Zonal Reflector incorporating rings," IEEE Trans. Antennas Propagat., vol AP- 43, pp. 350-354, April 1995.
[5] W. Menzel, D.Pilz, M. Al-Tikriti, ''Millimeter wave folded reflector antennas with hign gain, low loss, and low profile'', IEEE Antennas and Propagation Magazine, vol 44, pp.24-29, June 2002.
[6] B. Huder, W. Menzel, "Flat printed reflector antenna for mm-Wave applications," Electron. Lett., vol. 24, no. 6, pp. 318-319, March 1988.