Jung, M.

Lambrecht, A.

Schmitz, J.

Wiesbeck, W.

Zwick, T.

English

KITopen

Rotman-Lens as a True-Time-Delay Beamformer at Low UHF Frequencies   A. Lambrecht(1), T. Zwick(1), Senior, W. Wiesbeck(1), Fellow, J. Schmitz(2), M. Jung(2) (1) Institut für Höchsfrequenztechnik und Elektronik, Universität Karlsruhe (TH), Engesserstraße 5, 76131 Karlsruhe, Germany, http://www.ihe.uni-karlsruhe.de (2) Rheinmetall Waffe Munition GmbH, Heinrich-Ehrhardt-Str. 2, 29345  Unterlüß, Germany   Introduction  As a well-known broadband device with simple fabrication processes the Rotman lens is applied in various fields. Because it substitutes a whole “switched line length” concept in one passive structure it is investigated here for a true-time-delay beamformer in the frequency range of 450 MHz to 1000 MHz. The problem of the Rotman lens is, that it is a traveling wave structure, which has to be sized in the order of several wavelengths to give a proper functionality. This leads to hard requirements considering the size of the Rotman-lens, which will be determined mainly by cost and size of the available substrate. The design of Rotman lenses is hard at lower frequencies and this is the first time this structure was investigated for the low UHF band. In general there exist various methods to steer a beam in an antenna array [1]. To get the proper phase distribution along the used dimension of the array phase shifters are needed. Normally in narrow-band systems these are ferrite phase shifters or PIN-diode phase shifters, i.e. constant-phase-type (CPT) phase shifter. The problem forming a proper wave front in the desired direction becomes severe, when the bandwidth and the steering angle grow and true-time-delay (TTD) phase shifters are needed.  The CPT only works monochromatically or narrow-band. So for the phase front of the feeding there is only one pair of frequency and steering angle ! f0 ,"0  where it is consistent with the phase front of the radiated field. For another frequency and angles the following condition holds: ! f0 + "f( ) # sin $0 + "$( ) = f0sin $0( ) (1) So for a change in frequency we get for the change in angle: ! "# = arcsin $( ) %#0 $= f0f0 +"f&sin #0( ) (2) By taking the middle frequency as ! f0  and the maximum deviation at the end of the band ! "f =275 MHz an angle error of ! "#= –14.16° occurs. A TTD phase shifter does not fix the phase but, of course, the traveling time of the electromagnetic wave: ! "i=1c0N + 12# i$ % & ' ( ) d sin *0( )  (3) This equation assumes an antenna array with spacing of ! d  and ! N  elements. ! "i is the delay for the i-th element of the array, the reference point is the middle of the array. This leads to frequency dependent phasing of the array, compensating the frequency change of the traveling wave, leading to a stationary far-field solution. There exist various methods to realize a true-time-delay beamforming network, i.e. switching of transmission lines (microstrip, coaxial, waveguide), switching of optical lines, use of traveling wave structures (Rotman lens, other lenses). The system concept comprises a 1 x 4 phased array antenna, for which the Rot-man lens provides the ultra-wideband phasing of the antenna feeds. The lens is fed by SP9T PIN-switch, which provides the different scan angles positions. The system specifications define a scan angle range of ±40°, a frequency range of 450 MHz – 1000 MHz (bandwidth of 75%), desired VSWR of 2:1 and the num-ber of beam directions is 9. The system will be initially used in CW mode.  Realization of the Beamformer with a Rotman-Lens Design  The basic design equations for Rotman Lenses can be found in [2], [3]. These are based on ray-optical modeling of the parallel plate waveguide with feeding points on the arrayport and beamport contour [4]. There also exist some design rules for the optimum port layout, when using microstrip technology [5]. A numerical field solver is used to calculate the Rotman-Lens structure.  The prototype for the frequency range of 450 MHz to 1000 MHz is etched on an Arlon substrate with a dielectric constant of 10.9 and dimensions of 609.6 mm x 914.4 mm x 3.23 mm. In Fig. 1 the prototype is shown. So the available space is only ! 3"450MHz in the wave traveling direction (x-dimension), because the y-dimension (914.4 mm) has to be used for designing the dummyport region of the lens. Often Rotman-Lenses have extensively formed dummy regions; geometrical reasons restrict that here. The Fig. 2 shows the surface currents for excitation of BP1 and BP5. For BP1 a non-uniform distribution along the arrayport side is ob-servable. This is partly due to the small y-dimensions available for the dummy region, i.e. in terms of wavelengths on the beamport side 1.26λ450MHz / 2.81λ1000MHz and on the arrayport side 0.88λ450MHz / 1.95λ1000MHz. The structure has 14 dummyports on the sidewall-region and 3 dummyports between the array-ports. The lens without absorber attached to the dummyport region shows a sufficient matching of the beamports only from 600 MHz onwards; with the absorber (dark parts) the matching is better, approximately from 570 MHz onwards. The used absorber material is C-RAM FDSS®. An excellent phase behavior in the whole frequency range is achieved (see Fig. 4). The gain patterns show the stationary beams for BP1 and BP5. These gain patterns were synthesized by using measure-ment results. The grating lobe for higher frequencies visible in Fig. 4 left for BP1 is due to the pre-defined antenna array spacing of 30 cm, corresponding to the free-space wavelength at 1 GHz. In these gain patterns a measured Vivaldi an-tenna with a frequency dependent gain is used as single element for the array. This and the transmission variance of the Rotman-Lens lead to the gain deviation visible in Fig. 4 down left and down right.  Fig. 1 The prototype Rotman-Lens with N-Connectors and absorbing material (C-RAM FDSS®) in the dummy region.         Fig. 2 left: surface currents at 450 MHz, right: surface currents at 1000 MHz.   Fig. 3 left: return loss of beamports BP1 to BP9, right: return loss of arrayports AP1 to AP4. frequency in MHz frequency in MHz return loss in dB return loss in dB   Fig. 4 left: accumulated phase for BP1 and synthesized gain pattern, right: accumulated phase for BP5 and synthesized gain pattern. If the system operates in pulse-mode the time domain array factor gives the su-perposition of only 4 pulses from each sending antenna, so no grating lobe will occur. Nonetheless the appearance of grating lobes for CW-mode translates into additional dispersiveness for pulse-mode [6].  Conclusion  The Rotman-lens was investigated for the first time at low UHF frequencies. The lens structure has a very small dummyport region compared to wavelength. Fur-ther research will concentrate on improvements of the matching and hardware implementation into a whole array and pulse-mode operation.  References:   [1] Johnson, R. C., "Antenna engineering handbook", 3. ed. Norwich, NY : Knovel; New York [u.a.] : McGraw-Hill, 2006. [2] Rotman W., Turner R.F., "Wide-Angle Microwave Lens for Line Source Applications",  IEEE Transactions on Antennas and Propagation, 1963. [3] Hansen R. C., "Design Trades for Rotman Lenses", IEEE Transactions on Antennas and Propagation, vol. 39, no. 4, April 1991. [4] Simon P. S., "Analysis and Synthesis of Rotman Lenses", 22nd AIAA International Commu-nications Sattelite Systems Conference & Exhibit, 2004. [5] Hall L., Hansen H., Abbott D., "Rotman lens for mm-wavelengths", Proceeding of SPIE, vol. 4935, 2002. [6] W. Sörgel, C. Sturm, and W. Wiesbeck, "Impulse Responses of Linear UWB Antenna Arrays and the Application to Beam Steering", Proceedings 2005 IEEE International Conference on Ultra-Wideband, pp. 275-281, Zurich, Switzerland, September 2005 angle in degree frequency in MHz frequency in MHz angle in degree gain(ψ) in dBi accumulated phase in deg accumulated phase in deg gain(ψ) in dBi 

Rotman-Lens as a True-Time-Delay Beamformer at Low UHF Frequencies

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Rotman-Lens as a True-Time-Delay Beamformer at Low UHF Frequencies

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