A NEW TWO-FLARE-SHAPED UWB ANTENNA ELEMENT

A novel compact UWB antenna is based on using of two smoothly shaped flares of a simple geometrical structure. The first flare, larger in size, serves as an antenna base, reflector and one of the radiator arms. The second flare, smaller in size, operates as the other arm of the radiator. An apparent idea is exploited in this design that an “ideal” UWB antenna would operate as a broadband well-matched transformer for the current at its driving ports to the radiated fields that leave the antenna aperture. Besides terminal matching, the antenna would provide a necessary spatial distribution of the radiated energy. In order to create such a current-to-field transformer the flares of the proposed antenna are properly shaped and driven through direct connection to coaxial cables without balun. The antenna is studied through numerical full-wave simulations and tests with several prototypes. The achieved performance in the band >5:1 involves <-10dB return loss, >10dB front-to-back ratio, constant >5 dBi directive gain, constant beamwidth, and suitability for pulse transmission with minimized distortions. 1. Antennas for UWB Systems Previously and in the coming years, there is a great interest in short range high-speed data transmissions, high-resolution sensing and ranging using the Ultra-Wideband (UWB) technology [1-3]. Unlike traditional narrowband systems, their UWB counterparts operate by employing very short electrical pulses of nanosecond and subnanosecond duration resulting in very wide transmission bandwidths ranging from several hundred megahertzes up to several gigahertzes. Such huge bandwidths must be supported with suitable UWB antennas that behave unavoidably as bandpass filters [4,5]. As a result, UWB antennas must be properly designed to enhance the potential advantages of short-pulse signaling for communication, ranging, sensing and other applications. For many applications, minimal pulse distortion is the primary focus because useful information may be contained in the signal shape or precise timing of the signaling pulses. Minimized pulse distortion is equivalent to supporting clean wellconfined impulse responses in time-domain or “flat gain” and “linear phase” for frequency domain transfer functions [4]. Both of these quantities collect the key electrical features of individual antennas such as: (1) input impedance bandwidth and (2) radiation features (directive gain, pattern shapes, beamwidth, front-to-back ratio, etc.). UWB antenna designer may chose among known antenna solutions or create new ones to meet diverse system-related requirements, electrical, mechanical, manufacturing and cost constraints. Some authors considered historically an ideal UWB antenna that radiates time derivatives of the input signal [6] but this principle is not helpful to design practical UWB radiators. Rather other rational and physics-based guidelines could be more productive [7]. The presented antenna has been developed to provide a well-behaved radiator operating as a single element and/or array element in UWB time-domain radar and telecommunication systems. Specifically, the antenna must meet the following key functional demands: (1) broadband impedance matching to the 50 Ω; (2) direct coaxial cable feeding without balun; (3) minimized distortions for pulse transmission, (4) good radiation features to provide well-confined beams with minimum front-to-back ratio; (5) simplicity and compactness of geometrical shape, (6) suitability for easy prototyping and manufacturing at a reduced cost. Reviewing the UWB antenna related literature [7-13] leads to a conclusion that neither of such known radiators like those schematically sketched in Figure 1 can meet the above demands. In general, the impedance bandwidth is better supported in protruded antennas like TEM horn, tapered slot and log-periodic. The log-periodic dipole array is back-fire antenna and is highly dispersive. Directive radiation patterns are provided by using apertures of larger extent and/or backside reflectors, e.g. in impulse radiating antenna that is large. Furthermore, the use of a reflector to improve front-to-back ratio degrades impedance bandwidth, e.g. a broadband dipole over a ground plane. Also, the most of the antennas in Figure 1 excepting the trapezoidal one [9,10] are balance-fed structures that require balun for feeding through coaxial lines. As a rule, balun has limited bandwidth that impacts additionally on the whole achievable bandwidth. Figure 1. Sketches of canonical antenna shapes (from the left to the right): TEM-horn, tapered-slot (Vivaldi), dipole with backside reflector, trapezoidal antenna, impulse radiating antenna (reflector with feeding TEM-horn), log-periodic dipole array. 2. Novel UWB Antenna Design Concept This work deals with a new type of UWB directive low-dispersion antenna [14] that has some superior characteristics with respect to other known UWB radiators. A simplified geometrical sketch of this antenna is in Figure 2a and its first prototype made in Kiev in 2003 is pictured in Figure 2 b-c. The antenna radiates in its broadside direction, normal to its aperture. It is linearly-polarized, similar to that for the TEM horn, with the same flare orientation but with more efficient using of physical space when providing comparable terminal and radiation characteristics. As a matter of fact, an old and well-known idea, see e.g. [7], is exploited in the antenna design. This idea states that a well-behaved radiator must act as a well-matched transformer from the current at its driving port to the radiated fields leaving the antenna aperture and providing necessary distribution of the radiated energy in space. Such a current-to-field transformer is created by two smoothly shaped metal flares of the proper geometrical configuration. In particular, the first flare, larger in size, is called the “reflector”, Figure 2a. It serves simultaneously as an antenna base, its backside reflector and one of the radiator arms of an asymmetrical TEM-horn-like radiator. The second flare, smaller in size, is called the “tongue”, Figure 2a, and operates as second arm of the radiator. The antenna feeding network is demonstrated in Figure 2c where the 50-Ω UHF coax connector is mounted from its backside, connected by its ground contact to the reflector and by the central pin to the tongue.