Microwave Engineering Europe - April 2009 - (Page 23)

ANTENNAS 23 when the envelope of the element becomes square. Since sinusoids are folded monopoles, their unfolded length is constrained to be somewhat greater than λ0/4. This constraint imposes an equivalent constraint on the sum of the two sides of the envelope, because of spatial uniformity. Turning the envelope into a square produces the envelope with the shortest diagonal, hence the smallest spread on the PCB. To enforce the square envelope, the dimensions of the sinusoidal antenna are constrained according to (1): ( (1) II). We have studied the performance of the sinusoidal up to the 6th iteration. As is clearly seen in Table I, the size of the element saturates at 8.6 mm × 8.6 mm. Thus, in terms of miniaturization, there is little sense in designing beyond the 5th iteration. Because of the dual-mode operation, the reﬂection coefﬁcient at midband is S11 = -12 dB, i.e. the antennas are non-resonant. 4. Building compact multi-element antennas A. Initial considerations Antennas for wireless sensors cannot be characterized, either through simulation or measurement, by being located on an inﬁnite or large conducting ground plane. With miniature integrated antennas operating at 2–3 GHz, the ground plane dimensions will certainly be less than the operating wavelength and will closely approach the size of the antenna. In fact, the ground plane itself becomes an integral component of the radiating structure and must be included in determining the effective electrical size of the radiator [4]. It is well understood from small antenna theory that, if the size of an antenna is decreased arbitrarily, all important attributes will suffer [3]. For small radiators spread across a rich scattering environment, gain and radiation pattern seem to be irrelevant [9]. Radiation efﬁciency paints a more accurate picture of the antenna-under-test. To quantify the trade-off between fundamental antenna parameters, we used the ratio in (2) as a Figure-OfMerit (FOM) for the antennas we designed. The goal of the designer is RF testing out of control? Only Keithley gives you the RF test tools you need to rein in today’s devices and tame tomorrow’s challenges. 3. Numerical results The antennas were designed and simulated by means of a Transient Solver [8], which is part of a fullwave E/M simulator that uses the Finite Integration Technique (FIT) to reformulate Maxwell’s integral equations into the so-called ‘Maxwell Grid Equations’. In the time domain, by applying Yee’s spatial discretization and time-stepping scheme, FIT results in the same set of equations as FDTD. The numerical results in Fig. 3 and Fig. 4 show the merits of the sinusoidal scheme. The antennas are very wideband, covering the range 2.0–3.5 GHz. They clearly behave as dual-mode antennas. The achievable VSWR bandwidth is about 68 percent greater than the one reached by a meander-line monopole of corresponding size and radiation efﬁciency [8]. Moreover, their total efﬁciency is very good. These results are valid for the nominal dimensions of the GNDP (see Table I and Table MODEL 2920 RF SIGNAL GENERATOR MODEL 2820 RF SIGNAL ANALYZER ■ Test the most complex signal structures, including 802.11n WLAN MIMO and 802.16e Wave 2 WiMAX. Conﬁgure a 4x4 MIMO test system costeffectively. Generate and analyze signals up to 6 GHz repeatybly and accurately with our instruments’ software-deﬁned radio architecture. Reduce your time to market and cost of test with MIMO systems optimized for R&D and production test. ■ ■ ■ Figure 6: A perspective view of the 2-element compact array. The FR4 substrate is transparent so that the GNDP is visible. In this snapshot the interelement distance is 18 mm (0.15λ0). Go to www.keithley.com/tame and try a demo. www.keithley.com/contact info@keithley.de Microwave Engineering Europe ● April 2009 ● www.mwee.com http://www.keithley.com/tame http://www.keithley.com/contact http://www.mwee.com

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Microwave Engineering Europe - April 2009 News Contents Comment MMICs/RFICs: MMICs for Broadband Receiver Applications Antennas: Mutual Coupling Reduction in Compact Arrays for Wireless Sensors via a Pre-fractal Defected Ground Structure Repeatable Characterization of Distortion Caused by Nonlinearities in Wideband Communication Systems Products Calendar

Microwave Engineering Europe - April 2009

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