3392 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 58, NO. 10, OCTOBER 2010 simulated and measured efﬁciency is caused by the small opening in the Wheeler cap, Wheeler cap loss, possible shift of the DR after placing the cap atop the antenna, and the measurement/calibration errors. IV. CONCLUSION This communication introduced a high-efﬁciency on-chip antenna in SiGe MiMIC technology. The antenna consisted of an on-chip H-slot antenna and a rectangular dielectric resonator. A shielding mechanism was implemented to isolate the radiating section from the lossy sil- icon substrate. Using the Wheeler method a radiation efﬁciency of 48% was measured. This electrically small antenna has a relatively large bandwidth of 12% operating from 33 to 37 GHz. It was shown that adding the high-permittivity rectangular DR, improves the efﬁciency and matching of the antenna structure by 17 dB. Moreover, it was shown that removing the passivation layer on top of the slot improves the coupling between DR and H-slot antenna, which increases the ra- diation efﬁciency by 10%. This result was conﬁrmed by both measure- ments and simulations. REFERENCES [1] A. Shamim, L. Roy, N. Fong, and N. G. Tarr, “24 GHz on-chip an- tennas and balun on bulk Si for air transmission,” IEEE Trans. Antennas Propag., vol. 56, no. 2, pp. 303–311, Feb. 2008. [2] P. V. Bijumon, Y. Antar, A. P. Freundorfer, and M. Sayer, “Dielectric resonator antenna on silicon substrate for system on-chip applications,” IEEE Trans. Antennas Propag., vol. 56, no. 11, pp. 3404–3410, Nov. 2008. [3] N. Behdad, “Single- and dual-polarized miniaturized slot antennas and their applications in on-chip integrated radios,” presented at the IEEE Int. Workshop on Antenna Technology, iWAT 2009, Mar. 2–4, 2009. [4] B. G. Porter and S. S. Gearhart, “Impedance and polarization charac- teristics of H and IHI slot antennas,” IEEE Trans. Antennas Propag., vol. 48, no. 8, pp. 1272–1274, Aug. 2000. [5] K. M. Luk and K. W. Leung, Dielectric Resonator Antennas. New York: Research Studies Press, Jun. 2002. [6] K. Payandehjoo and R. Abhari, “Characterization of on-chip antennas for millimeter-wave applications,” presented at the Int. Symp. An- tennas Propag., Jun. 1–5, 2009. [7] H. A. Wheeler, “The radian sphere around a small antenna,” in Proc. IRE, Aug. 1959, vol. 47, no. 8, pp. 1325–1331. [8] A. D. Yaghjian and S. R. Best, “Impedance, bandwidth, and Q of an- tennas,” IEEE Trans. Antennas Propag., vol. 53, no. 4, pp. 1298–1324, Apr. 2005. Investigation Into the Effects of the Reﬂection Phase Characteristics of Highly-Reﬂective Superstrates on Resonant Cavity Antennas Alireza Foroozesh and Lotfollah Shafai Abstract—First we describe two frequency selective surfaces (FSSs), one capacitive and the other inductive, that are designed to exhibit identical high-reﬂection magnitude at an arbitrary frequency. These two FSSs are then employed as the superstrate of two RCAs having identical microstrip patch source. In order to determine the resonant conditions and obtain ap- proximate values for the antenna directivity, RCAs are initially analyzed using the well known simple ray-tracing method. Next, a full-wave analyzer (ANSOFT Designer v4.0), based on the method of moments (MoM), is uti- lized to thoroughly analyze the RCAs. Experimental results are provided to support the full-wave simulations, as well. In contrast to the prediction of the ray-tracing modeling, which is merely based on the reﬂection mag- nitude of the FSSs, it is pointed out that their phase properties have notice- able effects on the RCA gain. Second, two other RCAs are designed based on high permittivity and high permeability superstrates with identical con- trast. There, too, it is shown that the reﬂection phases of the RCA super- strates determine the air-gap heights which in turn affect the RCA gains. Index Terms—Antenna gain, antenna input impedance, antenna radia- tion patterns, frequency selective surface (FSS). I. INTRODUCTION A highly-reﬂective surface can be used as the antenna superstrate to substantially increase its directivity [1]. The phenomenon resulting in this signiﬁcant gain enhancement is based on multiple reﬂections be- tween the highly-reﬂective superstrate and the antenna ground plane similar to the Fabry-Perot resonator. Using a ray-tracing method, a simple formula has been derived in [1], which shows the relationship between the reﬂection magnitude of the superstrate and the relative in- crease in the antenna directivity by adding that superstrate. This simple relationship has been proven to be relatively accurate when highly-re- ﬂective capacitive FSSs, such as periodic patches or strips, have been employed as the RCA superstrates [2], [3]. However, the accuracy of this relationship has neither been studied nor veriﬁed in the literature, when inductive FSSs are used as the RCA superstrate. As it is known, the resonance length of the RCA is determined by the reﬂection phase of the FSS and the ground plane. It has been shown in [4] that for the RCAs whose superstrates are identical high-permittivity dielectrics but their ground planes are PEC and PMC surfaces, the RCA having PEC ground plane produces higher directivity and resonance length. Similar phenomena have been observed in [5], when metama- terial ground planes are utilized in designing RCAs. In [5], it has been shown that the antenna with shorter resonance length exhibits lower di- rectivity. Therefore, the purpose of this communication is to investigate the effects of the reﬂection phase of the FSS superstrates, as opposed to the ground planes carried out in [4] and [5], on the RCAs directivity. This effect is also studied for the RCAs having high permittivity or per- meability superstrate layers. Manuscript received November 06, 2009; revised March 12, 2010; accepted April 09, 2010. Date of publication July 01, 2010; date of current version Oc- tober 06, 2010. This work was supported in part by the Natural Science and Engineering Research Council of Canada (NSERC). A. Foroozesh and L. Shafai are with the department of Electrical and Com- puter Engineering, University of Manitoba, Winnipeg, MB R3T 5V6, Canada (E-mail: alireza@ee.umanitoba.ca, shafai@ee.umanitoba.ca). Color versions of one or more of the ﬁgures in this communication are avail- able online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/TAP.2010.2055810 0018-926X/$26.00 © 2010 IEEE