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To model the path-loss characteristics of microwave propagation in urban environments, we performed measurements at frequencies of 3.35, 8.45, and 15.75 GHz in metropolitan Tokyo. The actual breakpoint (BP) distance was shorter than the theoretical one because of the influence of vehicles, pedestrians, and other objects on the road. This effect can be characterized in terms,of the effective road height. We also found that the BP disappeared when the mobile-antenna height approached the effective road height. In non-line-of-sight (NLOS) areas, the corner losses and attenuation coefficients were greater than those of the UHF band. The attenuation coemcients increased with LOS distance between the base station and the corner into NLOS area. I. lntroductionMicrowave bands will likely be used for the fourthgeneration mobile-communication systems [ 11, but research still needs to be done on propagation characteristics. To model microwave path-loss characteristics. we performed measurements at the frequencies K.) of 3. 35, 8.45, and 15.75 GHz in urban environments.In line-of-sight (LOS) areas, we observed that the measured breakpoint (Rbp) was shorter than the theoretical one (R,) for all frequencies [2]. This is because of the bias of the ground level (h), which is caused by vehicles, pedestrians and other objects on the road [3]. This ground level can be referred to as the effective road height.We also found that the breakpoint (BP) disappeared when the mobile-antenna height (h,) approached h. Based on the BP characteristics in the microwave band, we give both upper-and lower-bound evaluations for the LOS path losses in urban environments.Path-loss characteristics in non-LOS (NLOS) areas have been studied for the UHF band [4] [5]. The path loss can be described with a constant attenuation coefficient, except at comers, where multipaths cause significant 'comer loss'. We compared these characteristics with those for the microwave band. Measurement DescriptionThe measurements were taken during the day in urban areas of Tokyo, where 10-to 15-story buildings line both sides of the streets. Figure 1 shows the experimental area. The base station (BS) was set up at two different locations: on a street 27-m wide (BSI) and on a street 11 -m wide (BS2). These streets were flat and straight with a LOS distance of about 1 km. The mobile station (MS) moved continuously along these streets and then turned (64 or 429 m from the BS2) onto NLOS streets whose widths were about 35 and 44 m (respectively). ElE €3 H 1.-0 in -64m -429 m Fig. 1 Measurement site in metropolitan Tohio.The transmitter of the measurement system is capable of transmitting a 10-W continuous wave in the microwave band. The transmitting BS had an antenna with a height ( h b ) of 4 m. The receiving MS was mounted on either a measuring vehicle or a handcart. A 2.7-m-high receiving

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A variable-capacitance (varactor) diode is widely applied in microwave devices, which utilize the characteristic property that the junction capacitance is varied by an externally applied voltage. In applications of capacitance-controlled devices, such as voltagecontrolled oscillators, the capacitance is varied by the control-bias voltage, whereas the variation in capacitance for microwave RF-signal voltages should be minimized in order to suppress higher-order distortions in the output signal. The capacitance-voltage characteristic of a varactor diode is characterized by a slope exponent ␥. This article discusses the slope exponent that gives a minimum of variation in capacitance for the microwave excitation in the circuit in which diodes are connected in anti-series. The analysis is based upon a time-domainoriented method, in which we first obtain the voltage and current waveforms by solving differential equations with respect to time and then calculate the frequency-domain quantities by Fourier analysis of voltage and current waveforms.