High-frequency effect due to the axial drift velocity of a plasma column,"[6] C. Yeh, "Scattering of, obliquely incldent microwaves by a , "Wave scattering from a oblique incidence," Proc. Inst. EZec. Eng., vol. 115, pp. 1446-circular cylmder coated w1t.h an inhomogeneous plasma at , "llelativistic treatment of a moving magnetoplasma medium," J . Inst. Eledron. Cmnmun. Eng. Jap., to be published.Abstract-A diversity-combining technique is described which reducts frequency distortion caused by multipath propagation in high-frequency single-or double-sideband voice radio transmission. Xanuscript. Calif. 94025. N. M. Kawachika was with the Radio Science Laborat,ory, Stanford Universit.v, Stanford, Calif. He is now with Kentron Hawaii, Ltd., Hondulu, Hawaii 96813.sions were used. It was found that substantial null filling occurred and that, as expected, the filling in is more effective when a pair of antennas is spaced along the line to the distant station than when the same pair is spaced transversely with respect to that line. It is concluded that the theory of this combining operation, as described in this paper, is correct to a fist order of accuracy.

A ray-tracing technique for simulating the performance of the mode-averaging diversity combinet (MADC) [Villard et al., 1972] is described, which extends the methods developed by Croft [1969] to the analysis of frequency-selective fading. It is shown that the computer procedure satisfactorily predicts observations made during a test representative of conditions encountered in practice. Using the parameters of this test, the method is applied to the original MADC configuration to investigate the dependence of performance on antenna spacing and ionospheric profile. Two MADC schemes which incorporate alternative methods of forming the running average are described. Their performance is also evaluated for the above parameters and shown to be roughly equal to that of the original. It is concluded that for the given parameters at least, a choice between the three alternative configurations can be made on the basis of convenience. INTRODUCTIONOne purpose of this paper is to describe a method for computer simulation of frequency-selective fading in HF signal propagation, based on realistic models of the ionosphere (electron density versus altitude). A further purpose is to apply the method in simulating and studying the action of a mode-averaging diversity combiner capable of reducing minima and nulls in the transmission passband caused by transmission-mode interference [Viilard et al., 1972]. These nulls are of practical importance because they affect the quality of complex signals, such as speech, having frequency components of which the duration is long compared with the differential time delay of multipath modes. EFFECT OF THE IONOSPHERE ON FADINGThe electron-density profile of the ionosphere determines the path or paths whereby a transmitted HF signal reaches its destination and thus determines the occurrence and severity of frequency-selective fading. For example, if the transmitted signal components received via the two paths in Figure 1 happen to be comparable in amplitude, then at fre-Copyright ¸ 1973 by the American Geophysical Union.quencies for which the difference in path lengths is equal to an odd number of half wavelengths, the components will almost completely cancel. In most cases the transmitted signals will reach their destination via two or more modes or hops. Possible angles of arrival of signals transmitted over a 1000-km path are indicated in Figure 2. In deriving this figure, a typical operating frequency and ionospheric profile have been assumed. Path lengths in general will differ, and frequency selective interference will result. An advantage of a computer simulation of fading is that a representative assortment of ionospheric conditions can be explored. In the study described here, the six possible models of the ionosphere shown in Figure 3 were used. Each model consists of a profile of electron density versus altitude up to 300 km or more. The profiles were those of Croit [1965, 1966] and are derived from rocket measurements. BASIS FOR COMPUTER SIMULATIONSThe ground range from transmitter...