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Synoptic studies of the D-region electron densities using the differential-absorption partialreflection experiment at 2.66 and 6.275 MHz are in progress at Ottawa (45øN, 76øW). The measurements are made from about one hour before sunrise to one hour after sunset throughout the winter and for 10 consecutive days each month during the other seasons. The methods employed to analyze data obtained over long periods are outlined in this paper and average electron densities showing diurnal behavior under quiet conditions are presented. Over the greater part of the day (i.e., for solar zenith angles less than about 75 ø ) the electron number densities show regular solar control with values increasing toward local noon. However, there is an asymmetry about local noon of up to 6 ø in solar zenith angle, the morning electron densities being lower than those at the corresponding time in the afternoon. The diurnal changes for solar zenith angles greater than 75 ø are more complicated. The formation over dawn of a C layer at heights below 75 km can be observed, and there is evidence that this layer decays after a solar zenith angle of about 90 ø. This decay continues until arrested by photoionization processes. 163 164 COYNE AND BELROSE DIURNAL AND SEASONAL VARIATION IN D REGION 165

The amplitude of weak echoes partially reflected from ionization irregularities in the height range 50-100 km are recorded. By utilizing two frequencies, 2.66 and 627 Mc/s, and recording the amplitudes of both the ordinary and extraordinary component waves, information is obtained about electron density and electron collision frequency in this height range under both normal and abnormal conditions. The method of analysis (discussed in detail in this paper) is essentially similar to that developed by Gardner and Pawsey, except that the generalized magnetoionic formulas, which take into account the energy dependence of the collision cross section, are employed. The results being obtained are believed to be more accurate in detail than those obtained by other ground-based techniques. Introduction. Our knowledge of electron densities and electron collision frequencies in the lower ionosphere (50-100 km) is relatively meager, partly because of observational limitations and partly because of difficulties in inter-preting the observational data that have been obtained. Under normal conditions the electron number densities are so low that low frequencies are needed for reflection in this height range. Despite an abundance of experimental data, few results have emerged from these studies. Under abnormal conditions the attenuation of radio waves propagating within the region can become so great that radio reflections, other than from the base of the ionosphere, are not obtained. The dynamic range of changes, even under normal conditions, is large, since the electron density and collision frequency change by about four orders of magnitude in the height range 50-100 km.The relatively high gas densities make environmental measurements with sounding rockets difficult, and most experiments have therefore been designed to provide information about the ionosphere at heights above 100 km. A few results have been obtained for the normal D region [Aikin et al., 1963; Kransnuskin and Kox With the University of Brisbane, Queensland, Australia (work performed while on sabbatical leave to DRTE during 1961). lesnikov, 1962], but these are less reliable than those obtained at times when radio-wave absorption was great [Kane, 1961; Jespersen et al., 1963].The magnitude of the electron density and its variation with height is roughly known, but superimposed on a general increase with height are important structural features, the dynamic changes of which cannot be studied by means of rockets. There is thus a need for a reliable means of studying the lower ionosphere by ground-based experiments. In recent years two new techniques have been developed for studying the D region. One of these, the ionospheric wave interaction experiment [Fejer, 1955; Barrington and Thrane, 1962] has given useful results, but there are major difficulties, both in the experimental technique and in the interpretation of the data. Another method utilizes the partial reflection of ordinary and extraordinary component waves from scatterers within the D region [Gardner ...

New high-voltage power lines are often built in close proximity to MF/AM broadcast antennas. The "skywire" which joins the tops of the power-line towers creates closed loops for RF current on the power line and its image in ground, and these loops can be resonant in the MF band. If the broadcast antenna operates on such a resonant frequency, it induces large currents on the skywires and power-line towers, and "reradiation" from these induced currents can considerably alter the broadcast antenna's radiation patterns. This paper uses computer modeling to systematically examine several techniques for "detuning" the power line, namely: isolating towers from the skywire, series insulators in the skywires, stub detuners on the skywires, and short capacitively terminated stub detuners on the towers. The mechanism which makes each detuner effective is seen from the RF current distribution computed for the power line and detuner. The bandwidth of each detuner is assessed. Consideration is given to the difficulties that can arise in the implementation of each detuner design on a real power line. All of these detuner designs have been tested by scale-model or full-scale measurements. Only when detuning has been made a systematic economical procedure can this special EMC problem be regarded as solved.