Fj is a function of frequency, varying from near unity (less than f5 dB) at frequencies below 10 MHz to about 0.1 (-20 dB) and 0.01 (-40 dB) at 100 MHz and 400 MHz, respectively. Since Fj is independent of distance there is no exponential attenuation of ultrashort-waves in the jungle. In fact the effect of the jungle on the direct wave must be almost identical to that on the reflected wave in order not to disturb the almost complete cancellation.
Part I of this paper first describes a method of measuring attenuation and field strength in the ultra‐short wave range. A résumé of some of the quantitative experiments carried out in the range between 17 mc. (17 meters) and 80 mc. (3.75 m.) and with distances up to 100 km, is then given. Two cases are included: (1) “Optical” paths over sea‐water and (2) “Non‐optical” paths over level and hilly country. An outstanding result is that the absolute values of the fields measured were always less than the inverse distance value. Over sea‐water, the fields decreased as the frequency increased from 34 mc. (8.7 m.) to 80 mc. (3.75 m.) while the opposite trend was found over land. As a rule, the signals received were very steady, but some evidence of slow fading was obtained for certain cases when the attenuation was much greater than that for free space. Part II gives a discussion of reflection, diffraction and refraction as applied to ultra‐short wave transmission. It is shown, (1) that regular reflection is of importance even in the case of fairly rough terrain, (2) that diffraction considerations are of prime importance in the case of non‐optical paths, and (3) that refraction by the lower atmosphere can be taken into account by assuming a fictitious radius of the earth. This radius is ordinarily equal to about 4/3 the actual radius. The experiments over sea‐water are found to be consistent with the simple assumption of a direct and a reflected wave except for distances so great that the curvature of the earth requires a more fundamental solution. It is shown that the trend with frequency to be expected in the results for a non‐optical path over land is the same as that actually observed, and that in one specific case, which is particularly amenable to calculation, the absolute values also check reasonably well. It is found both from experiment and from theory that non‐optical paths do not suffer from so great a disadvantage as has usually been supposed. Several trends with respect to frequency are pointed out, two of which, the “conductivity” and the “diffraction” trends, give decreased efficiency with increased frequency, and another of which, the “negative reflection” trend, gives increased efficiency with increased frequency under the conditions usually encountered. The existence of optimum frequencies is pointed out, and it is emphasized that they depend on the topography of the particular paths, and that different paths may therefore have widely different optimum frequencies. Thus, among the particular cases mentioned, the lowest optimum values vary from frequencies which are well below the ultra‐high frequency range up to 1200 mc. (25 cm.). For other paths the lowest optimum frequency may be still higher.
The theory of the exponential transmission line is developed. It is found to be a high pass, impedance transforming filter. The cutoff frequency depends upon the rate of taper.The deviation of the exponential line from an ideal impedance transformer may be decreased by an order of magnitude by shunting the low impedance end with an inductance and inserting a capacitance in series with the high impedance end. The magnitudes of these reactances are equal to the impedance level at their respective ends of the line at the cutoff frequency.For a two-to-one impedance transformer the line is 0.0551 wavelengths long at the cutoff frequency. For a four-to-one impedance transformer the line is 0.1102 wave-lengths long at the cutoff frequency, etc. , The results have been verified experimentally. Practical lines 50 meters and 15 meters long have been constructed which transform from 600 to 300 ohms over the frequency range from 4 to 30 me. with deviations from the ideal that are small compared with the deviations from the ideal of commercial transmission lines, either two-wire or concentric.When an exponential line is used as a dissipative load of known impedance instead of a uniform line it is possible to approach more nearly the ideal of constant heat dissipation per unit length. This makes it possible to use a shorter line.T H E exponential line may be defined as an ordinary transmission line in which the spacing between the conductors (or conductor size) is not constant but varies in such a way that the distributed inductance and capacitance vary exponentially with the distance along the line. That is, the impedance ratio for two points a fixed distance apart IS independent of the position of these two points along the line. A disturbance is propagated down an exponential transmission line in the same manner as it would be down a uniform line with the additional effect that the voltage is increased by the square root of the change in impedance level and the current is decreased by the reciprocal of this quantity.The exponential line has the properties of a high pass impedance transforming filter. The cutoff frequency depends upon the rate of • Presented before joint meeting of U.
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