Fading Period Research Articles

THE RATE OF FADING OF NATURAL DYES

Each dyed pattern was divided into six pieces one of which TIas alwaya protected from the action of light, while the others were exposed for different of time. The shortest period was about three weeks. Its fading power was measured by the fading of a number of dyes. The faded standards were then replaced by fresh patterns of the same dye an1 the second exposure continued Q~til the second set of standards had faded to the s~e extent as the first. The third exposure was equal to three of these standard fading periods (sfp). The fourth and fifth exposures were of seven and eleven sfp renpectively. The total time of exposure was about one year.

The fading of radio waves reflected from the ionosphere at oblique incidence

The fading of radio waves reflected from the ionosphere at oblique incidence is studied by the use of pulse transmissions on frequencies near 2 Mc/s. It is found that echoes reflected from a height of about 90 km have a fading period of about 1 min. Echoes from a greater height of about 120 km, which usually appear only during or immediately after magnetic disturbances, have a shorter fading period of about 6 sec. A clear distinction between reflections from these two levels is urged for an understanding of the fading of echoes from the E-region. The spatial characteristics of the diffraction pattern on the ground are studied by the use of three spaced receivers. Possible models for the ionospheric irregularities which would explain the results are considered. It is suggested that the irregularities at heights near 90 km have the form of horizontal discs, that the irregularities near 120 km are spherical, and that the irregularities near 300 km are elongated along the direction of the earth's magnetic field, with an axial ratio of about 3 : 1.

The fading of low-frequency radio waves reflected from the ionosphere

A study has been made of the fading of radio waves of frequency 70–200 kc/s reflected at steep incidence from the ionosphere. It is shown that the amplitude fading on 113 kc/s is sufficiently consistent to be represented by an average correlogram. Some measurements of the complex amplitude are reported and the correlograms deduced from them are compared with those derived from measurements of real amplitude. On all frequencies there is a component of fading with a correlation time (to e −1) of order 90 sec corresponding to a fading period of order 7.5 min and there is some evidence that this component of the fading is quasi-periodic. This quasi-periodic component is noticeable on a cross correlogram relating measurements made simultaneously on waves of quite different frequencies, from which it is deduced that it is caused by a quasi-sinusoidal irregularity in the ionosphere, which affects all the frequencies. Observations at three spaced receivers show that these irregularities have a spatial period of order 20 km and that they travel with speeds of about 35 m sec −1. This drift of the irregularities is an important cause of the fading. On the higher frequencies there is clear evidence of a more rapid component of fading with a correlation time of the order 50 sec.

Further Investigations of the Amplitude Variations of Downcoming Wireless Waves

Experiments are described for measuring the lateral deviation of wireless waves after reflection from the E and F regions of the ionosphere. It was found that the greatest lateral deviation observed, 20° or more, was that due to the e region, and the least, about 0·5°, was due to the normal E region in the case of a distant transmitter.The time variation of amplitude of a reflected wave was found to be consistent with a random scattering at the ionosphere.In the theoretical discussion it is shown that changing horizontal irregularities, ion clouds, are a very important cause of fading. Values are calculated for the average fading periods which would result from the horizontal winds in the neighbourhood of the E region known to exist from other evidence. These calculated periods agree with the observed and it is inferred that horizontal winds are a very important cause of fading.