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Amplitude fading of 17 Me/s CW signals is observed on 12 dipole antennas evenly spaced in two perpendicular rows each about 300 m long. Tho signals are ionospherically propagated ovor approximatoly east-wost 20 to 35 deg N latitudo paths of 1840, 3741, and 5724 km.. The number of hops on tho two shorter paths is determined by oblique, stopfrequency soundings. Periodic spatial fadin g patterns consisting of approx imately straight, parallel nulls ex ist 33 percent of t he t ime on t he 1840 km s ignal and 10 percent of t he time on t he 5740 km s ignal. Mean null spacing on t he 3741 km s ignal varies from 1.4 km d uring periods of 1F propagation to 0.4 km during periods of 3F' propagat ion. The period ic fading patterns arc shown to be caused by mul t ipath interforen ee between rays. A tendency for the nulls to have a d iagonal position, with respect to t he great circle connecting t he transmitter and receiveI', is found to be a result of s mall azimuthal ray dev iations caused by small transverse ionospheric t ilts. During two magnetic storms the lateral ionospher ic t ilt over t he 3741-km path appeared, from the fading pattern measurements, to be less than normal dur ing the flrst 10 h]', and so mew hat greater t han normal dur ing t he remainder . During one of these storms t he fading patterns were much more period ic than usual, implying that t he random var iat ions in ionization dens ity were s mallcr than usual. These res ul ts show that measurement of the d irection of fadin g patterns on the ground is a sens it ive technique for studying small ionospheric t ilts . In addition, it appears t hat d ual space-diversity reception, over paths where lateral ionospheric t il ts of one predo minant direction are enco untered, can be optimized by spacin g antennas a long a line normal to the predominant null direction.

Spatial properties of the amplitude fading of continuous HF radio waves