Diversity reception and automatic phase correction

Abstract

The interference between the direct ray and a ground-reflected ray gives rise at the receiver of a communication link to a sinusoidal field pattern in the vertical plane consisting of nodes and maxima. The position and pattern wavelength of this field depend on the receiver and transmitter heights and spacing and on propagation conditions via an effective curvature parameter, C. This parameter varies in time, and is the cause of most fading at a single antenna. Its upper limit, which may be as high as 2.5 for a small percentage of the time, determines minimum antenna heights for line-of-sight transmission under extreme conditions. C is normally about ¾. Its range of variation determines the optimum spacing of a pair of diversity antennae, and suitable design formulae are given. Experiments using a pair of mirrors, a varying transmitter frequency, and photographs of oscillograph traces indicate an extreme lower value of C over water of −1.5. A moving-film display shows that conditions can vary rapidly from minute to minute, although at other times the display is steady for hours at a time.An automatic phasing junction has been designed for insertion in the feed from two diversity antennae, the drive for the phasing element being taken from the receiver. A combined signal nowhere smaller than the greater of the received signals is obtained from the combining unit, and when the diversity spacing is chosen with regard to the extreme values of curvature obtained on the path, an excellent overall response is ensured. Some preliminary figures are given for the performance of an improved combining network and phase-control apparatus operating in conjunction with a height-diversity microwave system over a 69-mile overwater path.