A 7‐meter offset Cassegrainian antenna with a precise surface has been built and tested. Measurements using a terrestrial source were made and compared with calculations for 19, 28.5, and 99.5 GHz. Low sidelobe level (≲ − 40 dB) at one degree off the main beam and low cross polarization (≤ − 40 dB) throughout the main beam are achieved using a quasi‐optical 19/28.5‐GHz feed system that also demonstrates very low multiplexing loss (∼0.1 dB). The prime‐focus gain measurement at 99.5 GHz found the difference between the measured and calculated gains to be (0.79 ± 0.45) dB, which is consistent with the expected rms surface error (∼0.1 mm). Multiple‐beam operation accommodates both propagation experiments with the comstar beacons at 19 and 28.5 GHz and millimeter wave radio astronomy observations without physical disturbance of equipment.
A propagation experiment was performed with a coherent beam of optical-frequency radiation transmitted over a 10-meter path. The combination of short path length and short wavelength makes the experiment sensitive to the fine-scale structure of the turbulent atmosphere. From the propagation measuretnents, we are able to infer both the characteristic size of the small-scale eddies known as the microscale /o and the functional form of the spectrum of permittivity fluctuations at wave numbers in the region of dissipation (i.e., wave numbers of the order of the microscale).
NATURE OF EXPERI1VIENTThe theoretical calculations pertinent to this study are based on the expression for C•(p) the covariance of the logarithmic amplitudes of signals received at two points spaced a distance p apart and located in a plane perpendicular to the direction of propagation of the trans-,mitred wave. The expression is found in Tartarski [1961]. As Strohbehn [1970] points out, the covariance 'function C•(p), a measurable property of a propagated wave, is related to the permiStivity spectrum (•,(•), a property of the medium. Furthermore, when the Fresnel zone appropriate to the path (XL) •/• is of the same order as the microscale lo, the shape of the covariance function C•(p) depends strongly on the magnitude of to and, to a lesser extent, on the shape of the permittivity spectrum in the vicinity of the microscale (in the region of viscous dissipation). For this reason, the integral expression for Cx(p) has been evaluated by assuming different models of the pennittivity spectrum and many different values of the microscale lo. The computations were performed for a radiation wavelength of 6328 A and a path length of 10 meters, thus giving a Fresnel zone, (XL) •/•', of 2.5 mm, of the order of the microscale. The resultant evaluations of the integral are shown in Fi'gures i and 2, where the • NOw with Bell Telephone Laboratories, Incorporated, Holmdel, New Jersey 07733.
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