Search

This report presents the results of a 4-month study to reduce the sidelobe levels of a 38 GHz, 2 ft diameter Cassegrain reflector. Both partial and full circumferential shrouds were added to the main reflector. Various shielding techniques to reduce the horn and subreflector radiation characteristics were evaluated. Measured primary patterns for the various horn-subreflector combinations were used to compute the secondary reflector patterns to evaluate the sidelobe improvement. The computed patterns were compared with measured results. Keywords include: Cassegrain reflectors, Space frame radome, Horn-subreflector feed patterns, and Tunnel reflector antennas.

Atmospheric emission measurements have routinely been used to determine the total attenuation on an earth-space path at the centimeter and millimeter wavelengths. In the presence of rain, scattering effects have to be taken into account at frequencies 20 GHz when one interprets emission data to derive the total attenuation. If one uses the radiometric relation to derive the attenuation, assuming an absorptive medium, one finds that the predicted attenuation underestimates the true attenuation. The reason for this the rain scatters more emission energy out of the direct ray path than into it. Calculations of the atmospheric emission temperature and attenuation in light-to-moderate rain at different frequencies and rain rates are performed, taking into account molecular absorption, and varying cloud cover and ground temperature. The results show that for typically encountered meteorological conditions, the radiometric formula can be used to derive the total attenuation ( 10 dB) from emission measurements with accuracies of the order less than 1 dB, provided that the empirical constant in the radiometric formula, dependent on the ambient temperature, is a function of frequency and ground temperature.

The exact formulas for the bias and variance of the product estimator under simple random sampling are given. Its asymptotic normality is rigorously established under weak and interpretable regularity conditions on the finite populations. No superpopulation model is assumed. Hajek's projection method is the key tool in our proofs. (Author)