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The same kinds of restrictions apply to receiving antenna area as to the transmitting antenna. The useful area of a single element may be limited by allowable size, achievable tolerances, or (for coherent reception) achievable pointing accuracy. For a satellite receiving station, limiting values may not differ too much from those for a deep space vehicle. On the other hand, for a ground station, over-all aperture diameter will generally not be restricted by installation requirements; but a single aperture element will be subject to limitations from atmospheric phase distortion of the signal wavefront over the aperture.

Little quantitative information is available on such atmospheric anomalies as wavefront corrugation, tilt, or unequal illumination of the aperture. However, the nature of the restriction is suggested by available data from the NBS experiment at Maui, Hawaii (1956). The assumption is made that the wavefront distortions, corresponding to the phase deviations experimentally measured as a function of baseline, have an effect similar to dimensional deviations of the antenna surface caused by fabrication tolerances.

It is further assumed that the linear dimensions of the distortion are, to a first approximation, frequency-independent. A curve based on the worst data from the Maui experiment is plotted with other restrictions in Fig. 2.

In the optical region of the spectrum, atmospheric phase correlation length limits the effective aperture diameter for coherent detection. This correlation length is assumed to vary as λ and typical values have been given for various environmental conditions. The aperture area corresponding to a 3-dB signal decorrelation loss for typical daytime atmospheric turbulence is plotted in Fig. 2 for a 70-deg zenith angle. This restriction applies only to coherent reception.

Amplitude fluctuations are also present in the received wavefront as a result of atmospheric disturbances. These may require physical apertures considerably larger than the effective aperture limit to average reception over several amplitude correlation lengths. In the visible region, for example, where the effective aperture diameter for coherent detection is only a few cm, the amplitude correlation length is typically 10 cm and a 30 cm aperture is required to reduce rms deviations in received signal power to about 10 percent.

Transmitted power

Frequency-dependent restrictions on transmitter power are related to the method of power generation. At microwave and mm-wave frequencies, traveling-wave tubes are most satisfactory in providing high power. In general, however, the maximum power capability of TWT amplifiers is inversely proportional to frequency.

At optical frequencies, lasers represent the only power source having the spatial coherence necessary for high-gain (essentially diffraction-limited) apertures as well as the temporal coherence to permit the advantages of coherent detection. The principal limitation on power generated is heat dissipation in the laser cavity due to low conversion efficiencies. There is no significant systematic dependence on frequency, performance being related to the different specific laser mechanisms.

In either portion of the spectrum, an increase in power can always be achieved by paralleling units, although in the optical region phase-locking the various elements will be more difficult. Hence, the effective power limitation is set by weight, size, and power supply restrictions.

Atmospheric losses

Transmission losses occur in the atmosphere by absorption of electromagnetic radiation by the constituent gases (especially water vapor), and in the optical region, because of aerosol and molecular scattering. Absorption occurs in bands rather than as a systematic variation and becomes significant at frequencies higher than 10 Gc. Transmission “Windows” between the bands exist in the millimeter-wave region, but transmission at submillimeter waves is believed to be generally blocked.

Windows appear again in the infrared, while in the visible region absorption occurs only in well-separated lines. Scattering at low altitudes from aerosols generally accounts for the predominant loss in the visible region, with the transmission decreasing for shorter wavelengths.

Rain and fog begin to seriously affect transmission in the millimeter-wave region, with the fog losses increasing prohibitively at optical frequencies. Thus, an optical communication link would be effectively blacked out by cloud cover.