Putting all of the microwave emissions together into a single composite, the spectrum envelope near the main signal would be as shown in Figure 71. The three lobes in the center indicate the range of frequency over which the allocation for the main signal might be, for example, a 100-MHz region centered on the 2450-MHz industrial heating band. The three lobes centered on -400 MHz are the turn-on signals associated with each of the three running frequencies. These signals are non-existent during steady operational running. The solid envelope line is the approximate noise level for the random background noise as a function of frequency derived from measurements on a pulsed cold platinum cathode type of CFA. The solid-line envelope represents the current state of affairs in existing CFA's which have had no specific effort directed in their design or application to minimize or improve the RFI emission. The microwave properties of a typical CFA indicate a well-matched, operating passband in the mid-region of frequency and mismatches or resonances in the outer regions of the band. Sharp resonances tend to peak up background noise energy and produce concentrations of it at certain discrete frequencies. Most of them are the result of the metallic confines of the tube necessitated by the vacuum envelope in the natural atmosphere of the Earth. In a space application where the environment is under vacuum, the necessity for such an enclosure is removed and much of this resonance phenomenon will not exist. In any case, resonances can be considered a natural and routine problem that is encountered in every development for which relatively straightforward design steps are available. Looking towards what might be achievable with specific effort directed at optimization for the space requirement, certain beneficial trade-offs are possible which have not been required nor implemented for radar systems usage. Most of the presently available tubes have been built for usage in broadband radar. The design trade-off has been geared to other requirements, notably gain and bandwidth, rather than toward the RFI problem. This means that trade-offs can be made that have not been implemented before because they were not necessary, or they were not possible for other reasons. One of the available trade-offs for this requirement results from the narrow-band or singlefrequency operation. This means that tube performance need not be optimized over the frequency band. The optimization can be made over a narrow frequency range. With a narrower passband, the frequencies on either side of the main signal would be filtered more than they are in current designs, which would reduce, appreciably, the noise level away from the main signal. The outlook, therefore, would be to get improvements in the spectrum contour by tailoring the tube to this requirement. With additional filtering, it could be improved even more. The general tube-design concept would be to use all of the latitude available in the narrow-band space environment application, such as the use of unenclosed structures to eliminate resonant modes, no input or output windows with their associated resonances, and low gain which has the further effect of suppressing other signals. The trade-off would focus on the RFI question rather than upon the standard requirements in Earth-bound applications.
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