A second limitation on the number of tubes per crowbar is related to the performance degradation that occurs each time a crowbar fires. For a first order approximation we assume the following characteristics: • RF devices will have an arc rate of one every 3,000 hours • The maximum arc current per spark gap is 100, 000 amperes • The maximum design life for a spark gap is 100, 000 operations For 30 year life the spark gap must hold off voltage and trigger upon command for this period. If the triggered spark gap has a life of 100, 000 arc firings, then the number of allowable crowbar firings (over the 30 year life) would be: The number of tubes per spark gap is found to be: At 0. 3 amperes per tube this would result in a maximum current of 345 amperes per crowbar. A third consideration in the number and location of the crowbar units is their relationship to a subarray, a basic antenna building block as described in the section on Transmitting Antenna. The crowbar unit, in addition to being a protective element also incorporates the features of a low level switch gear. Since the lowest subintegral of tube groups is a subarray, it is desirable that a relationship exist between the number of crowbar units and the number of subarrays. To relate the number of subarrays with the number of crowbar units, the five step approximation to a 10 dB taper (covered in Transmitting Antenna section) has been chosen. Figure 5-16 indicates the relationship between the number of crowbars and the subarrays. As can be seen, this results in a maximum current of 259. 2 amperes per crowbar unit, conservatively within the postulated maximum of 345 amperes per crowbar.
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