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The advantages to be gained in the operation of radar systems at very high frequencies have led to the use of frequencies of several thousand megacycles. Operation at these frequencies has imposed serious problems in obtaining suitable tube behavior. Because of the difficulty in obtaining amplification at the transmission frequency, the r.f. section of the usual radar receiver consists of a crystal converter driven by a beating oscillator and operating directly into an i.f. amplifier. Since the midband frequency of the latter has commonly been either 30 or 60 Mc., it has been necessary to provide beating oscillators operating at frequencies differing from those of the transmitter by only a few per cent. For radar systems intended to operate at approximately 3000 Mc., which were under development in the early days of the war, it was found that triodes then available gave unsatisfactory performance. Attention shifted to the possibility of using velocity-modulated tubes, and the particular form known as the reflex oscillator came into general use. In this paper the requirements on beating-oscillator tubes for radar systems will be discussed, and the design features which have made the reflex oscillator eminently satisfactory in this application will be pointed out. Problems encountered in such oscillators will be outlined, and the solution in a number of cases is indicated. In some instances military requirements and expediency were in conflict with the optimum performance, and hence certain compromises were necessary.