The coaxial-line diode: a rectifier of microwaves

Abstract

The coaxial-line configuration of the diode is shown to provide a non-linear radial field and a long interaction path. These two features contribute to the relatively high detection efficiency of the device when operating at high microwave powers. At the signal levels at which it is usefully employed the low-signal theory does not apply. The theory which is discussed shows that electronic transit times are equal to several cycles of the microwave field. It also shows that some electrons which are caused by the negative phase of the microwave field to back-bombard the cathode play an important part in the process of rectification. The microwave power range in which the coaxial-line diode is employed is from 100 watts to 100kW. Its operation at these high power levels provides sufficiently high output voltages to produce useful deflections on normal high-speed oscillographs without intermediate amplification. It thus provides a direct and reproducible method of measuring peak microwave power and of observing magnetron pulse forms with high resolution. The inherent resolution is limited only by the self-capacitance and output impedance (both of which are low) and by the transit times of electrons in the diode. Resolutions of up to 10 -8 sec have been obtained. Coaxial-line diodes have been operated at frequencies from 3000 to 35000 Mc/s. In the process of rectification several harmonics are produced. The dimensions of the diode are not limited by the transit time of electrons in the microwave field; consequently the valve is easy to construct and does not involve unusually small electrode spacing.