What are the limitations of conventional tubes at microwave frequency?

Answer 1

2-3Figure 2-1B.-Interelectrode capacitance in a vacuum tube. 100 MEGAHERTZ.Figure 2-1C.-Interelectrode capacitance in a vacuum tube. INTERELECTRODE CAPACITANCE IN ATUNED-PLATE TUNED-GRID OSCILLATOR.A good point to remember is that the higher the frequency, or the larger the interelectrodecapacitance, the higher will be the current through this capacitance. The circuit in figure 2-1C, shows theinterelectrode capacitance between the grid and the cathode (Cgk) in parallel with the signal source. Asthe frequency of the input signal increases, the effective grid-to-cathode impedance of the tube decreasesbecause of a decrease in the reactance of the interelectrode capacitance. If the signal frequency is 100megahertz or greater, the reactance of the grid-to-cathode capacitance is so small that much of the signalis short-circuited within the tube. Since the interelectrode capacitances are effectively in parallel with thetuned circuits, as shown in figures 2-1A, B, and C, they will also affect the frequency at which the tunedcircuits resonate.Another frequency-limiting factor is the LEAD INDUCTANCE of the tube elements. Since the leadinductances within a tube are effectively in parallel with the interelectrode capacitance, the net effect is toraise the frequency limit. However, the inductance of the cathode lead is common to both the grid andplate circuits. This provides a path for degenerative feedback which reduces overall circuit efficiency.