Do Fleming valves fail at high frequencies?

In summary, solid state diode technology is not yet good enough to be practical for harvesting THz radiation from antennas. The reason is the power loss due to the electron velocity.
  • #1
awygle
13
0
So I've been reading up on optical rectennas, and the sources I read say that the reason such devices are not yet efficient enough to be practical isn't the antenna, it's the rectifier, since solid-state Schottky diodes of the types normally used don't work in the THz range of AC frequencies generated by the antennas without large power losses. Apparently MIM diodes have been shown to work up to about 150THz, but the ideal wavelengths of light for solar power would produce 190 - 750 THz.

My question, then, is in the topic title - do Fleming valves (vacuum-tube diodes) fail at high frequencies as well? Also, what precisely causes the power loss in solid-state diodes to happen in the first place?
 
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  • #2
Fleming valves won't work when the frequency gets too high.
It takes some amount of time for electrons to go from the cathode to the anode. If the voltage reverses before the electron reaches the anode, there is no current, so the valve doesn't work.

The electrons reach some velocity and when they strike the anode, the kenetic energy is transferred to the anode.
Some of the power vacuum tubes operated with the anode cherry red or even white hot. Very impressive.
 
  • #3
Carl Pugh said:
Some of the power vacuum tubes operated with the anode cherry red or even white hot. Very impressive.

Why the use of the PAST tense?
Big power triodes are still used in places - though the really big valves have water cooled anodes these days. Solid state does have its limitations, you know.:wink:
 
  • #4
There are several reasons why normal vacuum tube diodes are not very good at high frequencies.

There is the transit time problem Carl has mentioned. Electrons take time to pass from Cathode to Anode and the voltage causing them to flow may reverse direction before they get there, if the frequency is high enough.

There is the capacitance across the diode which bypasses the diode effect at high frequencies because it appears in parallel with the diode.

There is the necessary wire length to connect the diode to the pins at the base of the vacuum tube. This has quite a lot of inductance and this becomes significant at high frequencies.

I mentioned "normal" vacuum tube diodes because Klystrons and Magnetrons are diodes and actually work very well at microwave frequencies.
 
  • #5
Klystrons, TWTs and magnetrons work on a very different principle. Amplification is achieved by a combination of velocity modulation and a long drift time which gives 'bunching' (current modulation). Very different from the action of a grid in triode.
 

Related to Do Fleming valves fail at high frequencies?

1. What is a Fleming valve?

A Fleming valve, also known as a thermionic diode, is an early type of vacuum tube used in early radio communication systems. It was the first practical device capable of amplifying electrical signals and was an important precursor to the modern vacuum tube.

2. How do Fleming valves work?

Fleming valves work by using a heated cathode to emit electrons into a vacuum. These electrons are attracted to a positively charged anode, creating a flow of current. This current can then be controlled and amplified by applying a varying voltage to the grid, which is positioned between the cathode and anode.

3. What do you mean by "high frequencies" in relation to Fleming valves?

High frequencies refer to the rate at which the alternating current (AC) signal is changing. In the context of Fleming valves, this refers to AC signals with frequencies above 100 kHz, which is considered high frequency in comparison to the lower frequencies used in early radio communication systems.

4. Why do Fleming valves fail at high frequencies?

Fleming valves were designed to operate at lower frequencies and are therefore not optimized for high frequency signals. At high frequencies, the electrons emitted by the cathode have a shorter time to reach the anode before the signal changes direction. This can cause a decrease in the valve's amplification and can lead to distortion or failure of the signal.

5. Can anything be done to prevent Fleming valve failure at high frequencies?

Yes, there are some techniques that can be used to mitigate the effects of high frequencies on Fleming valves. These include using a higher voltage on the grid to increase the speed of the electrons, as well as adding special inductors or capacitors to the circuit to filter out high frequency components. However, these solutions are not always effective and the best way to ensure proper functioning at high frequencies is to use a valve that is specifically designed for such use, such as the later developed tetrode or pentode tubes.

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