Radar Level Transmitter is a technology that has revolutionized Level Measurement for the past 2 decades and it is one of the most widely used technology for Level Measurement as a whole. However, very rarely do we understand the Sensor behind the Radar Level Transmitter which is at the heart of all its functionalities and features. It is the sensor chip, which determines the first instance of the signal and the complex algorithms are only the second line of technology behind the final product.
At EIP, we believe in technology and sharing the same with our customers and have assembled a detailed article to towards understanding the briefly about the Magnetron and the Solid State Radar Level Transmitter with the below information.
Magnetron Radar Level Transmitter includes a vacuum tube that consists of a cavity which creates self-excited microwave oscillations. The electrons and magnetic field cross each other to produce a high-power output. The way oscillations are created is similar to how a whistle is blown and a tune is created. When a stream of electrons passes through a series of metal cavities with a magnetic field they oscillate the microwaves within.
The magnetron cannot control the frequency of the resonance, and it works solely as an oscillator. The frequency depends on the dimensions of the cavities that are created. Usually there are a series of cavities of spaces that are created for the resonance to happen.
Construction of a Magnetron Radar Level Transmitter
A magnetron is a diode since it has no grid. The anode is placed in a solid cylindrical block made of copper. At the centre is the filament and the cathode. These are supported by the filament leads which keep them fixed at one position. The cathode is made of a high-emission material and is heated indirectly. The cathode is surrounded by the cavities that serve as a parallel resonant circuit. The cavities can range from 8 up to 20 holes. The entire setup is arranged concentrically with air being removed creating a vacuum, so that the electrons move freely.
At times there could be a third diode that is placed in-between the anode and cathode. This provides an option to control the flow of electrons between the two diodes by varying the voltage on this third electrode.
The shape of the cavities are usually of either of these four types- slot, vane, rising sun, and hole-and-slot. The cavities are usually even numbered. The space between anode and cathode is called the interaction space and this aids in generating the required magnetic field.
Operation of a Magnetron
A magnetron produces DC to AC energy. It works in four phases.
- Generation and acceleration of the electron beam- this is generated in a dc field
- Velocity modulation- this is in an AC field
- Electron bunch formation
- Energy dispense- to the ac field
Generation and Acceleration
When cathode is heated, the electrons travel directly towards anode – from negative to positive. Currently no magnetic field exists. Then the magnetic field is introduced gradually. The electrons start to deflect gradually. The stronger the magnetic field gets, the more deflection in the path of electrons. At a point, the magnetic field is so strong that electrons travel in a circular path as they are being deflected by the anode. This happens as the electric field and magnetic field are both resisting each other. The magnetic field reached a high value, the electric field in the plate dropped to a very small value. The values at which the anode current is halted are called Hull cut-off magnetic field value and cut-off voltage.
This value is controlled to make the electrons keep travelling in circular motion creating oscillations.
The velocity of the electrons varies based on the AC and DC fields. The DC field is conducted radially while the AC field extends within the adjacent slots. The electrons that are more positively charged reach a higher tangential speed while the electrons that are more negatively charged drop their speed or slow down.
Electron Bunch Formation & Energy Dispense
This is the stage where the electron bunch is formed which is also known as the Space-Charge Wheel. At the Velocity modulation stage, the electrons move in bunches. The difference in speed of electrons results in a movement of electrons that resembles the blades of the wheel. Hence, the name space-charge wheel. This enables the electrons to deliver a continuous charge of energy that can sustain the RF oscillations. Now, since electrons gain energy moving in opposite charge while lose energy moving in the same charge, this maintains a continuous flow of energy that has an optimisation level of 80%.
Now, since magnetron radar level transmitter uses diodes, there is a considerable amount of wear and tear that happens over time. Hence, an alternative to magnetrons is the Solid State Radar Level Transmitter. These have replaced magnetron radar level transmitters largely in all capacity as the technology evolves. However, the magnetrons are still continued to be used as it provides for a much more economical solution.