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How to Test a Microwave’s Magnetron

Fred’s Appliance Academy January 6, 2020 Video Leave a Comment

How to Test a Microwave's Magnetron

Testing a magnetron from a microwave with an analog meter.

Note before testing any components inside of a microwave, be sure that the unit is unplugged and the high voltage capacitor has been discharged.

When testing a magnetron, keep in mind that there is no complete way to test the magnetron electrically, but there are some things to look for that clearly show a magnetron needs to be replaced.

Looking at the magnetron itself. Making sure that the magnets inside are not cracked. The antenna on the other side is not bent or damaged and it could be burnt. If we do end up with a burnt antenna, we will want to check the waveguide. If the waveguide is damaged, you’ll want to call your manufacturer and get the unit condemned.

The only test with a meter that we could do with the magnetron is read less than one between ohm between FA, and F. Which is our filament and filament anode. Set your meter to R times one. Calibrate and place your meter leads across F and FA. Looking for less than one ohm. You’ll also want to set your meter to R times 10K and calibrate. Checking each terminal of the magnetron to the magnetron itself. Looking for no reading. If you should get a reading, the magnetron has shorted. If you get no reading, the magnetron is open.

I took apart a microwave, and when I saw the magnetron, I conviniently remembered that I had heard that magnetrons were dangerous. I decided to research this a bit further (I know, great timing) and I found that some magnetrons contains berilyum oxide, which is fatal if you breathe it in. I also read that it is dangerous in this way only if it’s crushed, then inhaled. (It is also lethal if if you ingest it, but I’m not planning on doing that).

Since we stopped using that microwave, I haven’t dropped it on the floor or anything like that, so does that mean that it is safe to handle? How could the magnetron become dangerous? What precautions should I take to make sure that I’m safe?

3 Answers 3

Some magnetrons use beryllium oxide as the “ceramic” looking insulators inside of the ring magnets on both the “Stem” and the “Antenna” ends. Reference the image below, the beryllium oxide parts are the pink items in the middle. They are totally inert if undisturbed.

How to Test a Microwave's Magnetron

Not all magnetrons use that for the insulators, but it’s virtually impossible to tell if they did so you must assume they do. It has to get airborne to become dangerous. So just don’t go crushing and snorting the ceramic dust and you will be fine. If you do happen to break one, don’t use a vacuum cleaner, clean up with a damp rag and get ALL of the dust, then dispose of the rag while still wet by putting it in a plastic zip-lock bag.

I take apart magnetrons from old microwaves that I get for free and harvest the magnets, they are cool and powerful. I then put that center assembly into a thick plastic zip-lock bag before disposing of it.

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Microwaves work by creating radio frequency energy using oscillating magnets. It’s easy to take their operation for granted until they stop working as efficiently as they normally do. Issues with operation, noise and odor often indicate that the microwave’s components are failing. In most cases, a failing microwave should be replaced or repaired by a trained technician.

Before Troubleshooting

Before troubleshooting your microwave, take some safety precautions. Microwaves work using high voltage that can lead to electric shock or death. Before examining any electrical components, make sure the microwave is not connected to a power supply. Never touch any wiring within the microwave. This should be done by a trained technician. Remove jewelry such as watches and never attempt to disassemble your microwave. If there is smoke or fire coming from your microwave, disconnect it from the power supply and replace the microwave immediately.

Food Test

The easiest way to determine whether your microwave is failing is to try to heat a food item. If the microwave doesn’t heat, or it heats more slowly than it should, this indicates that it is failing. Often reduced heat or no heat at all is caused by a failing magnetron, which is a tube that is part of the microwave’s high-voltage system. If you are unable to adjust the power level, the problem may be within the circuit board.

Sounds

Loud noises such as bangs, buzzes, rattles and knocking while the microwave is in operation could indicate several things. Noise can result from debris stuck in the turntable. If cleaning the turntable doesn’t stop the noise, the problem may be a malfunctioning power diode, capacitor or magnetron, which means it’s time to replace your microwave.

Operational Issues

Watch the microwave during use. If there is no light when the microwave is in operation but it heats food properly without noise or odors, the bulb is burned out and should be replaced. The microwave’s internal parts are not failing. However, if the microwave vibrates or shakes or you see sparks inside, the microwave is failing. Smoke is a sign that that problem is severe. If you notice any of these things while the microwave is in operation, disconnect the power at the outlet or the main breaker and contact a technician for service or replace the microwave. Other indicators of a failing microwave include a stuck turntable or a flickering display. If carousel does not turn, the motor that operates it may have malfunctioned. A flickering display could indicate a failing electrical system.

Odors

Burning odors such as melting plastic or burned wires when the microwave is in use indicate that the electrical components within your microwave are failing. Do not attempt to repair or troubleshoot electrical issues with your microwave, especially if you smell burned plastic or wiring. Either replace the appliance or contact a trained technician to repair it.

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Renee Miller began writing professionally in 2008, contributing to websites and the “Community Press” newspaper. She is co-founder of On Fiction Writing, a website for writers. Miller holds a diploma in social services from Clarke College in Belleville, Ontario.

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How to Test a Microwave's Magnetron
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How to Test a Microwave's Magnetron
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How to Test a Microwave's Magnetron

Ever wonder if your ‘700 watt’ microwave oven’s actually cooking anywhere near its rating? Does it seem slower these days than when it was new?

Here’s a quick & easy way to find out what’s taking place in there:

Fill a Microwave-safe container with 1 carefully measured liter of water, preferably at room temperature, about 70F, and measure its temperature (Fahrenheit) as accurately as possible. Write that temp. down, and place the container in the MW. Set the timer for 2:03, and hit start. (If yours is a ‘mechanical timer’ type, with just a dial, use a stopwatch – this must be accurately timed.)

Carefully measure the ‘end’ temperature and multiply the difference by 19.4. The result is the approximate energy gain in watts. (I know you’re wondering about those 3 seconds – it takes about that long for the magnetron tube’s filament to heat and start to ‘fire’, and we want exactly 2 minutes of heating)

Keep in mind that it’s normal for a microwave to produce less energy as it ages (hmmm… sounds familiar somehow!), but your results should be within about 50-75 watts of the rating.

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This test is generally made only when a customer complains that the food appears to be under-cooked or takes a long time to cook thoroughly.

There have been a number of attempts to quickly determine if the oven is performing to its specified power. However, the simplest and perhaps the most reliable involve heating a known volume of water for a specified time, and noting the temperature change following the heating process.

There two arrangements for this type of test; JIS and IEC705.

The JIS system was originally provided for ovens manufactured up to 1990, while the IEC705 arrangement was established for ovens manufactured after 1990.

The IEC705 arrangement is aimed more at a laboratory environment where mains voltage and ambient conditions are controlled.

This test is designed to provide a guide to the general efficiency of the heating system and to indicate gross loss of power output. It is not a laboratory test and as such has a wide toleerance in terms of results

2 x 500ml plastic beakers
1 x Accurate thermometer
1 x Flat stirrer

  1. Fill each beaker with 500ml of water at 20 C° +/- 5 C°
  2. Check the precise temperature of each and if there is a difference find the average by adding the two values together and dividing the result by 2.
  3. Place the beakers in the centre of the cooking area and set the oven to full power and switch on. Allow the oven to operate for precisely 60 seconds, allowing 2 seconds for the magnetron to reach operating temperature.
  4. Remove the beakers from the oven and stir each beaker immediately before taking the temperature of each, repeating step 2.
  5. Subtract the average starting temperature from the average final temperature and multiply the result by 70 to give the value representing the oven power in Watts (JIS)

Starting temperature: (19.3 + 19.9) /2 = 19.6 C°
Final temperature: (27.1 + 30.5) /2 = 28.8 C°
Difference: 28.8 – 19.6 = 9.2 C°
Power output: 9.2 x 70 = 644 Watts JIS

IEC705

  1. Proceed as for JIS test, but with water at 10 C° +/- 2 C°
  2. On reaching step 5, multiply the difference between the averaged values by 71.15 to give the power output in Watts IEC705

Starting temperature: (8.8 + 11.0) /2 = 9.9 C°
Final temperature: (19 + 20) /2 = 19.5 C°
Difference: 19.5 – 9.9 = 9.6 C°
Power output: 9.6 x 71.15 = 683.04 Watts IEC

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Your microwave may be equipped with various types of thermal fuses to monitor and control the work of the internal parts, such as Magnetron, Blower Motor, Fan Motor, etc.

How to Test a Microwave's MagnetronHow to Test a Microwave's Magnetron

Thermal fuses can be non-resettable and resettable which is also called thermal cutoff switch or thermostat. The newer models of microwaves and other appliances tend to use the resettable thermal fuses.

In your microwave, each thermal fuse controls specific internal part and monitors the temperature increase and decrease of that component. All thermal fuses, except the one that controls the fan motor, should have continuity. To test the fuse for continuity, you will need to disconnect one of its terminals and check it with multimeter.

The thermal fuse that control the magnetron can completely shut down the microwave. Its location varies depending on the type of the microwave:

In Countertop Microwave: Usually It is mounted on top of the microwave shell near the magnetron

Over the range Microwaves: Usually it is screwed directly to the magnetron

To test the thermal fuse control the magnetron, please disconnect one of the terminals of the fuse and check it for continuity with a multimeter.

Magnetron Failure.

The magnetron can fail in several ways, many of which produce obvious visual symptoms that can be seen and require no testing with a meter.

Components in a magnetron.

How to Test a Microwave's Magnetron

When the magnetron must be replaced, here are some Magnetron Replacement Considerations

  1. Be careful not to strike or touch the antenna dome area
  2. Be sure to transfer any add-on parts, such as an air duct or thermal fuse or cutouts
  3. Ensure that the wire mesh RF gasket is intact and in place
  4. Examine the rim of the opening where the magnetron dome is to be inserted into the waveguide. Smooth out any irregularities, such as dents, pits, and burns. The rim surface should be bare metal, smooth to the touch. Use light-grade sandpaper – do not use steel wool.
  5. If there is evidence of poor terminal connections (i.e., discoloured, burned, pitted connectors), repair or replace the slip-on connectors on the filament leads
  6. If possible, perform an RF leakage check around the magnetron

Following is a pictorial list of common magnetron failures that are visible, along with their respective symptoms and solutions.

Terminals showing signs of burning.

Insulator breakdown begins with a tiny burned spot on the magnetron insulator, then with each subsequent cook cycle, progressively produces stronger arcing and burning, eventually leaving clear visual evidence of the failure as shown in the picture to the right

Symptoms: Loud hum, no heat, arcing sound, electrical burning smell

Solution: Replace the magnetron and change the terminals making sure they fit properly.

Cracked Magnet(s)

This is caused by the magnetron over heating, some instances this is because of reflected microwave energy.

How to Test a Microwave's Magnetron

Symptoms: Weak or no heat, magnetron gets extremely hot (overheats), intermittent arcing or “snapping” sound

Solution: Replace the magnetron and check why the magnetron has overheated.

Burnt or melted antenna cap.
Burned Dome (or Antenna) caused by arcing due to reflected microwave energy . When this occurs, check for a stalled or arcing stirrer blade or non-rotating antenna assembly. In many commercial models with multiple magnetrons, the cooking tray must be raised to inspect the condition of the lower antenna assembly.

How to Test a Microwave's Magnetron or you may see this inside the cavity How to Test a Microwave's Magnetron

Symptoms: Weak or no heat, arcing sound during cook cycle

Solution: Replace the magnetron and, if necessary, the respective antenna or stirrer assembly. Clean the cavity wear the arcing has happen. To much carbon build up will cause the arcing to return within a very short period of time..
===============================================================================

Loose Magnetron terminals

Loose Magnetron Filament Connectors / Discoloration of the connector(s) or plastic insulator(s)
If the connectors that slip onto the magnetron filament terminals become loose or are improperly crimped, it causes a build up of resistive heat. As this occurs the connection further deteriorates causing the following visual symptoms Small blackened pits in the magnetron terminal(s)
Melted, and decayed appearance
Also, as noted above, an unusually fearsome spark is produced when discharging the capacitor.
Symptoms: Intermittent and/or low heat initially, then eventually no heat

Solution: Repair defective terminals as follows:
Either (1) Clean the burned/pitted magnetron terminals and replace the slip-on connectors, making sure they fit tightly on the terminals; or (2) Cutaway burned wire and connector(s). (Make sure there is enough remaining wire to reach with some slack) Clean terminals to prepare for soldering. Solder the filament leads directly to the magnetron. Be careful not to apply soldering heat any longer than necessary.

How to Test a Microwave's Magnetron

This article is part of Epi Loves the Microwave, our exploration (vindication?) of the appliance everybody loves to hate.

It’s frighteningly easy to overcook—or undercook—food in your microwave. And that’s no surprise: Unlike cooking something on your stovetop, it’s much more fussy to check in on food when you need to stop the microwave, open the door, check on the food, close the door, and press start again. Way more work than just lifting the lid on a simmering pot.

The other major roadblock to perfectly cooked food in the microwave? The microwave itself. Or at least its wattage. No matter how good your microwave recipe is, if it was tested with an 800-watt machine, and yours is 1,200-watt, then you’re for sad, shriveled food. Unless, of course, you Know Your Wattage.

You can find your microwave’s wattage by looking at its manual. Because you know where that is, right? If you’d rather not rummage through dusty toaster oven boxes and tangled laptop cords to find it, just try to find the wattage on the machine itself. It’s usually on a label right on the door or inside it. And if all else fails, it’s easy to test wattage on your own. Wattage ranges from 600 to 1,200 and is a pretty good indicator of a microwave’s power (the higher the wattage, the more powerful it is).

To find an approximation of your machine’s wattage, fill a microwave-safe liquid measuring cup with 1 cup cold water. Microwave on High and keep an eye on it, noting how long it takes for the water to come to a boil:

1 1/2 minutes: 1,200 watts

2 minutes: 1,000 watts

2 1/2 minutes: 800 watts

3 minutes: 700 watts

4 minutes: 600 watts

Armed with that info, you can adjust your microwave’s power level—or cooking time to suit the wattage that’s specified in your recipe of choice.

To match a lower wattage

To match the power of a microwave with a lower wattage, simply divide the desired wattage by your microwave’s wattage. Shift the decimal two places to the right to get the percentage power you should set your machine to. So if you have a 1,000-watt microwave and are following a recipe that calls for a 600-watt one, you should set the power to 60%. If your machine is 1,200-watts and needs to mimic a 800-watt one, you should set the power to 70% because you’ll round the 66.67% up to 70%.

To mimic a higher wattage

You can’t approximate the power produced by a machine with a higher wattage; you can only increase the cooking time. You’ll need about 10 additional seconds for every 100 watts for every minute of cook time. So if something takes 2 minutes in a 1,200-watt microwave on 100% power, it’ll take 2 minutes plus 20 seconds in an 1,000-watt one.

Even with the ability to adjust your microwave’s power, there are other factors that affect how a microwave heats and thaws. As always, size matters. (And yes, the bigger, the more powerful.) Different technologies also affect power. Convection microwaves blow hot air around the food, which causes it to heat much more quickly than in conventional models. Microwaves with “inverter” technology offer more even heating at lower power levels. Put it this way: If you set a regular microwave on 50% power, it will approximate that by switching between 100% power and zero power throughout the cook time. With an inverter microwave, the power is at 50% the whole time, which is especially helpful for thawing.

What your microwave can’t do is control the power coming to it. On days when your whole power grid is under stress (think super-hot summer days) or when you’ve got a lot going on (vacuuming while doing your laundry and running your air conditioning), your microwave may not be getting—or emitting—as much as power as usual.

Your best bet for controlling the machine’s power is standing by its side. Set the timer in short increments and keep checking on its progress. You won’t end up overcooking your food (and robbing it of precious nutrients), and you’ll get to know your microwave’s power in the process.

Appliance Express August 23, 2017 Microwave Repair Leave a Comment

How to Test a Microwave's Magnetron

A blown fuse is one of the most common problems a microwave can have, but that’s just the indication that something has gone wrong. A blown fuse really means one of your electrical components has broken or failed, and the next step is to find out which piece that is. Here’s how:

What do you do know that you know the inline fuse is blown?

If the fuse is blown, that’s usually caused by another malfunction.

WARNING: Do not attempt unless you have training in appliance repair. Microwaves can have 2000 volts of electricity in the high voltage circuit and can easily lead to injury or death. Be sure to discharge the capacitor before messing with anything inside the microwave. If you do not how to discharge a capacitor safely, STOP RIGHT NOW.

Use a resettable fuse before testing so you don’t run through new fuses while testing primary malfunctions, but you can test with new fuses if that is more convenient. Then, you need to repower it: this means putting your microwave back together again so you can plug it in safely. Even though a microwave can run without the cover, you want to fully reattach it for your safety.

Isolate the high voltage circuit by removing one lead off the power supply leading to the transformer. Once the power is back on, test it by microwaving something like a small bowl of water or anything that lets you easily test the heating power. In the most common scenario, your microwave will start up and run with no issue. This verifies that nothing that takes just 120 volts like your door switches and electronic control is causing the issue.

WARNING: Do not attempt unless you have training in appliance repair. Microwaves can have 2000 volts of electricity in the high voltage circuit and can easily lead to injury or death. Be sure to discharge the capacitor before messing with anything inside the microwave. If you do not how to discharge a capacitor safely, STOP RIGHT NOW.

Microwave Capacitor: Test the capacitor by first discharging it. Isolate the capacitor and check for resistance on the most sensitive scale. Your meter needs to be using a 9-volt battery. Place your meter leads on the ends of the cap and you should see a quick rise and fall on your meter. If the resistance stays high, the capacitor is stuck open and needs to be replaced. If the resistance never changes, the capacitor is shorted and needs replaced.

Microwave Diode: Isolate the capacitor and check for resistance on the most sensitive scale. Your meter needs to be using a 9-volt battery. Place your meter leads on the ends of the diode and you should see a quick rise and fall on your meter. If the resistance stays high, the diode is stuck open and needs to be replaced. If the resistance never changes, the diode is shorted and needs replaced. This needs to be checked on both ends of the diode, and it should only rise from one end since diodes only allow electricity from one direction.

Microwave High Voltage transformer: Transformers are very similar but we recommend you pull the wiring schematic to locate what wire does what. From there, you will measure the proper resistance for each line when the microwave has no power. If the resistance deviates strongly from these amounts, then the transformer is broken and must be replaced.

Microwave Magnetron: There is no way to accurately bench check a magnetron. Inspect the component for physical damage and rule out the other three components to determine if the magnetron is at fault.

If you want to learn more about diagnosing common electrical problems in your appliances or you’re looking for specific parts, check out Appliance Express for more information.

How to Test a Microwave's Magnetron

Microwave ovens are a testament to man’s ingenuity and creativity. For someone to have the foresight to imagine that food can be cooked without ever being placed in an oven or near a fire is astonishing. The microwave oven is a marvel of science, available right in our kitchens.

So, you’re here because your microwave oven isn’t working as well as it used to or it isn’t working at all. Many times, you can quickly and inexpensively fix the problem by yourself. You will need to have a volt-ohm meter, which you can pick up at most hardware centers. A notebook to record your process is a valuable assistant when it comes to trying to remember which screw goes where. As always, unplug the appliance before troubleshooting unless otherwise noted. Now, let’s see if we can fix that problem.

Note: Before touching any internal parts, be sure to discharge the capacitor. The capacitor stores additional voltage and can hurt you even if the unit is unplugged. To discharge a capacitor safely, you will need the following: a screwdriver, a wire-wound resistor with a 2 watt-20,000 ohm rating, and a pair of jumper wires with alligator clips on the ends. Clip a wire to each end of the resistor. Clip one wire to the metal shaft of the screwdriver. Clip the other wire to one of the capacitor’s terminals. Now, touch the other terminal with the tip of the screwdriver. There may be a small spark. If the capacitor has three terminals, do the same process with the middle terminal and each outside terminal.

Your Microwave Oven Won’t Run at All

Unplug the power cord and check for voltage at the outlet. First, inspect the cord for any damage or burn marks. Because of all the safety devices in a microwave oven, any one of them could be the cause, but before you can look more closely, you will need to remove the outside shell of the microwave. Unscrew the screws underneath and on the back that hold the shell in place and slide it off.

Check whether or not the fuse is blown by removing it with a set of fuse pullers. Place it on a paper towel so it doesn’t roll away, and then with your VOM set to RX1, place a probe on each end of the fuse. The reading should be zero. If not, replace the fuse with an identical one.

Your door switch could be the problem. Locate the door switches and remove the leads. With the VOM on RX1, probe the terminals. The reading should be infinity with the door open and zero with it closed. If not, replace it. Make sure to check both door switches.

Also, it could be a bad fan motor. Locate the fan and remove the leads. Once again, with the VOM set to RX1, probe the terminals. If the reading is infinity, then it is bad and needs replacing.

Your Microwave Keeps Blowing Fuses

Check the door switch as described above. The capacitor or diode may be bad. Discharge the capacitor as described earlier in the article, and then test it by removing the leads and setting the VOM to RX100. Probe the terminals. The reading should start in the low ohms and increase toward infinity. Reverse the probes and re-test. The reading should do the same thing, otherwise, you’ve found the problem.

To test the diode, disconnect the diode from both the appliance and the capacitor. With the VOM set to RX100 as before, probe the wires. Then reverse the probes and read again. You should get infinity for one reading and low ohms for the other reading. Another cause could be a faulty magnetron, but due to the sensitivity of that piece, it’s best left to a professional.

Your Microwave Oven Cooks Slowly or Unevenly

Check the voltage at the outlet supplying power. If it is lower than 115 volts, there is a problem with your electrical service or breaker. A bad turntable motor may also be the cause. To check it, turn the microwave over onto its top and remove the bottom grill. Set the VOM to RX1, remove one lead from the motor terminals, and probe the terminals. If the reading is infinity, then replace the motor. The magnetron and the waveguide may also be the culprits here, but they need to be serviced by a professional.

Your Microwave Runs but Won’t Cook Anything

For this problem, first, check the thermal cutoffs for both the oven and the magnetron. The thermal cutoffs are little disc-shaped devices with a wire connecting the two of them. Remove a lead and set the VOM to RX1 again before probing the terminals to look for a reading of zero. If the reading isn’t right, then it will need replacement. You’ll need to check both thermal cutoffs.

If these are okay, check the capacitor and diode as described above. The magnetron or transformer could also be bad, but they need to be serviced by a professional.

These are the easiest and least expensive repair situations for problems with a microwave oven. Any issues not covered here will require a professional in most cases. As always, have the make and model numbers handy when heading to the parts shop for replacements. If your microwave isn’t the only appliance giving you headaches, this website has repair and information guides for many of them. Pick your next project, heat up a cup of coffee in your now-working microwave, and read on.

Looking to purchase a new microwave? Check out our Microwaves Buyer’s Guide.

The vacuum-tube cavity magnetron is nearly obsolete (except for the millions in consumer microwave oven. Its development was key to highly effective WWII radar, and it also led to other RF/microwave vacuum-tube devices.

Vacuum tubes are so “yesterday,” aren’t they? They have been rendered obsolete and supplanted by solid-state devices for many reasons, except in some highly specialized applications such some radar transmitters. Similarly, the venerable cathode ray tube (CRT) which was used for decades in home TVs, oscilloscopes, user consoles, monitors, and all sorts of displays has been replaced by flat-screen units

Certainly, CRTs are gone, but there is still one vacuum tube which survives with wide use in a specific application — although it has been largely obsoleted in many others. How so? If you have a microwave oven in your kitchen, you have a vacuum tube called a magnetron in your house. Yet this humble, unassuming active device also changed the course of World War II, in the opinion of many experts and historians.

Q: What is a magnetron?

A: A magnetron is a specialized vacuum tube which does one thing: it is a power oscillator source for frequencies of several hundred MHz to several GHz. Depending on size and other factors, it can produce tens and hundreds of watts to kilowatts.

Q: Why even study this unique and somewhat obsolete device?

A: There are at least three reasons: it is still in widespread use, and millions are made every year; large ones are used for radar and broadcast operations; and it taught scientists and engineers about electronic devices which use electromagnetic principles and combine RF electric and magnetic fields, and more, resulting in important RF/microwave devices such as the traveling wave tube (TWT).

Q: What is the physics principle and basic arrangement of the magnetron?

A: Unlike an oscillator built around a resonant circuit made of discrete inductors and capacitors, a magnetron uses a unique physical structure in conjunction with a combination of electric fields, electron motion and magnetic fields in a confined metal cavity. While the magnetron is a vacuum tube, it is very much unlike a conventional vacuum tube, which uses electrons emitted from a heated cathode and which travels in a straight line to the positive charged anode, with their travel path modulated by the electric field of an intervening grid.

There is no magnetic aspect to a conventional vacuum tube. In contrast, the magnetron is a “crossed field” device which uses an electric field in conjunction with a magnetic field with their field-energy lines at right angles to each other. (The name “magnetron” is a combination of “magnetic” and “electronic”)

Q: How does the magnetron work?

A: Analysis of the magnetron can range from a qualitative explanation to highly technical analysis using advanced electromagnetic field theory and math. We’ll use the more qualitative approach.

Q: What is the physical arrangement of the magnetron?

How to Test a Microwave's Magnetron Fig 1:The vacuum-tube magnetron uses resonant cavities in its anode into which electrons, emitted by a heated cathode, are directed by a powerful static magnetic field at right angles. (Image: Hyperphysics/Georgia State University)

A: The basic, first magnetron – and there are many variations, of course – used a solid block of copper (for thermal dissipation) drilled with holes (called cavities) (Figure 1). The size of these cavities is critical to establishing the operating frequency of the magnetron. This physical construction and arrangement is radically different from the glass-envelope vacuum tube which had been used in an attempt to generate the short wavelengths efficiently and high frequencies needed for RF/microwave designs (1 GHz = 1000 MHz = 0.3 meters = 30 cm).

Q: How does this arrangement function when energized?

A: The cathode at the center (which is heated by a filament) emits electrons, in the same manner as the cathode of a glass vacuum tube, but that’s where their similarities end. These electrons would normally be attracted to, and travel as radial spokes, to the outer ring as an anode, which is positively charged (like the plate of a tube). However, there is a powerful static magnetic field (blue lines) aligned with the axis of the magnetron core. This field causes the electrons to instead travel in a circular flow pattern to the outer ring (red lines). The magnetic field was originally developed by electromagnets, but as more-power permanent magnets were developed years later, these were used instead.

Q: It seems as if all that has been done is to shift the static electric flow, and there is no oscillation – so how does the magnetron produce oscillation?

A: The magnetic field deflects the electrons and they “sweep” around the circle. In doing so, they “pump” at the natural resonant frequency of the cavities. The resulting current around the cavities causes them to radiate electromagnetic energy at the resonant frequency of the cavities.

Q: Is that all? How is this resonant energy made useful?

A: From a physics standpoint, work is done on the electrons, and they absorb energy from applied the power supply applied to the anode. The electrons continue to sweep and reach an energy level at which there is an excess negative charge, and that charge is pushed back around the cavity. This, in turn, imparts energy to the oscillation at the natural frequency of the cavity (pumping). The cavity is analogous to a resonant LC tank: the positive charged field is along one edge of the open side of a cavity, and the negative charged field is aligned along the other edge, so the separated row functions as a capacitor with a vacuum-gap for spacing.

Q: How is the oscillating energy extracted from the magnetron cavity and put to use in a system?

A: A coaxial coupling with a precisely sized probe is inserted sideways into one cavity to capture the energy from the block, Figure 2; it functions as a receiving antenna for electromagnetic energy.

How to Test a Microwave's Magnetron Fig 2: A frequency-matched probe is inserted into an opening in one of the cavities to intercept and extract the oscillating RF energy in the magnetron. (Image: EU Radar Tutorial)

Q: What sets the frequency of the magnetron oscillations?

A: The size and arrangement of the cavities sets the frequency, as they act as the resonant chambers. Magnetrons generally have a small adjustment screw to change the cavity size so the physical dimensions can be adjusted to resonate at the exact desired frequency despite inevitable manufacturing tolerances. Note that a magnetron is a fixed-frequency device and is not tunable, although there are some advanced and more complicated versions which have a modest tuning range.

Part 2 of this FAQ will look back at the history and role of the magnetron, and ahead to its future and possible demise.

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