Microwaves from the wave guide

[gary350]

How much does microwaves spread when they come from the wave guide?

I saw this in college text book 1970 the longer wave guide prevents the waves from spreading wider than a shorter wave guide.

 

[debe]

Depends on the wave guide. This is a small Furuno radar antenna., its center fed slotted wave guide aprox 18 inches long & gives a vert 25 degrees & horizontal 5 degrees. Narrower horizontal they use a mutch longer antenna.

 

[gary350]

College class we had a microwave transmitter and receiver first test was across the room then 2nd test was to another building 2nd floor about 1 block away. We had lots of attachments, receiver had a funnel on the wave guide. I remember adding more extensions to transmitter it worked better receiver picked up a stranger signal. After college I worked with another guy Bob we drover all over several States servicing microwave towers. I never did get a close up look at how those antennas worked it looked like some type of funnel about 300 ft up both transmitter & receiver. Wow that was 50 years ago.

Wave guide in 25 year old microwave ovens were longer full length of the oven. Wave guide in the new micro wave ovens are only 6" long. I have several old microwave ovens that were trashed because they don't work but I see they all work if I by pass the circuit board. Circuit board is what goes bad. I used metal case to build project yesterday. Free metal, LOL.

I used a microwave oven to do a short test. I made a 2 ft long wave guide extension then aimed it at a slice of bread 30 ft away on the wooden wall other side of room. I pushed the normally open switch for 10 seconds the slice of bread 30 ft away was about 20 degrees warmer. It works. I worry microwaves could reflect off of certain objects like metal but I have no metal walls in the work shop.

I need to do something these microwaves they been laying in a pile for 5 years.

 

[unclejed613]

first of all you don't want to be radiating a kilowatt signal into a room you are in (or near for that matter)... you can cause tissue damage to yourself, and one of the first place this shows up is in your eyes in the form of cataracts. also be aware that your waveguide extension was unterminated, and depending on the actual length of it could cause a serious impedance mismatch, which is not good for the magnetron. every dimension inside a microwave oven cavity is a carefully calculated thing. if you look at metal racks for use inside the oven, they all have identical dimensions for the length and width of the gridwork, which is actually a series of 1/4 wave shorted stubs (which is a high impedance at 2450Mhz), which will absorb very little energy. there is an analogy with the waveguide as a series of quarter wave shorted stubs, where you begin with an open wire transmission line (2 parallel wires, in this case they are also 1/4 wavelength apart). then supports are provided to hold the two wires 1/4 wavelength off the ground, and the supports are made of 1/4 wave shorted stubs, which allow the wires to be supported, and the shorted ends of the stubs can be laid on a grounded surface without affecting the signal on the transmission line. for added strength, the vertical sides can become metal walls, and the short circuit of the stub can become a metal floor, so you end up with a solid U shaped channel. for protection against weather, an upside down channel of the same dimensions can be laid on top of the channel, so now you have a piece of rectangular tubing with a 2:1 ratio of height versus width, and this is a wave guide.

one thing required to efficiently couple a wave guide into open space is some way of matching the impedance of the waveguide to that of open space, the simplest way to do this is the use of a horn antenna (the "funnel" you mentioned) if you don't have this at the end of the waveguide, some of the signal will be reflected back down the waveguide to the magnetron, and can damage the magnetron.

to protect yourself, the best thing would to run your experiments inside a faraday cage made of metal window screen (with the screen grounded)... with you outside the faraday cage...

 

[shortbus=]

to protect yourself, the best thing would to run your experiments inside a faraday cage made of metal window screen (with the screen grounded)... with you outside the faraday cage...

Oh sure! you just don't want him to win top prize in the Darwin Awards this year.

 

[Reloadron]

Oh sure! you just don't want him to win top prize in the Darwin Awards this year.

How do I defeat these darn door interlocks? I want to better see my hot dogs cooking in there.

Ron

 

[Reloadron]

Somewhere buried in my piles of old books I have a book titled Microwave Theory and Applications written by . During the early 80s I attended a seminar in Washington DC presented by Mr. Adam. He was a lead engineer for HP at the time and the guy who developed what we called the HP cookie jar (frequency meters). If you can find a copy of his book it is an excellent read and well illustrated with drawings. The guy was pure genius. BY 1990 my microwave days ended as I moved on but I remember the sign on our microwave lab entrance which read "If it's below 1 Gig it's DC. Somewhere my autographed book lies around here.

Ron

 

[Visitor]

Years ago, the microwave at the office was acting up. We took off the cover while it was on a table under a fluorescent work light and pushed the start button. The door was properly shut during this test.

This caused the fluorescent tubes 3' above the oven to glow and flicker. We decided not to try that again

 

[Nigel Goodwin]

Having repaired huge numbers of microwaves over the years, and leakage tested huge numbers, I can confirm that there's no leakage above the permitted levels even with the top removed. Obviously you don't normally test for leakage with the top OFF, but when you've got a leakage tester (and you spend a considerable time next to running microwaves with the tops off) it's nice to check these things

I suggest Gary takes them all to the dump and gets rid of them, he's not safe round anything remotely dangerous.

 

[gary350]

first of all you don't want to be radiating a kilowatt signal into a room you are in (or near for that matter)... you can cause tissue damage to yourself, and one of the first place this shows up is in your eyes in the form of cataracts. also be aware that your waveguide extension was unterminated, and depending on the actual length of it could cause a serious impedance mismatch, which is not good for the magnetron. every dimension inside a microwave oven cavity is a carefully calculated thing. if you look at metal racks for use inside the oven, they all have identical dimensions for the length and width of the gridwork, which is actually a series of 1/4 wave shorted stubs (which is a high impedance at 2450Mhz), which will absorb very little energy. there is an analogy with the waveguide as a series of quarter wave shorted stubs, where you begin with an open wire transmission line (2 parallel wires, in this case they are also 1/4 wavelength apart). then supports are provided to hold the two wires 1/4 wavelength off the ground, and the supports are made of 1/4 wave shorted stubs, which allow the wires to be supported, and the shorted ends of the stubs can be laid on a grounded surface without affecting the signal on the transmission line. for added strength, the vertical sides can become metal walls, and the short circuit of the stub can become a metal floor, so you end up with a solid U shaped channel. for protection against weather, an upside down channel of the same dimensions can be laid on top of the channel, so now you have a piece of rectangular tubing with a 2:1 ratio of height versus width, and this is a wave guide.

one thing required to efficiently couple a wave guide into open space is some way of matching the impedance of the waveguide to that of open space, the simplest way to do this is the use of a horn antenna (the "funnel" you mentioned) if you don't have this at the end of the waveguide, some of the signal will be reflected back down the waveguide to the magnetron, and can damage the magnetron.

to protect yourself, the best thing would to run your experiments inside a faraday cage made of metal window screen (with the screen grounded)... with you outside the faraday cage...

I wish I had a lower power microwave unit 1400 watts it dangerous. That is way I used a spring loaded N.O. push button on switch no matter what happens i can turn it off fast. I have been wanting to use 1 of the 1400KW transformers to build a spot welder but the project I needed it for has already been built so I don't need spot welder anymore. I have replaced secondary coil on several transformers to get different voltage.

What you mention makes good sense now because I remember college microwave unit came with several extensions each was a multiple length of the others. I don't remember the exact length guessing the longest were 8" and one was 4". The funnel shape ends were several different lengths too, 1 funnel lowered signal at receiver, 1 funnel no change in signal at receiver, 1 funnel stronger signal at receiver. Instructor knew nothing about microwave & 3 of us stayed after class several times to experiment with it.

None of these junk microwaves have metal racks. Are all home microwaves the same frequency? Wonder if I can use wire rack measurement from another microwave to do math to see if my home made wave guide is the correct length? It probably makes so difference is wave guide is improved I have no way to test it. I don't live in town I decided it is much safer to play with 1400 watts outside. I like to set on patio an watch birds there is a tree limb 100 ft away birds like to land on so I aimed wave guide at the tree limb. A sparrow landed on tree limb and I pushed the button. After 1 second bird fell off tree limb like a rock end over end after falling 8 ft it came to life and flew away. Interesting no bad effects I watched the bird continue to fly back and forth to the same tree limb before going to the bird house.

I have a tree limb 25 ft up that shades the garden it needs to be cut off but I can't reach it. Electric chain saw is too heavy to lift up on 25 ft pole. Pole with no chain saw is heavy. I wonder if i microwave the tree limb in 1 spot limb will get 200 degrees hot and die.

 

[Reloadron]

Yes, 2.450 GHz. That is :

S band Band

2 to 4 GHz Frequency Range

7.5 cm to 15 cm Wavelength

The microwave region is broken down into bands and the funnel looking thing on a microwave antenna is the feed horn.

Wavelength is calculated the same way. Divide the speed of light by the frequency to get wavelength. Speed of light expressed as meters / second or 300,000,000 works close enough.

Ron

 

[shortbus=]

I suggest Gary takes them all to the dump and gets rid of them, he's not safe round anything remotely dangerous.

I have a tree limb 25 ft up that shades the garden it needs to be cut off but I can't reach it. Electric chain saw is too heavy to lift up on 25 ft pole. Pole with no chain saw is heavy. I wonder if i microwave the tree limb in 1 spot limb will get 200 degrees hot and die.

I think you're correct about that!

 

[unclejed613]

Are all home microwaves the same frequency?

according to an oven magnetron data sheet, about 2455Mhz, and according to one of the charts the tolerance is +/- 10Mhz.. you might want to check and see if any of your wifi or bluetooth devices have bit the dust since you began messing around with magnetrons. you can find the spec sheet at: https://www.relltubes.com/filebase/en/src/Datasheets/2M256-spec.pdf

most oven magnetrons are very similar, so a couple of takeaways, max VSWR=4 (there's a Rieke diagram thaqt's similar to a Smith chart). operating into a waveguide with no load (either some kind of antenna or dummy load)reflects power back into the magnetron and can destroy it. the "effective range" of the RF to heat organic matter will be limited by the inverse square law, and this is why there aren't any "microwave death ray weapons". as for the bird, 100 feet from a nonterminated waveguide to the branch, you probably didn't cause any heating, but you may have interfered with the bird's "compass" and disoriented it... from what i've read birds have a way of sensing the earth's magnetic field, and migratory birds seem to follow the field lines when they migrate.

 

[ChrisP58]

That is way I used a spring loaded N.O. push button on switch no matter what happens i can turn it off fast.

That's called a dead man switch. When you're dead because it killed you, it will automatically turn off.

 

[gophert]

That's called a dead man switch. When you're dead because it killed you, it will automatically turn off.

Unless he grabs his eyes as the microwaves super heat the water-filled balls and bends forward in pain and expires on top of a simple NO button and the microwaves continue running until someone walks in to find his then-charred body and, as they scream, they grab their eyes in pain...