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Microwave leak detector

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G^3

Sun Mar 08 2009, 03:04AM

Registered Member #97 Joined: Thu Feb 09 2006, 05:40PM
Location:
Posts: 61

I built a microwave leak detector that consists of a schottky diode in anti-parallel with an led as described on http://www.repairfaq.org/sam/micfaq.htm#micsimkleak. The only problem is it doesn't seem to work. I assume that my microwave leaks some because it is old and when I use it my wireless internet really slows down. I tried all along the door seam in various positions, but nothing seemed to work. Any hints?

Thanks

Proud Mary

Sun Mar 08 2009, 03:19AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The LED thing looks like a gimmick to me.

Just make a quarter-wave antenna out of stout wire, couple that to a microwave diode, followed by an op amp like OP-07 and a meter, and stick the whole thing in a diecast metal box - except the antenna, of course!

Or you could cut a stripline out of Vero board, and rectify and amplify the output of that for the meter.

G^3

Sun Mar 08 2009, 09:21PM

Registered Member #97 Joined: Thu Feb 09 2006, 05:40PM
Location:
Posts: 61

If I used Intensity (w/m^2)=.5*c*epsilon naught*(E)^2 where E is the electric field required to light an LED or about 1Volt/(6cm) I get an intensity of ~0.4W/m^2 or about .04mw/cm^2 and a leaky microwave is considered dangerous at 100 times that level. I guess it doesn't work, but I would like to know why.

P.S. I am using a 1n5819 and a random red LED.

Bjørn

Mon Mar 09 2009, 03:31AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Back in the days when many of the 4HV members were experimenting with microwaves the LED/diode detector was common. If I recall correctly they used a diode that was 50-100 times faster than the 1n5819. Remember that the signal is 2.45 GHz, a medium fast or fast diode will be too slow.

Wolfram

Mon Mar 09 2009, 07:05AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

BjÃ¸rn wrote ...

Back in the days when many of the 4HV members were experimenting with microwaves the LED/diode detrector was common. If I recall correctly they used a diode that was 50-100 times faster than the 1n5819. Remember that the signal is 2.45 GHz, a medium fast or fast diode will be too slow.

Spot on. If I'm not mistaken, the diode most commonly used was the 1N5711

.

A.M.

klugesmith

Mon Feb 22 2010, 07:46PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

G^3 wrote ...
... I assume that my microwave leaks some because it is old and when I use it my wireless internet really slows down. I tried all along the door seam in various positions, but nothing seemed to work. Any hints?

Is it common for wireless routers to suffer interference from microwave ovens, even when the latter are in compliance with leakage standards? My guess is yes, I think Netgear WGR614v4 (for example) depends on unlicensed radio communication in the 2450 MHz ISM band. Just found a discussion here:

My dad's tenant complained that his built-in MWO has been interfering with his wireless Internet connection, and with a digital electronic scale on the kitchen counter. I dusted off my old Holaday survey meter

Verified it qualitatively by jamming paper behind the door of MWO at home to induce leakage. Took the meter to tenants house this morning. Confirmed the interference, and found that it went down when the MWO door was pressed just right. But the meter showed no measurable leakage, even on its most sensitive range (FSR = 2 mW/cm^2).

I told the tenant there was no safety hazard even for his wife & baby -- that I would have no problem sleeping next to the MWO -- and that I would look into the interference thing. Any ideas, other than f***ing with the door hinge adjustment and hoping not to make things worse? The wireless router is at the far end of house, and the troubled computer location is in kitchen about 8 feet away from the MWO.

[edit] p.s. Initially, the "robust 1mA meter movement" in the Holaday instrument was not moving freely. It got draggy in the bottom 20% of the range, with no visible mechanical interference. After taking it apart to the point of seeing both front and rear bearings, and looking for a dust fiber stuck in the gap, I found the probem with the aid of a 10x loupe. A splinter of ferrous metal about 1/2 mm long (.020") was stuck to the edge of moving coil. In the magnetic field it kept a radial orientation, and could not be dislodged with duster spray or a thin stiff wire. Eventually I brushed it away (?) by passing a narrow strip of paper through the gap.

Proud Mary

Tue Feb 23 2010, 08:55AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Perhaps the rectifier diode is working partly in varactor mode, rich in harmonics.

radiotech

Tue Feb 23 2010, 07:17PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

My diode was 1N21. It comes in a neat ceramic package and was in old radars. Narda used to furnish them wrapped in lead foil. The probes in my Narda 8700 uses thermocouples, which is why one shouldn't drop the probe. Only one way to learn this. ref Narda test set manual.

G^3

Fri Oct 15 2010, 06:19AM

Registered Member #97 Joined: Thu Feb 09 2006, 05:40PM
Location:
Posts: 61

I bought some 1n5711's and made a simple detector and it worked. I have a new microwave and to light the LED I had to wedge a plastic bag folded over a few times into the door.

Strangely the LED on my detector fried and is now an open circuit.

Adam Munich

Fri Oct 15 2010, 07:18AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I've used an AM radio to detect microwave leaks before. Tune it to an empty place on the band and put on some earphones. Turn up the volume as high as it goes and fire up the microwave. If you hear a 60hz hum it's leaky.

If it fried the LED then that's quite a leaky oven.

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