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'[OT]:Microwave Oven'
2001\08\24@132408 by robertf

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Hello,

I was working on a timer ckt
on top of the microwave oven
in the kitchen( only place
with grounded outlet) when the
wife needed to use it. My
voltmeter was on top and it
started giving me a reading of
about a few tenths of a volt.
The dvm lead was just lying
over the top of the door. This
caught my attention and I
started to run the leads
around the door seal. It reads
when the leads are by it and
no reading when I pull it
away.  Does this mean it is
leaking?

Regards,

Robert Francisco

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2001\08\24@145546 by Thomas C. Sefranek

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robertf wrote:

>Hello,
>
>I was working on a timer ckt
>on top of the microwave oven
>in the kitchen( only place
>with grounded outlet) when the
>wife needed to use it. My
>voltmeter was on top and it
>started giving me a reading of
>about a few tenths of a volt.
>The dvm lead was just lying
>over the top of the door. This
>caught my attention and I
>started to run the leads
>around the door seal. It reads
>when the leads are by it and
>no reading when I pull it
>away.  Does this mean it is
>leaking?
>
Probably not, I'd guess you were measuring the magnetic field of the
power transformer.
You meter has almost no chance of detecting the frequencies of the oven.

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2001\08\24@154953 by Chris Carr

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{Quote hidden}

Thomas C Sefranek wrote

> Probably not, I'd guess you were measuring the magnetic field of the
> power transformer.
> You meter has almost no chance of detecting the frequencies of the oven.
>
> >
Sorry to disagree. but any electrical non-linearities within the measurement
system can cause the meter to register a microwave signal. The Microwave
signal bathing the inside of a microwave oven consists of a 2450 MHz carrier
pulsed at your line frequency 50 or 60 Hz. Old microwave ovens are
particularly prone to leaky door seals And rusty self tapping screws, purely
as part of the wearing out process. It is possible for the rectified
Microwave leakage (Rust Bolt Effect) to be of sufficient power to register
on the unconnected leads of a high input impedance dvm.

If I was you I would play it safe and get your oven checked out for leakage.

Regards

Chris Carr

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2001\08\24@170136 by Nigel Goodwin

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In message <005001c12cd5$b7e244c0$faba7ad5@dougal>, Chris Carr
<nyedspamspam_OUTBTINTERNET.COM> writes
>Sorry to disagree. but any electrical non-linearities within the measurement
>system can cause the meter to register a microwave signal. The Microwave
>signal bathing the inside of a microwave oven consists of a 2450 MHz carrier
>pulsed at your line frequency 50 or 60 Hz. Old microwave ovens are
>particularly prone to leaky door seals And rusty self tapping screws, purely
>as part of the wearing out process. It is possible for the rectified
>Microwave leakage (Rust Bolt Effect) to be of sufficient power to register
>on the unconnected leads of a high input impedance dvm.

I repair microwave ovens (amongst other items), leakage is
EXTREMELY!!!!! rare - I've only ever seen two which leaked, both were
the same Philips model with the door hinging downwards. However, both
were still well within permitted UK limits (but not USA ones!). We are
legally obliged to check every microwave we take apart, so I test a good
many every year, and it's not a problem!.

I would agree that it's more likely the large magnetic field from the
transformer affecting your meter.

>If I was you I would play it safe and get your oven checked out for leakage.

If it will reassure you, get it tested, but it's unlikely to be leaking.
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2001\08\24@232516 by Douglas Wood

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Yes. But only a very, *VERY* little. My dad worked for Litton Microwave for
11 or 12 years. Supposedly, you can run a small fluorescent tube around the
door of an old microwave oven and the tube will grow if there's too much
leakage.

Douglas Wood
Software Engineer
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{Original Message removed}

2001\08\25@020156 by Nigel Goodwin

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In message <007901c12d15$acdb4960$spamBeGonecde51f18spamBeGonespamkc.rr.com>, Douglas Wood
<TakeThisOuTdbwoodEraseMEspamspam_OUTKC.RR.COM> writes
>Yes. But only a very, *VERY* little. My dad worked for Litton Microwave for
>11 or 12 years. Supposedly, you can run a small fluorescent tube around the
>door of an old microwave oven and the tube will grow if there's too much
>leakage.

That sounds unlikely, UK legislation requires leakage to be below 5mW at
5cm distance, USA regulations require below 0.5mW at 5cm - I wouldn't
swear to the distance (although I think it's correct), but the power
values are exact. All Microwaves are built to the USA standard anyway,
almost always on our leakage tester (calibrated yearly, as required by
UK regulations) on a scale of 1-5mW the needle never moves at all.

BTW, the tester has a plastic cone on the front, which maintains the
correct distance, you can remove this and test at zero distance - still
no readings!.
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2001\08\25@062923 by Peter L. Peres

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Your MW is not leaking, if it would be the DVM would go 'bananas'. The
small leakage is normal. There is no such thing as 'perfect shielding' and
with 700W inside it takes better than 1:1000 shielding to keep total
radiation under 0.7W. That is very good shielding. The real leakage is
probably 50mW or so. The DVM is sensitive and picks this up.

Peter

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2001\08\25@132135 by Nigel Goodwin

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In message <Pine.LNX.4.10.10108251305570.576-100000EraseMEspam.....plp.plp.home.org>,> Peter L. Peres <EraseMEplpspamACTCOM.CO.IL> writes
>Your MW is not leaking, if it would be the DVM would go 'bananas'. The
>small leakage is normal. There is no such thing as 'perfect shielding' and
>with 700W inside it takes better than 1:1000 shielding to keep total
>radiation under 0.7W. That is very good shielding. The real leakage is
>probably 50mW or so. The DVM is sensitive and picks this up.

The permissible leakage is 5mW in the UK, and only 0.5mW in the USA,
ovens are almost always (even 20 year old ones) well below 0.5mW.
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2001\08\25@161240 by robertf

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The reading on my dvm does
increase when it is nearby the
'guts' of the over( heavier
side), less away from it.
So it's probably the
transformer.

Thanks for all the input.

Regards,

Robert Francisco
{Original Message removed}

2001\08\25@202326 by Kathy Quinlan

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Yes, but do not forget that the spec also indicates for a given area at a
given distance, here in AU and I think the same in the UK it is <5mw /cm2 at
5cm from surface being measured.

Regards,

Kat.

{Original Message removed}

2001\08\26@085707 by Bob Ammerman

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Now this is starting to make sense. The units of measure (W/cm^2) (specified
at a given distance) are what I'd expect.

Let's take a 'real world' example.

Assume a cube shaped microwave about 50cm on a side.

A surface 5cm from it (at the measurement distance) would be a cube 60cm on
a side. The total surface area of such a cube would be:

        6 * 60cm * 60cm = 21600cm^2

At 5mw/cm^2 this would allow for a total (evenly distributed) leakage of
10.8 watts, or about 1.4% of the output of a 700 watt microwave. This makes
sense.

Even at 0.5mw/cm^2 we can have a leakage of 1.08 watts.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)


{Original Message removed}

2001\08\26@172741 by Peter L. Peres

picon face
> leakage

0.5mW ERP isotropic point radiator? 0.5 mW/cm^2 at 5cm? 0.5mW/cm^2 at 5 cm
with a good 1 meter total seam length around the door, taken as a swath
only 1 cm wide totals 100cm x 0.5mW ~= 50mW total radiated (I am making
some assumptions about the radiation starting as a line and spreading to a
swath 1 cm wide exactly where the detector is).

0.5mW isotropic point radiator for an oven rated 700W that costs $150 is
something I really want to see ;-). It means an attenuation of 7x10^2 /
5x10^-4 ~= 1.4x10^6 = 1,400,000 times. That would be just too neat to be
achievable with a simple hinged door and some stamped steel and colored
glass.

Peter

PS: If you set up 2.4GHz video links and have the equipment you will 'see'
and cuss each and every MW in the neighbourhood (appear as mains noise
bars on AM links).

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2001\08\26@181530 by Jim

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    "That would be just too neat to be
     achievable with a simple hinged door
     and some stamped steel and colored
     glass.

Ever heard of, or seen, a "choke joint"?

Jim


{Original Message removed}

2001\08\27@022648 by Nigel Goodwin

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In message <011a01c12e7c$7a7820c0$0100a8c0@piii500a>, Jim
<RemoveMEjvpollspam_OUTspamKILLspamDALLAS.NET> writes
>     "That would be just too neat to be
>      achievable with a simple hinged door
>      and some stamped steel and colored
>      glass.
>
>Ever heard of, or seen, a "choke joint"?

Personally I've always heard it called a 'choke seal'. We get a lot of
people asking for their new ovens to be tested as they can see light
through the door seal, or steam escapes.

Another point to bear in mind is that you don't have 700W wildly trying
to find it's way out of the cavity - most is absorbed by the load in the
oven (which should never be used empty).
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2001\08\27@104017 by Jim

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"The invention of the 'choke joint'."

Excerpted from:
www.ieee.org/organizations/history_center/oral_histories/transcripts/
ramsey1991.html

``Ramsey: ... Eventually we learned the art. That's one of the things we
were learning. Initially, most of the laboratory waveguide work was being
done by the three centimeter [10 GHz] group because the ten centimeter [3
GHz] group was still dominantly working with the coax lines.   [snip]  I
remember for example just one of the problems. When one just wanted to
fasten two waveguides together, one either butted them together and hoped
they wouldn't spark, but you could see those little sparks at the joint. Or,
else one had fingers inside where you put them together, but these tended to
spark and destroy the impedance matching.

Bryant: Spring fingers?

Ramsey: Spring fingers, yes. Then we tried the trick of using a butt joint
with a **choke**, which worked quite well. So we shifted over to that. As
far as I know that was my invention, although Shep Roberts in the Radiation
Lab joined in that work. ''

** End of excerpt **

Jim


{Original Message removed}

2001\08\27@120011 by Harold M Hallikainen

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On Sun, 26 Aug 2001 08:53:33 -0400 Bob Ammerman <EraseMERAMMERMANspamspamspamBeGonePRODIGY.NET>
writes:
> Now this is starting to make sense. The units of measure (W/cm^2)
> (specified
> at a given distance) are what I'd expect.
>
> Let's take a 'real world' example.
>
> Assume a cube shaped microwave about 50cm on a side.
>


       How about we use a sphere instead, since the entire surface of the
sphere is the same distance from the center. If you put a 1 watt
isotropic radiator at the center of a 1 meter radius sphere, you'd have 1
watt spread over the surface area of the sphere or 79.577mW/m^2.

Harold


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2001\08\27@120155 by Harold M Hallikainen

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On Sat, 25 Aug 2001 06:58:18 +0100 Nigel Goodwin <RemoveMEnigelgKILLspamspamLPILSLEY.CO.UK>
writes:
{Quote hidden}

       These units don't make sense to me... Could it be mw/cm^2 or "power
density?"

Harold



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2001\08\27@173659 by Peter L. Peres

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> choke joint

Yes but not 1:1.4E6 attenuation in one step. They usually have several
impedance steps afair? In my non-extensive experience achieving >50 dB
attenuation of nearly anything in 'one step' is no go (assuming it is not
tuned). Untuned one-step attenuators seldom go beyond 30 dB assuming that
they are to be reproducible and untuned.

Even a simple cover joint with wire-clad gasket has two or three steps
(depending on how you count).

Apropos attenuation, any idea why a cellular phone works perfectly (S
meter at 3 of 5 ?!) in a 4 x 2 x 2 meter welded steel safe with the door
closed? Not all the seams are welded perfectly (this is not a submarine)
but there are not enough bare wires to conduct the cellular signal inside
at that level I think ?!

Peter

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2001\08\27@175324 by Barry Gershenfeld

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>Apropos attenuation, any idea why a cellular phone works perfectly (S
>meter at 3 of 5 ?!) in a 4 x 2 x 2 meter welded steel safe with the door
>closed?
>Peter

Well, no, but I just gotta ask you how you know this :)

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2001\08\27@180338 by Jim

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Peter:

  "Yes but not 1:1.4E6 attenuation in one step. They
    usually have several impedance steps afair? In my
    non-extensive experience achieving >50 dB attenuation
    of nearly anything in 'one step' is no go (assuming it is
    not tuned). Untuned one-step attenuators seldom go
    beyond 30 dB assuming that they are to be reproducible
    and untuned."

Two cases that work against any assumptions posted above:

1) I'm holding in my hand a Harris GSSD/PRD Instruments
   "Type X-101" , an X-band (10 GHz nominal) variable
    attenuator.

  Is is marked from 0 dB through to 60 dB. In my use of these
  and similar device made by HP I do not ever recall having
  'trouble' with the higher attenuation values.

  Of course, this does not use a 'switchable resistor' bank
  either ...

2) The attenuator on any of the Hewlett Packard 608 series
   of sig gens - these are marked on the dial from +7 through
   to -127 dBm. It does this in a continuous, un-stepped fashion
   with one mechanism - I have repaired this model before and
   can testify to the fact that it is one 'piece' which serves to
   couple ever-progressively less energy as the movable 'piece'
   is pulled ever further from it's mating assembly.

  I can attest that this device operates as billed too. These *is*
  leakage from the oscillator portion of this instrument, but it is
  *not* from the attenuator. Testing a fully shielded radio apparatus
   like the GE Mastr Pro line presents no problem from the leakage,
  testing a Japanese made HT or a poorly shielded 'scanner ' is
  another story.

Peter:
   "Even a simple cover joint with wire-clad gasket has two or
    three steps (depending on how you count).

To what is this applicable? (Microwave oven or choke joint)?

Jim

{Original Message removed}

2001\08\27@181830 by Jim

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Peter:
     "Apropos attenuation, any idea why a cellular phone works
      perfectly (S meter at 3 of 5 ?!) in a 4 x 2 x 2 meter welded
      steel safe with the door closed? Not all the seams are welded
      perfectly (this is not a submarine) but there are not enough
      bare wires to conduct the cellular signal inside at that level I
      think ?!"

I once tested a "screen room" where facilites had simply run a LAN
connection into screen the room WITHOUT A FILTER. By placing
my 2-way UHF ham transceiver next to this LAN wire I could bring
up the UHF repeater that was on-site. All other facilities connections
(AC power) were through filters.

Standing in the *middle* of the SR - it was not possible to 'hit' the
machine.

I suspect that much of your 'leakage' in the safe was owing
to poor contact around the door of the safe -  that coupled
with perhaps, a nearby cell site. All it would take is three or
four inches (at that frequency) of even 'poor' metal to metal
contact from the door to the body of the safe and you have
an 'aperature' through which RF energy can propagate.

Was there on-site "cell extender" (bascically two cell-band amps,
appropriately duplexed, to create a bi-lateral amplifier like the
dB Products "Prism" series) with one of the coverage antennas
placed nearby the vault? I offer this as a possible reason - not
knowing where your 'vault' was or the signal level outside the
vault ...

Jim



{Original Message removed}

2001\08\27@182648 by Nigel Goodwin

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In message <KILLspam20010827.085759.-403285.0.haroldhallikainenspamBeGonespamjuno.com>,> Harold M Hallikainen <EraseMEharoldhallikainenspamEraseMEJUNO.COM> writes
>> That sounds unlikely, UK legislation requires leakage to be below
>> 5mW at
>> 5cm distance, USA regulations require below 0.5mW at 5cm - I
>> wouldn't
>> swear to the distance (although I think it's correct), but the power
>> values are exact. All Microwaves are built to the USA standard
>> anyway,
>> almost always on our leakage tester (calibrated yearly, as required
>> by
>> UK regulations) on a scale of 1-5mW the needle never moves at all.
>>
>
>        These units don't make sense to me... Could it be mw/cm^2 or "power
>density?"

Possibly? - I can't say I've ever paid much attention, you have to use
an approved meter, calibrated yearly, and it has to read under 5mW (or
0.5mW in the USA). Presumably there's more to the specification, but
it's not of any use in practice - unless you are manufacturing test
equipment?.

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2001\08\27@212411 by Kathy Quinlan

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A couple of points:

A: A microwave leakage is specified in AU and UK as to be less than 5mW per
cm2 at a distance of 5cm from the surface being tested.

B: Most modern Microwaves have a 3 stage "filter" to stop leakage, I can not
remember what they were or how they operated, as I did the theory 8 years
ago when here in Perth AU you had to have a radiation permit to service a
microwave oven.

Regards,

Kat.

{Original Message removed}

2001\08\29@161112 by Peter L. Peres

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> Another point to bear in mind is that you don't have 700W wildly trying
> to find it's way out of the cavity - most is absorbed by the load in the
> oven (which should never be used empty).

You wish ;-). Power distribution depends on the impedance ratios of the
'hole' and of the food. I suspect that running the oven with the door open
will reduce its 'food' power by 50 to 80%. This because any MW still
heating it will pass through it once instead of many times and the power
distribution would correspnd to that of a 'hole' the size of the open door
with the food obstructing part of it. Ok, I admit this is extreme.

Peter

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2001\08\29@161120 by Peter L. Peres

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> Peter:
>    "Even a simple cover joint with wire-clad gasket has two or
>     three steps (depending on how you count).
>
> To what is this applicable? (Microwave oven or choke joint)?
>
> Jim

Both or none. I meant the cover joints normally used in RF proof equipment
boxes, with a channel and an o-ring clad with wire (and sometimes
flashing), as compared to oven doors. The simple box joints get it down by
40-55 dB in VHF/UHF in my experience, and they are more complex than the
oven door joint.

I believe what you say about the single-'step' continuously variable
attenuators you mention, they can be very good, it's just that as you
said, there are other factors (such as oscillator leakage) which prevent
them from working perfectly in real life. There is also the small issue of
the price tag of those parts and their 'wide' availability.

It is trivial to design a T-pad attenuator with resistors for say 80 dB in
one stage, but due to real life parasitics, radiative coupling, ground
conduction, etc, etc it will do 80 dB only at DC. This is what I meant wrt
'not possible to attenuate in 1 stage'. The 30 dB I gave is a rule of
thumb that comes from my experience with real life equipment and
attenuators, in the sense that a 30 dB T, L or PI assymetrical attenuator
calculated and implemented as designed will indeed attenuate 30 dB +/- 4
dB in the frequency band considered. Above that, the ifs become too many.

If I'm not wrong the ARRL handbook also gives (or gave ?) a DIY attenuator
schematic, and there too the first two stages are 26 dB, not a single 50
dB stage.

Peter

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2001\08\29@161124 by Peter L. Peres

picon face
>>Apropos attenuation, any idea why a cellular phone works perfectly (S
>>meter at 3 of 5 ?!) in a 4 x 2 x 2 meter welded steel safe with the door
>>closed?
>>Peter
>
>Well, no, but I just gotta ask you how you know this :)

Ever got a call while doing things inside the safe - well strongbox - at
work?

Peter

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2001\08\29@161129 by Peter L. Peres

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> Was there on-site "cell extender" (bascically two cell-band amps,
> appropriately duplexed, to create a bi-lateral amplifier like the
> dB Products "Prism" series) with one of the coverage antennas
> placed nearby the vault? I offer this as a possible reason - not
> knowing where your 'vault' was or the signal level outside the
> vault ...
>
> Jim

There are cellular towers all around me there, we are in 'downtown'. They
put them on the roofs and move them all the time so I can't keep track but
there was one definitely within 100 meters from where I was. There is no
repeater that I know of there.

It is hard to judge the field outside, we are in a well-covered city and
the S meter stays at 5 (maximum) all the time while you are not in a
basement or such. I haven't played with graphite sheet attenuators and
such to estimate it.

Peter

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2001\08\29@163504 by David VanHorn

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At 10:26 PM 8/28/01 +0300, Peter L. Peres wrote:
> >>Apropos attenuation, any idea why a cellular phone works perfectly (S
> >>meter at 3 of 5 ?!) in a 4 x 2 x 2 meter welded steel safe with the door
> >>closed?
> >>Peter
> >
> >Well, no, but I just gotta ask you how you know this :)
>
>Ever got a call while doing things inside the safe - well strongbox - at
>work?

UHF signals are coming into the box on the skins of wires, conduits, etc.
In one case, I saw a "safe" (room with a vault door) that had a hole above
the ceiling that you could crawl through. It was where a jewelery shop was
keeping it's goods at night.

Things aren't always what they seem.
(Neither was their alarm system)
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2001\08\29@181559 by Nigel Goodwin

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In message <RemoveMEPine.LNX.4.10.10108282151280.619-100000spamspamBeGoneplp.plp.home.org>,> Peter L. Peres <spamBeGoneplp@spam@spamspam_OUTACTCOM.CO.IL> writes
>> Another point to bear in mind is that you don't have 700W wildly trying
>> to find it's way out of the cavity - most is absorbed by the load in the
>> oven (which should never be used empty).
>
>You wish ;-). Power distribution depends on the impedance ratios of the
>'hole' and of the food. I suspect that running the oven with the door open
>will reduce its 'food' power by 50 to 80%. This because any MW still
>heating it will pass through it once instead of many times and the power
>distribution would correspnd to that of a 'hole' the size of the open door
>with the food obstructing part of it. Ok, I admit this is extreme.

Well the power rating of a microwave oven is measured by the temperature
rise of a certain quantity of water (with a specified salinity, and
starting temperature within a defined range) with the oven on full power
for a certain time (usually 87 seconds for 2 x 500ml containers). So a
700w oven is rated as 700w into the load, the actual RF output of the
magnetron isn't considered.

I'm sure I don't need to mention that a microwave won't work with the
door open, and requires at least three safety switches bypassing to
allow it to do so!.
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2001\08\29@182224 by Jim

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Peter:
     "Both or none. I meant the cover joints
      normally used in RF proof equipment
      boxes, with a channel and an o-ring clad
      with wire (and sometimes flashing), as
      compared to oven doors. The simple box
      joints get it down by 40-55 dB in VHF/UHF
      in my experience, and they are more
      complex than the oven door joint."

I've had extensive experience sweeping and tuning both
commercial and amatuer band duplexers at between
50 MHz and 1300 MHz. I've also designed, modeled
and built duplexers for use in this same range of
frequencies. (A 'duplexer' allows the simultaneous use of
an antenna on two different frequencies - most normally
one frequency is receive and the other is transmit. The
duplexer supplies the much needed islolation between
the two thereby allowing simultaneous, concurrent
operation as required in a "repeater".)

These duplexers exhibit attentuation values from 40 dB
(the simpler designs) to greater than 95 dB attenuation
(the better-built commerical designs by the likes of
dB Products and Collins). It requires a great deal of
skill and the proper equipment using  double-shielded
(or "Super-flex" Heliax) at times to assure one is not
simply seeing coaxial cable leakage (at 150 MHz and
up) as opposed to DUT (device under test) 'leakage'.

Some of this product uses construction techniques *far*
simpler than you describe (i.e. an aluminum cylinder with
a tight-fitting cast end-cap fitted with coupling loops and
pop riveted in place,  *no* gaskets, no finger stock, etc.)
- with much better isolation and shielding results than you
describe.

It makes me wonder - what it is you were using to
make these measurements? Were the contacting
'metal' surfaces painted? Were you using double-
shielded coaxial cable on your test/measurement
instruments?

Jim

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2001\08\30@054413 by Roman Black

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David VanHorn wrote:

> >Ever got a call while doing things inside the safe - well strongbox - at
> >work?
>
> UHF signals are coming into the box on the skins of wires, conduits, etc.
> In one case, I saw a "safe" (room with a vault door) that had a hole above
> the ceiling that you could crawl through. It was where a jewelery shop was
> keeping it's goods at night.
>
> Things aren't always what they seem.
> (Neither was their alarm system)


Ha ha! We once looked at renting a business premises
that had been a Bank for many years. It had a huge
impressive steel strongroom door with enormous lock,
looked great to the customers. It was like a small
room, but the entire strongroom was just double brick
wall and the wall to the next shop was the same double
brick wall!! Maybe 2 minutes for any meathead with a
sledgehammer. Was most amusing. The door must have been
worth a lot of money though. :o)
-Roman

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2001\08\30@083403 by Alan B. Pearce

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>but the entire strongroom was just double brick
>wall and the wall to the next shop was the same double
>brick wall!! Maybe 2 minutes for any meathead with a
>sledgehammer. Was most amusing. The door must have been
>worth a lot of money though. :o)

Sounds like it was sold to the bank by a cellphone aerial salesman. ;))

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