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'[OT] Radiation from microwave oven'
2010\10\10@062511 by ivp

face picon face
Hi all,

I have subscriber TV (Sky digital) and now know that some channels
are affected by the microwave oven. In fact before I figured out it
was the oven causing the disturbance (seems obvious now) I rang
Sky and they correctly identified a particular group of channels that
would be similarly affected by whatever it was. The effect is that the
sound and picture breaks up and/or freezes until the oven is turned off

My niece was watching a film and I didn't want to disturb her viewing
pleasure so placed a metal tray in front of the receiver, which is enough
to prevent the disruption. I explained to her what the tray was for and
went about cooking

During an ad break I took the tray away and turned the empty oven
on to show her what happens. The odd thing I discovered though is
that an empty oven does not interfere with the receiver. Put a cup of
water in there though and as usual the reception goes to the dogs

Any thoughts about what could be radiating ?

I've looked around a little but the only mention I've found of "no load"
is on the Wikipedia page

"Another hazard is the resonance of the magnetron tube itself. If the
microwave is run without an object to absorb the radiation, a standing
wave will form. The energy is reflected back and forth between the
tube and the cooking chamber"

but that doesn't help any. I'd have thought if microwaves were
leaking the effect would be seen whether there was anything in the
oven

TIA

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2010\10\10@095252 by John Gardner

picon face
Perhaps the device senses an empty oven & does'nt power up?

Jac

2010\10\10@151941 by ivp

face picon face
> Perhaps the device senses an empty oven & does'nt power up?

Well, possible but perhaps unlikely. The oven is knocking on 25
years old and is basic as. It's not digital for example. Two rotary
dials - power and time - and a Start switch. That's yer lot

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2010\10\10@153104 by Richard Prosser

picon face
Joe,
A couple of thoughts on the subject.

I don't know what the Sky frequencies are but believe them to be well
separated from the microwave oven frequency at 2.4GHz. The oven
frequency is chosen as this is a major water absorption peak - which
is exactly what you don't want for a radio transmission.

I'm also a bit interested in the steel plate thing. This would
indicate that the receiver is detecting the oven directly, rather than
it being picked up in the LNB at the antenna. Since the LNB converts
the incoming signal to an IF for retransmission to the decoder box it
would appear that this band is the one being interfered with.

It seems to me more likely that the microwave is generating RF noise
from it's power supply etc and this is what's causing the problem. I
know that older microwaves produced the high voltage from a large
power transformer, but I'm not so sure about modern ones. Our
microwave, for example, just doesn't feel heavy enough to have a large
transformer in it. So I'm guessing that modern microwaves have a more
electronic method of generating the high voltage and this is what's
causing the problem.  Also, I'm again guessing, that there is
protection against high VSWR conditions and that this protection is
cutting back the power under unloaded conditions. This could be
checked fairly easily by using an ammeter or power meter on the power
cable.

Not sure this helps at all but it could be that a decent power supply
filter on the power line to the microwave would fix the problem.

Richard

On 10 October 2010 22:25, ivp <spam_OUTjoecolquittTakeThisOuTspamclear.net.nz> wrote:
{Quote hidden}

>

2010\10\10@153623 by Richard Prosser

picon face
Joe,

Just seen your latest post - probably not an electronic boost system then.

Maybe switching noise from the high voltage diiodes??

It still would be worthwhile trying a power supply filter on the AC..

RP


On 11 October 2010 08:19, ivp <.....joecolquittKILLspamspam@spam@clear.net.nz> wrote:
{Quote hidden}

>

2010\10\10@154057 by Michael Watterson

face picon face
 On 10/10/2010 20:31, Richard Prosser wrote:
{Quote hidden}

Oven is 2.5GHz.
max Satellite IF frequency = 12.6GHz - 10.6 GHz approx = 2GHz

I'm puzzled

2010\10\10@155549 by jim

flavicon
face
You probably have a bad safety switch.  There are usually two or three on
the door.  One near the latch, and one or two around the door that are
activated when the door opens and closes.  If any of these switches aren't
working, they will keep it from powering up.  There is also a fuse that is
rated at a few amps and about 300volts or so in the primary circuit.  Make
sure you replace this fuse with the same voltage and current rating.  DO NOT
use one rated at 125.250 volts.  It won't work for long and it could create
a shock/fire hazard.  
I used to work on microwave ovens many years ago.  But they started making
them more reliable and much less expensive, so it became economically
impractical to repair them.  It's cheaper to just buy a new one now days.
Unless of course you can fix it yourself.  The switches I mention above are
typically just microswitches.  They control only primary voltage, so
virtually any microswitch will work.  They big thing is the fuse.  That is
the most critical.

Regards,

Jim  
{Original Message removed}

2010\10\10@155652 by jim

flavicon
face
Microwave ovens operate at 2450 Mhz. or 2.45 Ghz. Typically.

Regards,

Jim

-----Original Message-----
From: .....piclist-bouncesKILLspamspam.....mit.edu [EraseMEpiclist-bouncesspam_OUTspamTakeThisOuTmit.edu] On Behalf Of
Richard Prosser
Sent: Sunday, October 10, 2010 2:31 PM
To: Microcontroller discussion list - Public.
Subject: Re: [OT] Radiation from microwave oven

Joe,
A couple of thoughts on the subject.

I don't know what the Sky frequencies are but believe them to be well
separated from the microwave oven frequency at 2.4GHz. The oven
frequency is chosen as this is a major water absorption peak - which
is exactly what you don't want for a radio transmission.

I'm also a bit interested in the steel plate thing. This would
indicate that the receiver is detecting the oven directly, rather than
it being picked up in the LNB at the antenna. Since the LNB converts
the incoming signal to an IF for retransmission to the decoder box it
would appear that this band is the one being interfered with.

It seems to me more likely that the microwave is generating RF noise
from it's power supply etc and this is what's causing the problem. I
know that older microwaves produced the high voltage from a large
power transformer, but I'm not so sure about modern ones. Our
microwave, for example, just doesn't feel heavy enough to have a large
transformer in it. So I'm guessing that modern microwaves have a more
electronic method of generating the high voltage and this is what's
causing the problem.  Also, I'm again guessing, that there is
protection against high VSWR conditions and that this protection is
cutting back the power under unloaded conditions. This could be
checked fairly easily by using an ammeter or power meter on the power
cable.

Not sure this helps at all but it could be that a decent power supply
filter on the power line to the microwave would fix the problem.

Richard

On 10 October 2010 22:25, ivp <joecolquittspamspam_OUTclear.net.nz> wrote:
{Quote hidden}

>

2010\10\10@161200 by ivp

face picon face
> I'm also a bit interested in the steel plate thing. This would
> indicate that the receiver is detecting the oven directly, rather than
> it being picked up in the LNB at the antenna

That's true. It appears to be quite a line-of-sight thing between the
oven and the decoder (ie set-top box next to the TV), across the
kitchen and the lounge, about 10m. A set-top box further down
the house is not interfered with. The actual transmission receiver is
a dish on the roof, sorry if that caused confusion

If I'm between the oven (close to the oven), and the TV the effect
is very much lessened but you need to find a position and stay still. A
metal tray in front of the set-top box though is an effective blocker

Knowing that the oven does this is only a minor inconvenience,
but finding that an empty oven doesn't interfere has me curious

Joe

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2010\10\10@161534 by ivp

face picon face
> You probably have a bad safety switch

Hi Jim. How does that explain that an empty oven doesn't
produce the interference that an oven with a cup of water
inside does ?

I should have a look around for the manual to see if the
oven can detect no load. You'd wonder though, if it does,
why bother turning on the light, fan and turntable

???

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2010\10\10@162620 by jim

flavicon
face
Maybe with water (or other load) inside, the variations of absorption of the
RF energy causes a slight modulation (AM) that is picked up as interference..

Jim

{Original Message removed}

2010\10\11@023209 by Ruben Jönsson

flavicon
face
> Joe,
> A couple of thoughts on the subject.
>
> I don't know what the Sky frequencies are but believe them to be well
> separated from the microwave oven frequency at 2.4GHz. The oven
> frequency is chosen as this is a major water absorption peak - which
> is exactly what you don't want for a radio transmission.

There is an ISM band between 2.4-2.5GHz. That's where WLAN, Bluetooth, zigbee and other IEEE 802.11 uses operates. Which would be another bad reason to put cable TV frequencies there.

Joe, if you want to know what really happens I think the only way to be sure is to use a spectrum analyzer with an appropriate antenna and look at what happens when running the oven with and without load.
/Ruben
==============================
Ruben Jönsson
AB Liros Electronic
Box 9124, 200 39 Malmö, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
@spam@rubenKILLspamspampp.sbbs.se
==============================

2010\10\11@030914 by Michael Watterson

face picon face
 On 11/10/2010 07:31, Ruben Jönsson wrote:
>> Joe,
>> A couple of thoughts on the subject.
>>
>> I don't know what the Sky frequencies are but believe them to be well
>> separated from the microwave oven frequency at 2.4GHz. The oven
>> frequency is chosen as this is a major water absorption peak - which
>> is exactly what you don't want for a radio transmission.
> There is an ISM band between 2.4-2.5GHz. That's where WLAN, Bluetooth, zigbee
> and other IEEE 802.11 uses operates. Which would be another bad reason to put
> cable TV frequencies there.
>
> Joe, if you want to know what really happens I think the only way to be sure is
> to use a spectrum analyzer with an appropriate antenna and look at what happens
> when running the oven with and without load.
>
> /Ruben
> ==============================

Yes as I said earlier, there is nothing on Satellite IF above 2000MHz usually. Sat IF is 950MHz to 2100MHz.
The actual signal picked up by Dish is 10.7 GHz to 12.6Ghz band.

I've heard of interference of WiFi by Microwave, Though I'd wonder about the screening. Ours doesn't affect WiFi.

Power is usually a big transformer. So I'm puzzled

2010\10\11@124729 by Alan Melia

flavicon
face
Maybe http://www.metageek.net/products/wi-spy might give you some of an idea
of what is going on.
Alan Melia
w: http://www.melmac.co.uk

The information contained in this message may be confidential information
subject to the terms and conditions of a confidentiality agreement between
Melmac Solutions Ltd. and you or your employer.  If you have received this
message in error please notify the sender immediately and delete the email
from your computer. You should not copy or forward this email, use it for
any purpose, or disclose its contents to any other person. 
{Original Message removed}

2010\10\12@024901 by Sean Breheny

face picon face
Two possibilities off the top of my head:

1) Adding a load changes the frequency of the magnetron and whatever
it is interfering with in the box is specific to a certain frequency
range.

2) The magnetron may not be able to oscillate at all with no load

Am I correct that all of the RF connections to the set top box are
shielded cable? If so, this seems a bit scary - I wouldn't think that
the box should be susceptible to such a small signal as can leak out
of a properly-shielded microwave oven. Even if the IF input were right
at 2.4GHz, there should be a continuous shield from the LNB to the set
top box and it should take significant power to inject through this
shielding.

Do you have a different microwave oven you could try? Do you have wifi
equipment? Does it operate properly when the microwave is running?

Sean



On Sun, Oct 10, 2010 at 6:25 AM, ivp <KILLspamjoecolquittKILLspamspamclear.net.nz> wrote:
{Quote hidden}

>

2010\10\12@040431 by alan.b.pearce

face picon face
> Am I correct that all of the RF connections to the set top box are
> shielded cable?

Probably wouldn't matter if they were because in my experience set top
boxes tend to be a plastic box with no aquadag or similar RF shielding
sprayed on the inside, so the electronics is 'exposed' to the
interference.
-- Scanned by iCritical.

2010\10\12@043715 by Michael Watterson

face picon face
 On 12/10/2010 09:05, RemoveMEalan.b.pearceTakeThisOuTspamstfc.ac.uk wrote:
>> Am I correct that all of the RF connections to the set top box are
>> shielded cable?
> Probably wouldn't matter if they were because in my experience set top
> boxes tend to be a plastic box with no aquadag or similar RF shielding
> sprayed on the inside, so the electronics is 'exposed' to the
> interference.
The tuner is in a Metal Can.

There is no way a Satellite Receiver should be susceptible to 2.3 to 2.6GHz

You need a Spectrum Analyser that covers 900MHz to 3GHz to see what is happening

2010\10\12@071816 by Olin Lathrop

face picon face
alan.b.pearce@stfc.ac.uk wrote:
> Probably wouldn't matter if they were because in my experience set top
> boxes tend to be a plastic box with no aquadag or similar RF shielding
> sprayed on the inside, so the electronics is 'exposed' to the
> interference.

Sounds like a a little alumninum foil in the right place is a easy fix.


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2010\10\12@133433 by Marcel Duchamp

picon face
On 10/11/2010 11:48 PM, Sean Breheny wrote:
>
> Am I correct that all of the RF connections to the set top box are
> shielded cable? If so, this seems a bit scary - I wouldn't think that
> the box should be susceptible to such a small signal as can leak out
> of a properly-shielded microwave oven. Even if the IF input were right
> at 2.4GHz, there should be a continuous shield from the LNB to the set
> top box and it should take significant power to inject through this
> shielding.
>

Where I work, the microwave oven would frequently knock out our 2.4 gig wireless lan network.  It made me wonder the same thing..

2010\10\12@142602 by jim

flavicon
face

All,

To me, this makes some sense.  A microwave oven operates at 2450
Mhz(2.45Ghz) which puts it in band.
A typical microwave generates between 500 and 1k watts.  Some of this
can leak out of the case and  ride on the power lines powering the oven.  The powerlines then radiate
the energy, and cause trouble.  Especially if the lines run close to a LAN cable junction, switch,
router or hub.  There are components
in both the microwave oven and in the switch/hub or router that
generally take care of any RFI on the
power lines, but with the strength of the microwave energy to start
with, some of it can get through by
brute strength (RF Field Strength).  And, the older the microwave, the
less effective these protection
schemes are, if the particular oven even has them. Some of the older
ones doon't have the protection that
the newer ones do.  And the older ones technology isn't as advanced as
the more current ones.
So, taking all of this into account, it seems to reason that what is
being seen could happen.
Why it happens only when there is a load though is a puzzler.  I still
believe it has to do with modulation
effects.
   Regards,

Jim

> ---{Original Message removed}

2010\10\12@143502 by Herbert Graf

picon face
On Tue, 2010-10-12 at 11:26 -0700, spamBeGonejimspamBeGonespamjpes.com wrote:
>  So, taking all of this into account, it seems to reason that what is
> being seen could happen.
>  Why it happens only when there is a load though is a puzzler.  I still
> believe it has to do with modulation
>  effects.

The great thing about magnetrons is they are very small and cheap to
build for their amount of power handling.

The bad thing about magnetrons is their oscillating frequency isn't
constant, it changes with the temp of the magnetron.
That would explain why you observe interference with loaded but not when
unloaded.

TTYL

2010\10\12@151841 by Sean Breheny

face picon face
Yes, but the equipment which the OP is having trouble with is NOT wifi
but rather a satellite TV receiver which does not operate at 2.4GHz.
Moreover, the interference doesn't seem to be entering through the
antenna (which is on the roof) or through the power line but by direct
line of sight from the microwave since he can stop the interference by
simply holding a metal sheet between the oven and the set-top box.

I am not surprised that a microwave would interfere with 2.4GHz wifi -
the oven is allowed to radiate a few 100 mW I think, so that would
overpower the majority of wifi cards/routers and even access points. I
am surprised, though, that 100mW or even 1W would be enough to
interfere with a device operating on a different frequency band and
whose antenna is not the entry point for the interference.

Joe (OP), have you tried holding that metal shielding plate at various
distances from the microwave but along the  line of sight? It would be
interesting to see if it still prevents the interference when you are
holding it near the microwave oven instead of near the set-top box.

Sean


On Tue, Oct 12, 2010 at 1:34 PM, Marcel Duchamp
<TakeThisOuTmarcel.duchampEraseMEspamspam_OUTsbcglobal.net> wrote:
{Quote hidden}

>

2010\10\12@153859 by jim

flavicon
face


Sean,

If the RF field strength is high enough, it can overpower the frontend
of the RF chain in the satellite
receiver, regardless of whether it is at the actual operating frequency
or not.  

Regards,

Jim

> ---{Original Message removed}

2010\10\12@163942 by ivp

face picon face
> Joe (OP), have you tried holding that metal shielding plate at various
> distances from the microwave but along the  line of sight? It would be
> interesting to see if it still prevents the interference when you are
> holding it near the microwave oven instead of near the set-top box

Surrounding the microwave oven with several cooking trays (~1mm
thick, 12" x 16") is much less effective than one tray in front of the set-
top box

AIUI, the microwave oven enclosure is supposed to be a Faraday
cage at microwave frequencies, for the obvious reason. So what is
being radiated could be longer wavelength RF ?

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2010\10\12@164130 by Michael Watterson

face picon face
 On 12/10/2010 19:26, RemoveMEjimspamTakeThisOuTjpes.com wrote:
{Quote hidden}

So why does it affect the 950Mhz to 2100Mhz (really actually only upto 2000MHz) Satellite IF? High level signal and all the IF to Baseband (Zero IF I &Q)) of the Satellite receiver is in the small metal can on F-Connector in "receiver" the rest of the box is PSU, DSP, Digital to Analogue and GUI CPU etc

2010\10\12@165845 by Richard Prosser

picon face
Joe,

A longer wavelength could certainly be being radiated - for example
the power supply diodes are a possibility even if there is minimal
electronics elsewhere.

Also, the radiation may not be from the oven itself, it may be
injecting signal into the AC wiring & radiating from there  There's
also a possibility that any built-in filter has become bad over time -
or, more likely, nothing much was fitted in the first place.

Have you gat a suitable AC filter you can place close to the oven?

RP

On 13 October 2010 09:39, ivp <joecolquittEraseMEspam.....clear.net.nz> wrote:
{Quote hidden}

>

2010\10\12@173656 by Sean Breheny

face picon face
I agree, Jim, but if that were the case you would expect that it
wouldn't matter whether Joe held a metal tray near the set-top box,
since the antenna is  on the roof.

Sean


On Tue, Oct 12, 2010 at 3:38 PM,  <EraseMEjimspamjpes.com> wrote:
{Quote hidden}

>> ---{Original Message removed}

2010\10\12@174511 by Herbert Graf

picon face
On Tue, 2010-10-12 at 17:36 -0400, Sean Breheny wrote:
> I agree, Jim, but if that were the case you would expect that it
> wouldn't matter whether Joe held a metal tray near the set-top box,
> since the antenna is  on the roof.

The LNB isn't the only place RF can get into the box.

Obviously the plate tells us RF is getting in through the case.
Considering the abysmal sheilding I've seen in many STBs this doesn't
surprise me in the least.

I admit, it's odd that the STB is sensitive to 2.4GHz, but given enough
wattage any receive is sensitive.

I wonder: is the STB grounded well?

TTYL

2010\10\12@181053 by jim

flavicon
face


I would almost wager that if you move the pan to different places in
front of the set-top box,
the interference would come and go.  I say this because he may have it
in just the right spot
front of the box to null out the signal.  If it were moved further
away, it might let the interference
come back.  Move a little further away, and the interference is stopped
again.  
 


Regards,

Jim

> ---{Original Message removed}

2010\10\13@111302 by Oli Glaser

flavicon
face


--------------------------------------------------
From: "Sean Breheny" <RemoveMEshb7EraseMEspamEraseMEcornell.edu>
Sent: Tuesday, October 12, 2010 8:18 PM
To: "Microcontroller discussion list - Public." <RemoveMEpiclistspam_OUTspamKILLspammit.edu>
Subject: Re: [OT] Radiation from microwave oven

> Joe (OP), have you tried holding that metal shielding plate at various
> distances from the microwave but along the  line of sight? It would be
> interesting to see if it still prevents the interference when you are
> holding it near the microwave oven instead of near the set-top box.

Interesting thread this..
It does indeed seem that some scientific investigation is needed, to prove/disprove the various postulations presented. A decent SA would be very handy here, and a directional antenna. Plus monitoring the current (radiation frequency if SA available) etc when the oven has nothing in it as suggested would be a start towards figuring out why it does not happen without anything in the oven.
If you don't have a SA, maybe you could knock together a RF power meter of some sort.

2010\10\13@235928 by Justin Richards

face picon face
My explanation of interference when there is a load (product getting heated).

The load I understand act as a receiving antenna and the load has its
electrons accelerated backwards and forwards generating heat .  As it
is receiving it is also capable of transmitting.  I suspect that the
shielding on the door is passing a little RF from the load as it is at
the right height sitting on the plate.

When there is no load there is no RF directed at the door.

This assumes that the door shielding is not 100% effective as a piece
of sheet steel.

On 13 October 2010 23:12, Oli Glaser <RemoveMEoli.glaserTakeThisOuTspamspamtalktalk.net> wrote:
{Quote hidden}

>

2010\10\14@042254 by ivp

face picon face

> Interesting thread this..

Interesting in a "Flip, wish I hadn't noticed that" way ;-))

> It does indeed seem that some scientific investigation is needed, to
> prove/disprove the various postulations presented

Not really in a position to do that right now. I don't have the gear to
do any tests, but know someone who does, who I see infrequently.
The phenomenum will still be there when I get around to it. Might
borrow the neighbour's microwave though, see what happens

Probably wouldn't recognise anything useful if I found it anyway.
Could I be the next 'Pons', '&' or 'Fleischmann' ?

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2010\10\14@044832 by alan.b.pearce

face picon face
> On Tue, 2010-10-12 at 17:36 -0400, Sean Breheny wrote:
> > I agree, Jim, but if that were the case you would expect that it
> > wouldn't matter whether Joe held a metal tray near the set-top box,
> > since the antenna is  on the roof.
>
> The LNB isn't the only place RF can get into the box.
>
> Obviously the plate tells us RF is getting in through the case.
> Considering the abysmal sheilding I've seen in many STBs this doesn't
> surprise me in the least.
>
> I admit, it's odd that the STB is sensitive to 2.4GHz, but given
enough
> wattage any receive is sensitive.
>
> I wonder: is the STB grounded well?

My suspicion would be that there are tracks on the PCB that are just the
right length to be 1/4 wavelength aerials at whatever is being radiated
from the microwave. Then there will be transmission line effects that
will cause high voltage nodes that will create the equivalent of the
"police radio getting into the hifi at the party" effect.
-- Scanned by iCritical.

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