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 <joecolquitt@clear.net.nz> wrote:
> 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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